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Using SELinux

Red Hat Enterprise Linux 9

Basic and advanced configuration of Security-Enhanced Linux (SELinux)

Red Hat Customer Content Services

Abstract

This title assists users and administrators in learning the basics and principles upon which SELinux functions and describes practical tasks to set up and configure various services.

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Chapter 1. Getting started with SELinux

Security Enhanced Linux (SELinux) provides an additional layer of system security. SELinux fundamentally answers the question: May <subject> do <action> to <object>?, for example: May a web server access files in users' home directories?

1.1. Introduction to SELinux

The standard access policy based on the user, group, and other permissions, known as Discretionary Access Control (DAC), does not enable system administrators to create comprehensive and fine-grained security policies, such as restricting specific applications to only viewing log files, while allowing other applications to append new data to the log files.

Security Enhanced Linux (SELinux) implements Mandatory Access Control (MAC). Every process and system resource has a special security label called an SELinux context. A SELinux context, sometimes referred to as an SELinux label, is an identifier which abstracts away the system-level details and focuses on the security properties of the entity. Not only does this provide a consistent way of referencing objects in the SELinux policy, but it also removes any ambiguity that can be found in other identification methods. For example, a file can have multiple valid path names on a system that makes use of bind mounts.

The SELinux policy uses these contexts in a series of rules which define how processes can interact with each other and the various system resources. By default, the policy does not allow any interaction unless a rule explicitly grants access.

Note

Remember that SELinux policy rules are checked after DAC rules. SELinux policy rules are not used if DAC rules deny access first, which means that no SELinux denial is logged if the traditional DAC rules prevent the access.

SELinux contexts have several fields: user, role, type, and security level. The SELinux type information is perhaps the most important when it comes to the SELinux policy, as the most common policy rule which defines the allowed interactions between processes and system resources uses SELinux types and not the full SELinux context. SELinux types end with _t. For example, the type name for the web server is httpd_t. The type context for files and directories normally found in /var/www/html/ is httpd_sys_content_t. The type contexts for files and directories normally found in /tmp and /var/tmp/ is tmp_t. The type context for web server ports is http_port_t.

There is a policy rule that permits Apache (the web server process running as httpd_t) to access files and directories with a context normally found in /var/www/html/ and other web server directories (httpd_sys_content_t). There is no allow rule in the policy for files normally found in /tmp and /var/tmp/, so access is not permitted. With SELinux, even if Apache is compromised, and a malicious script gains access, it is still not able to access the /tmp directory.

Figure 1.1. An example how can SELinux help to run Apache and MariaDB in a secure way.

SELinux_Apache_MariaDB_example

As the previous scheme shows, SELinux allows the Apache process running as httpd_t to access the /var/www/html/ directory and it denies the same process to access the /data/mysql/ directory because there is no allow rule for the httpd_t and mysqld_db_t type contexts. On the other hand, the MariaDB process running as mysqld_t is able to access the /data/mysql/ directory and SELinux also correctly denies the process with the mysqld_t type to access the /var/www/html/ directory labeled as httpd_sys_content_t.

Additional resources

  • selinux(8) man page and man pages listed by the apropos selinux command.
  • Man pages listed by the man -k _selinux command when the selinux-policy-doc package is installed.
  • The SELinux Coloring Book helps you to better understand SELinux basic concepts.
  • SELinux Wiki FAQ

1.2. Benefits of running SELinux

SELinux provides the following benefits:

  • All processes and files are labeled. SELinux policy rules define how processes interact with files, as well as how processes interact with each other. Access is only allowed if an SELinux policy rule exists that specifically allows it.
  • Fine-grained access control. Stepping beyond traditional UNIX permissions that are controlled at user discretion and based on Linux user and group IDs, SELinux access decisions are based on all available information, such as an SELinux user, role, type, and, optionally, a security level.
  • SELinux policy is administratively-defined and enforced system-wide.
  • Improved mitigation for privilege escalation attacks. Processes run in domains, and are therefore separated from each other. SELinux policy rules define how processes access files and other processes. If a process is compromised, the attacker only has access to the normal functions of that process, and to files the process has been configured to have access to. For example, if the Apache HTTP Server is compromised, an attacker cannot use that process to read files in user home directories, unless a specific SELinux policy rule was added or configured to allow such access.
  • SELinux can be used to enforce data confidentiality and integrity, as well as protecting processes from untrusted inputs.

However, SELinux is not:

  • antivirus software,
  • replacement for passwords, firewalls, and other security systems,
  • all-in-one security solution.

SELinux is designed to enhance existing security solutions, not replace them. Even when running SELinux, it is important to continue to follow good security practices, such as keeping software up-to-date, using hard-to-guess passwords, and firewalls.

1.3. SELinux examples

The following examples demonstrate how SELinux increases security:

  • The default action is deny. If an SELinux policy rule does not exist to allow access, such as for a process opening a file, access is denied.
  • SELinux can confine Linux users. A number of confined SELinux users exist in the SELinux policy. Linux users can be mapped to confined SELinux users to take advantage of the security rules and mechanisms applied to them. For example, mapping a Linux user to the SELinux user_u user, results in a Linux user that is not able to run unless configured otherwise set user ID (setuid) applications, such as sudo and su.
  • Increased process and data separation. The concept of SELinux domains allows defining which processes can access certain files and directories. For example, when running SELinux, unless otherwise configured, an attacker cannot compromise a Samba server, and then use that Samba server as an attack vector to read and write to files used by other processes, such as MariaDB databases.
  • SELinux helps mitigate the damage made by configuration mistakes. Domain Name System (DNS) servers often replicate information between each other in what is known as a zone transfer. Attackers can use zone transfers to update DNS servers with false information. When running the Berkeley Internet Name Domain (BIND) as a DNS server in Red Hat Enterprise Linux, even if an administrator forgets to limit which servers can perform a zone transfer, the default SELinux policy prevents zone files [1] from being updated using zone transfers, by the BIND named daemon itself, and by other processes.


[1] Text files that include information, such as host name to IP address mappings, that are used by DNS servers.

1.4. SELinux architecture and packages

SELinux is a Linux Security Module (LSM) that is built into the Linux kernel. The SELinux subsystem in the kernel is driven by a security policy which is controlled by the administrator and loaded at boot. All security-relevant, kernel-level access operations on the system are intercepted by SELinux and examined in the context of the loaded security policy. If the loaded policy allows the operation, it continues. Otherwise, the operation is blocked and the process receives an error.

SELinux decisions, such as allowing or disallowing access, are cached. This cache is known as the Access Vector Cache (AVC). When using these cached decisions, SELinux policy rules need to be checked less, which increases performance. Remember that SELinux policy rules have no effect if DAC rules deny access first. Raw audit messages are logged to the /var/log/audit/audit.log and they start with the type=AVC string.

In RHEL 9, system services are controlled by the systemd daemon; systemd starts and stops all services, and users and processes communicate with systemd using the systemctl utility. The systemd daemon can consult the SELinux policy and check the label of the calling process and the label of the unit file that the caller tries to manage, and then ask SELinux whether or not the caller is allowed the access. This approach strengthens access control to critical system capabilities, which include starting and stopping system services.

The systemd daemon also works as an SELinux Access Manager. It retrieves the label of the process running systemctl or the process that sent a D-Bus message to systemd. The daemon then looks up the label of the unit file that the process wanted to configure. Finally, systemd can retrieve information from the kernel if the SELinux policy allows the specific access between the process label and the unit file label. This means a compromised application that needs to interact with systemd for a specific service can now be confined by SELinux. Policy writers can also use these fine-grained controls to confine administrators.

If a process is sending a D-Bus message to another process and if the SELinux policy does not allow the D-Bus communication of these two processes, then the system prints a USER_AVC denial message, and the D-Bus communication times out. Note that the D-Bus communication between two processes works bidirectionally.

Important

To avoid incorrect SELinux labeling and subsequent problems, ensure that you start services using a systemctl start command.

RHEL 9 provides the following packages for working with SELinux:

  • policies: selinux-policy-targeted, selinux-policy-mls
  • tools: policycoreutils, policycoreutils-gui, libselinux-utils, policycoreutils-python-utils, setools-console, checkpolicy

1.5. SELinux states and modes

SELinux can run in one of three modes: enforcing, permissive, or disabled.

  • Enforcing mode is the default, and recommended, mode of operation; in enforcing mode SELinux operates normally, enforcing the loaded security policy on the entire system.
  • In permissive mode, the system acts as if SELinux is enforcing the loaded security policy, including labeling objects and emitting access denial entries in the logs, but it does not actually deny any operations. While not recommended for production systems, permissive mode can be helpful for SELinux policy development and debugging.
  • Disabled mode is strongly discouraged; not only does the system avoid enforcing the SELinux policy, it also avoids labeling any persistent objects such as files, making it difficult to enable SELinux in the future.

Use the setenforce utility to change between enforcing and permissive mode. Changes made with setenforce do not persist across reboots. To change to enforcing mode, enter the setenforce 1 command as the Linux root user. To change to permissive mode, enter the setenforce 0 command. Use the getenforce utility to view the current SELinux mode:

# getenforce
Enforcing
# setenforce 0
# getenforce
Permissive
# setenforce 1
# getenforce
Enforcing

In Red Hat Enterprise Linux, you can set individual domains to permissive mode while the system runs in enforcing mode. For example, to make the httpd_t domain permissive:

# semanage permissive -a httpd_t

Note that permissive domains are a powerful tool that can compromise security of your system. Red Hat recommends to use permissive domains with caution, for example, when debugging a specific scenario.

Chapter 2. Changing SELinux states and modes

When enabled, SELinux can run in one of two modes: enforcing or permissive. The following sections show how to permanently change into these modes.

2.1. Permanent changes in SELinux states and modes

As discussed in SELinux states and modes, SELinux can be enabled or disabled. When enabled, SELinux has two modes: enforcing and permissive.

Use the getenforce or sestatus commands to check in which mode SELinux is running. The getenforce command returns Enforcing, Permissive, or Disabled.

The sestatus command returns the SELinux status and the SELinux policy being used:

sestatus
SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             targeted
Current mode:                   enforcing
Mode from config file:          enforcing
Policy MLS status:              enabled
Policy deny_unknown status:     allowed
Memory protection checking:     actual (secure)
Max kernel policy version:      31
Warning

When systems run SELinux in permissive mode, users and processes might label various file-system objects incorrectly. File-system objects created while SELinux is disabled are not labeled at all. This behavior causes problems when changing to enforcing mode because SELinux relies on correct labels of file-system objects.

To prevent incorrectly labeled and unlabeled files from causing problems, SELinux automatically relabels file systems when changing from the disabled state to permissive or enforcing mode. Use the fixfiles -F onboot command as root to create the /.autorelabel file containing the -F option to ensure that files are relabeled upon next reboot.

Before rebooting the system for relabeling, make sure the system will boot in permissive mode, for example by using the enforcing=0 kernel option. This prevents the system from failing to boot in case the system contains unlabeled files required by systemd before launching the selinux-autorelabel service. For more information, see RHBZ#2021835.

2.2. Changing to permissive mode

Use the following procedure to permanently change SELinux mode to permissive. When SELinux is running in permissive mode, SELinux policy is not enforced. The system remains operational and SELinux does not deny any operations but only logs AVC messages, which can be then used for troubleshooting, debugging, and SELinux policy improvements. Each AVC is logged only once in this case.

Prerequisites

  • The selinux-policy-targeted, libselinux-utils, and policycoreutils packages are installed on your system.
  • The selinux=0 or enforcing=0 kernel parameters are not used.

Procedure

  1. Open the /etc/selinux/config file in a text editor of your choice, for example:

    # vi /etc/selinux/config
  2. Configure the SELINUX=permissive option:

    # This file controls the state of SELinux on the system.
    # SELINUX= can take one of these three values:
    #       enforcing - SELinux security policy is enforced.
    #       permissive - SELinux prints warnings instead of enforcing.
    #       disabled - No SELinux policy is loaded.
    SELINUX=permissive
    # SELINUXTYPE= can take one of these two values:
    #       targeted - Targeted processes are protected,
    #       mls - Multi Level Security protection.
    SELINUXTYPE=targeted
  3. Restart the system:

    # reboot

Verification

  1. After the system restarts, confirm that the getenforce command returns Permissive:

    $ getenforce
    Permissive

2.3. Changing to enforcing mode

Use the following procedure to switch SELinux to enforcing mode. When SELinux is running in enforcing mode, it enforces the SELinux policy and denies access based on SELinux policy rules. In RHEL, enforcing mode is enabled by default when the system was initially installed with SELinux.

Prerequisites

  • The selinux-policy-targeted, libselinux-utils, and policycoreutils packages are installed on your system.
  • The selinux=0 or enforcing=0 kernel parameters are not used.

Procedure

  1. Open the /etc/selinux/config file in a text editor of your choice, for example:

    # vi /etc/selinux/config
  2. Configure the SELINUX=enforcing option:

    # This file controls the state of SELinux on the system.
    # SELINUX= can take one of these three values:
    #       enforcing - SELinux security policy is enforced.
    #       permissive - SELinux prints warnings instead of enforcing.
    #       disabled - No SELinux policy is loaded.
    SELINUX=enforcing
    # SELINUXTYPE= can take one of these two values:
    #       targeted - Targeted processes are protected,
    #       mls - Multi Level Security protection.
    SELINUXTYPE=targeted
  3. Save the change, and restart the system:

    # reboot

    On the next boot, SELinux relabels all the files and directories within the system and adds SELinux context for files and directories that were created when SELinux was disabled.

Verification

  1. After the system restarts, confirm that the getenforce command returns Enforcing:

    $ getenforce
    Enforcing
Note

After changing to enforcing mode, SELinux may deny some actions because of incorrect or missing SELinux policy rules. To view what actions SELinux denies, enter the following command as root:

# ausearch -m AVC,USER_AVC,SELINUX_ERR,USER_SELINUX_ERR -ts today

Alternatively, with the setroubleshoot-server package installed, enter:

# grep "SELinux is preventing" /var/log/messages

If SELinux is active and the Audit daemon (auditd) is not running on your system, then search for certain SELinux messages in the output of the dmesg command:

# dmesg | grep -i -e type=1300 -e type=1400

See Troubleshooting problems related to SELinux for more information.

2.4. Enabling SELinux on systems that previously had it disabled

To avoid problems, such as systems unable to boot or process failures, follow this procedure when enabling SELinux on systems that previously had it disabled.

Warning

When systems run SELinux in permissive mode, users and processes might label various file-system objects incorrectly. File-system objects created while SELinux is disabled are not labeled at all. This behavior causes problems when changing to enforcing mode because SELinux relies on correct labels of file-system objects.

To prevent incorrectly labeled and unlabeled files from causing problems, SELinux automatically relabels file systems when changing from the disabled state to permissive or enforcing mode.

Before rebooting the system for relabeling, make sure the system will boot in permissive mode, for example by using the enforcing=0 kernel option. This prevents the system from failing to boot in case the system contains unlabeled files required by systemd before launching the selinux-autorelabel service. For more information, see RHBZ#2021835.

Procedure

  1. Enable SELinux in permissive mode. For more information, see Changing to permissive mode.
  2. Restart your system:

    # reboot
  3. Check for SELinux denial messages.For more information, see Identifying SELinux denials.
  4. Ensure that files are relabeled upon the next reboot:

    # fixfiles -F onboot

    This creates the /.autorelabel file containing the -F option.

    Warning

    Always switch to permissive mode before entering the fixfiles -F onboot command. This prevents the system from failing to boot in case the system contains unlabeled files. For more information, see RHBZ#2021835.

  5. If there are no denials, switch to enforcing mode. For more information, see Changing SELinux modes at boot time.

Verification

  1. After the system restarts, confirm that the getenforce command returns Enforcing:

    $ getenforce
    Enforcing
Note

To run custom applications with SELinux in enforcing mode, choose one of the following scenarios:

  • Run your application in the unconfined_service_t domain.
  • Write a new policy for your application. See the Writing a custom SELinux policy section for more information.

Additional resources

2.5. Disabling SELinux

When SELinux is disabled, SELinux policy is not loaded at all; it is not enforced and AVC messages are not logged. Therefore, all benefits of running SELinux are lost.

Important

Red Hat strongly recommends to use permissive mode instead of permanently disabling SELinux. See Changing to permissive mode for more information about permissive mode.

Prerequisites

  • The grubby package is installed:

    $ rpm -q grubby
    grubby-version

Procedure

To permanently disable SELinux:

  1. Configure your bootloader to add selinux=0 to the kernel command line:

    $ sudo grubby --update-kernel ALL --args selinux=0
  2. Restart your system:

    $ reboot

Verification

  • After reboot, confirm that the getenforce command returns Disabled:

    $ getenforce
    Disabled

2.6. Changing SELinux modes at boot time

On boot, you can set several kernel parameters to change the way SELinux runs:

enforcing=0

Setting this parameter causes the system to start in permissive mode, which is useful when troubleshooting issues. Using permissive mode might be the only option to detect a problem if your file system is too corrupted. Moreover, in permissive mode, the system continues to create the labels correctly. The AVC messages that are created in this mode can be different than in enforcing mode.

In permissive mode, only the first denial from a series of the same denials is reported. However, in enforcing mode, you might get a denial related to reading a directory, and an application stops. In permissive mode, you get the same AVC message, but the application continues reading files in the directory and you get an AVC for each denial in addition.

selinux=0

This parameter causes the kernel to not load any part of the SELinux infrastructure. The init scripts notice that the system booted with the selinux=0 parameter and touch the /.autorelabel file. This causes the system to automatically relabel the next time you boot with SELinux enabled.

Important

Red Hat does not recommend using the selinux=0 parameter. To debug your system, prefer using permissive mode.

autorelabel=1

This parameter forces the system to relabel similarly to the following commands:

# touch /.autorelabel
# reboot

If a file system contains a large amount of mislabeled objects, start the system in permissive mode to make the autorelabel process successful.

Additional resources

  • For additional SELinux-related kernel boot parameters, such as checkreqprot, see the /usr/share/doc/kernel-doc-<KERNEL_VER>/Documentation/admin-guide/kernel-parameters.txt file installed with the kernel-doc package. Replace the <KERNEL_VER> string with the version number of the installed kernel, for example:

    # dnf install kernel-doc
    $ less /usr/share/doc/kernel-doc-4.18.0/Documentation/admin-guide/kernel-parameters.txt

Chapter 3. Managing confined and unconfined users

The following sections explain the mapping of Linux users to SELinux users, describe the basic confined user domains, and demonstrate mapping a new user to an SELinux user.

3.1. Confined and unconfined users

Each Linux user is mapped to an SELinux user using SELinux policy. This allows Linux users to inherit the restrictions on SELinux users.

To see the SELinux user mapping on your system, use the semanage login -l command as root:

semanage login -l
Login Name           SELinux User         MLS/MCS Range        Service

__default__          unconfined_u         s0-s0:c0.c1023       *
root                 unconfined_u         s0-s0:c0.c1023       *

In Red Hat Enterprise Linux, Linux users are mapped to the SELinux default login by default, which is mapped to the SELinux unconfined_u user. The following line defines the default mapping:

__default__          unconfined_u         s0-s0:c0.c1023       *

Confined users are restricted by SELinux rules explicitly defined in the current SELinux policy. Unconfined users are subject to only minimal restrictions by SELinux.

Confined and unconfined Linux users are subject to executable and writable memory checks, and are also restricted by MCS or MLS.

To list the available SELinux users, enter the following command:

$ seinfo -u
Users: 8
   guest_u
   root
   staff_u
   sysadm_u
   system_u
   unconfined_u
   user_u
   xguest_u

Note that the seinfo command is provided by the setools-console package, which is not installed by default.

If an unconfined Linux user executes an application that SELinux policy defines as one that can transition from the unconfined_t domain to its own confined domain, the unconfined Linux user is still subject to the restrictions of that confined domain. The security benefit of this is that, even though a Linux user is running unconfined, the application remains confined. Therefore, the exploitation of a flaw in the application can be limited by the policy.

Similarly, we can apply these checks to confined users. Each confined user is restricted by a confined user domain. The SELinux policy can also define a transition from a confined user domain to its own target confined domain. In such a case, confined users are subject to the restrictions of that target confined domain. The main point is that special privileges are associated with the confined users according to their role.

3.2. SELinux user capabilities

The SELinux policy maps each Linux user to an SELinux user. This allows Linux users to inherit the restrictions of SELinux users.

You can customize the permissions for confined users in your SELinux policy according to specific needs by adjusting the booleans in the policy. You can determine the current state of these booleans by using the semanage boolean -l command.

Table 3.1. Roles of SELinux users

UserDefault roleAdditional roles

unconfined_u

unconfined_r

system_r

guest_u

guest_r

 

xguest_u

xguest_r

 

user_u

user_r

 

staff_u

staff_r

sysadm_r

unconfined_r

system_r

sysadm_u

sysadm_r

 

root

staff_r

sysadm_r

unconfined_r

system_r

system_u

system_r

 

Note that system_u is a special user identity for system processes and objects, and system_r is the associated role. Administrators must never associate this system_u user and the system_r role to a Linux user. Also, unconfined_u and root are unconfined users. For these reasons, the roles associated to these SELinux users are not included in the following table Types and access of SELinux roles.

Each SELinux role corresponds to an SELinux type and provides specific access rights.

Table 3.2. Types and access of SELinux roles

RoleTypeLog in using X Window Systemsu and sudoExecute in home directory and /tmp (default)Networking

unconfined_r

unconfined_t

yes

yes

yes

yes

guest_r

guest_t

no

no

yes

no

xguest_r

xguest_t

yes

no

yes

web browsers only (Firefox, GNOME Web)

user_r

user_t

yes

no

yes

yes

staff_r

staff_t

yes

only sudo

yes

yes

auditadm_r

auditadm_t

 

yes

yes

yes

secadm_r

secadm_t

 

yes

yes

yes

sysadm_r

sysadm_t

only when the xdm_sysadm_login boolean is on

yes

yes

yes

  • Linux users in the user_t, guest_t, and xguest_t domains can only run set user ID (setuid) applications if SELinux policy permits it (for example, passwd). These users cannot run the su and sudo setuid applications, and therefore cannot use these applications to become root.
  • Linux users in the sysadm_t, staff_t, user_t, and xguest_t domains can log in using the X Window System and a terminal.
  • By default, Linux users in the staff_t, user_t, guest_t, and xguest_t domains can execute applications in their home directories and /tmp.

    To prevent them from executing applications, which inherit users' permissions, in directories they have write access to, set the guest_exec_content and xguest_exec_content booleans to off. This helps prevent flawed or malicious applications from modifying users' files.

  • The only network access Linux users in the xguest_t domain have is Firefox connecting to web pages.
  • The sysadm_u user cannot log in directly using SSH. To enable SSH logins for sysadm_u, set the ssh_sysadm_login boolean to on:

    # setsebool -P ssh_sysadm_login on

Alongside with the already mentioned SELinux users, there are special roles, that can be mapped to those users using the semanage user command. These roles determine what SELinux allows the user to do:

  • webadm_r can only administrate SELinux types related to the Apache HTTP Server.
  • dbadm_r can only administrate SELinux types related to the MariaDB database and the PostgreSQL database management system.
  • logadm_r can only administrate SELinux types related to the syslog and auditlog processes.
  • secadm_r can only administrate SELinux.
  • auditadm_r can only administrate processes related to the Audit subsystem.

To list all available roles, enter the the seinfo -r command:

seinfo -r
Roles: 14
   auditadm_r
   dbadm_r
   guest_r
   logadm_r
   nx_server_r
   object_r
   secadm_r
   staff_r
   sysadm_r
   system_r
   unconfined_r
   user_r
   webadm_r
   xguest_r

Note that the seinfo command is provided by the setools-console package, which is not installed by default.

Additional resources

  • seinfo(1), semanage-login(8), and xguest_selinux(8) man pages

3.3. Adding a new user automatically mapped to the SELinux unconfined_u user

The following procedure demonstrates how to add a new Linux user to the system. The user is automatically mapped to the SELinux unconfined_u user.

Prerequisites

  • The root user is running unconfined, as it does by default in Red Hat Enterprise Linux.

Procedure

  1. Enter the following command to create a new Linux user named example.user:

    useradd example.user
  2. To assign a password to the Linux example.user user:

    passwd example.user
    Changing password for user example.user.
    New password:
    Retype new password:
    passwd: all authentication tokens updated successfully.
  3. Log out of your current session.
  4. Log in as the Linux example.user user. When you log in, the pam_selinux PAM module automatically maps the Linux user to an SELinux user (in this case, unconfined_u), and sets up the resulting SELinux context. The Linux user’s shell is then launched with this context.

Verification

  1. When logged in as the example.user user, check the context of a Linux user:

    $ id -Z
    unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

Additional resources

  • pam_selinux(8) man page.

3.4. Adding a new user as an SELinux-confined user

Use the following steps to add a new SELinux-confined user to the system. This example procedure maps the user to the SELinux staff_u user right with the command for creating the user account.

Prerequisites

  • The root user is running unconfined, as it does by default in Red Hat Enterprise Linux.

Procedure

  1. Enter the following command to create a new Linux user named example.user and map it to the SELinux staff_u user:

    useradd -Z staff_u example.user
  2. To assign a password to the Linux example.user user:

    passwd example.user
    Changing password for user example.user.
    New password:
    Retype new password:
    passwd: all authentication tokens updated successfully.
  3. Log out of your current session.
  4. Log in as the Linux example.user user. The user’s shell launches with the staff_u context.

Verification

  1. When logged in as the example.user user, check the context of a Linux user:

    $ id -Z
    uid=1000(example.user) gid=1000(example.user) groups=1000(example.user) context=staff_u:staff_r:staff_t:s0-s0:c0.c1023

Additional resources

  • pam_selinux(8) man page.

3.5. Confining regular users

You can confine all regular users on your system by mapping them to the user_u SELinux user.

By default, all Linux users in Red Hat Enterprise Linux, including users with administrative privileges, are mapped to the unconfined SELinux user unconfined_u. You can improve the security of the system by assigning users to SELinux confined users. This is useful to conform with the V-71971 Security Technical Implementation Guide.

Procedure

  1. Display the list of SELinux login records. The list displays the mappings of Linux users to SELinux users:

    # semanage login -l
    
    Login Name    SELinux User  MLS/MCS Range   Service
    
    __default__   unconfined_u  s0-s0:c0.c1023       *
    root          unconfined_u  s0-s0:c0.c1023       *
  2. Map the __default__ user, which represents all users without an explicit mapping, to the user_u SELinux user:

    # semanage login -m -s user_u -r s0 __default__

Verification

  1. Check that the __default__ user is mapped to the user_u SELinux user:

    # semanage login -l
    
    Login Name    SELinux User   MLS/MCS Range    Service
    
    __default__   user_u         s0               *
    root          unconfined_u   s0-s0:c0.c1023   *
  2. Verify that the processes of a new user run in the user_u:user_r:user_t:s0 SELinux context.

    1. Create a new user:

      # adduser example.user
    2. Define a password for example.user:

      # passwd example.user
    3. Log out as root and log in as the new user.
    4. Show the security context for the user’s ID:

      [example.user@localhost ~]$ id -Z
      user_u:user_r:user_t:s0
    5. Show the security context of the user’s current processes:

      [example.user@localhost ~]$ ps axZ
      LABEL                           PID TTY      STAT   TIME COMMAND
      -                                 1 ?        Ss     0:05 /usr/lib/systemd/systemd --switched-root --system --deserialize 18
      -                              3729 ?        S      0:00 (sd-pam)
      user_u:user_r:user_t:s0        3907 ?        Ss     0:00 /usr/lib/systemd/systemd --user
      -                              3911 ?        S      0:00 (sd-pam)
      user_u:user_r:user_t:s0        3918 ?        S      0:00 sshd: example.user@pts/0
      user_u:user_r:user_t:s0        3922 pts/0    Ss     0:00 -bash
      user_u:user_r:user_dbusd_t:s0  3969 ?        Ssl    0:00 /usr/bin/dbus-daemon --session --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only
      user_u:user_r:user_t:s0        3971 pts/0    R+     0:00 ps axZ

3.6. Confining an administrator by mapping to sysadm_u

You can confine a user with administrative privileges by mapping the user directly to the sysadm_u SELinux user. When the user logs in, the session runs in the sysadm_u:sysadm_r:sysadm_t SELinux context.

By default, all Linux users in Red Hat Enterprise Linux, including users with administrative privileges, are mapped to the unconfined SELinux user unconfined_u. You can improve the security of the system by assigning users to SELinux confined users. This is useful to conform with the V-71971 Security Technical Implementation Guide.

Prerequisites

  • The root user runs unconfined. This is the Red Hat Enterprise Linux default.

Procedure

  1. Optional: To allow sysadm_u users to connect to the system using SSH:

    # setsebool -P ssh_sysadm_login on
  2. Create a new user, add the user to the wheel user group, and map the user to the sysadm_u SELinux user:

    # adduser -G wheel -Z sysadm_u example.user
  3. Optional: Map an existing user to the sysadm_u SELinux user and add the user to the wheel user group:

    # usermod -G wheel -Z sysadm_u example.user

Verification

  1. Check that example.user is mapped to the sysadm_u SELinux user:

    # semanage login -l | grep example.user
    example.user     sysadm_u    s0-s0:c0.c1023   *
  2. Log in as example.user, for example, using SSH, and show the user’s security context:

    [example.user@localhost ~]$ id -Z
    sysadm_u:sysadm_r:sysadm_t:s0-s0:c0.c1023
  3. Switch to the root user:

    $ sudo -i
    [sudo] password for example.user:
  4. Verify that the security context remains unchanged:

    # id -Z
    sysadm_u:sysadm_r:sysadm_t:s0-s0:c0.c1023
  5. Try an administrative task, for example, restarting the sshd service:

    # systemctl restart sshd

    If there is no output, the command finished successfully.

    If the command does not finish successfully, it prints the following message:

    Failed to restart sshd.service: Access denied
    See system logs and 'systemctl status sshd.service' for details.

3.7. Confining an administrator using sudo and the sysadm_r role

You can map a specific user with administrative privileges to the staff_u SELinux user, and configure sudo so that the user can gain the sysadm_r SELinux administrator role. This role allows the user to perform administrative tasks without SELinux denials. When the user logs in, the session runs in the staff_u:staff_r:staff_t SELinux context, but when the user enters a command using sudo, the session changes to the staff_u:sysadm_r:sysadm_t context.

By default, all Linux users in Red Hat Enterprise Linux, including users with administrative privileges, are mapped to the unconfined SELinux user unconfined_u. You can improve the security of the system by assigning users to SELinux confined users. This is useful to conform with the V-71971 Security Technical Implementation Guide.

Prerequisites

  • The root user runs unconfined. This is the Red Hat Enterprise Linux default.

Procedure

  1. Create a new user, add the user to the wheel user group, and map the user to the staff_u SELinux user:

    # adduser -G wheel -Z staff_u example.user
  2. Optional: Map an existing user to the staff_u SELinux user and add the user to the wheel user group:

    # usermod -G wheel -Z staff_u example.user
  3. To allow example.user to gain the SELinux administrator role, create a new file in the /etc/sudoers.d/ directory, for example:

    # visudo -f /etc/sudoers.d/example.user
  4. Add the following line to the new file:

    example.user ALL=(ALL) TYPE=sysadm_t ROLE=sysadm_r ALL

Verification

  1. Check that example.user is mapped to the staff_u SELinux user:

    # semanage login -l | grep example.user
    example.user     staff_u    s0-s0:c0.c1023   *
  2. Log in as example.user, for example, using SSH, and switch to the root user:

    [example.user@localhost ~]$ sudo -i
    [sudo] password for example.user:
  3. Show the root security context:

    # id -Z
    staff_u:sysadm_r:sysadm_t:s0-s0:c0.c1023
  4. Try an administrative task, for example, restarting the sshd service:

    # systemctl restart sshd

    If there is no output, the command finished successfully.

    If the command does not finish successfully, it prints the following message:

    Failed to restart sshd.service: Access denied
    See system logs and 'systemctl status sshd.service' for details.

3.8. Additional resources

Chapter 4. Configuring SELinux for applications and services with non-standard configurations

When SELinux is in enforcing mode, the default policy is the targeted policy. The following sections provide information on setting up and configuring the SELinux policy for various services after you change configuration defaults, such as ports, database locations, or file-system permissions for processes.

You learn to change SELinux types for non-standard ports, to identify and fix incorrect labels for changes of default directories, and to adjust the policy using SELinux booleans.

4.1. Customizing the SELinux policy for the Apache HTTP server in a non-standard configuration

You can configure the Apache HTTP server to listen on a different port and to provide content in a non-default directory. To prevent consequent SELinux denials, follow the steps in this procedure to adjust your system’s SELinux policy.

Prerequisites

  • The httpd package is installed and the Apache HTTP server is configured to listen on TCP port 3131 and to use the /var/test_www/ directory instead of the default /var/www/ directory.
  • The policycoreutils-python-utils and setroubleshoot-server packages are installed on your system.

Procedure

  1. Start the httpd service and check the status:

    # systemctl start httpd
    # systemctl status httpd
    ...
    httpd[14523]: (13)Permission denied: AH00072: make_sock: could not bind to address [::]:3131
    ...
    systemd[1]: Failed to start The Apache HTTP Server.
    ...
  2. The SELinux policy assumes that httpd runs on port 80:

    # semanage port -l | grep http
    http_cache_port_t              tcp      8080, 8118, 8123, 10001-10010
    http_cache_port_t              udp      3130
    http_port_t                    tcp      80, 81, 443, 488, 8008, 8009, 8443, 9000
    pegasus_http_port_t            tcp      5988
    pegasus_https_port_t           tcp      5989
  3. Change the SELinux type of port 3131 to match port 80:

    # semanage port -a -t http_port_t -p tcp 3131
  4. Start httpd again:

    # systemctl start httpd
  5. However, the content remains inaccessible:

    # wget localhost:3131/index.html
    ...
    HTTP request sent, awaiting response... 403 Forbidden
    ...

    Find the reason with the sealert tool:

    # sealert -l "*"
    ...
    SELinux is preventing httpd from getattr access on the file /var/test_www/html/index.html.
    ...
  6. Compare SELinux types for the standard and the new path using the matchpathcon tool:

    # matchpathcon /var/www/html /var/test_www/html
    /var/www/html       system_u:object_r:httpd_sys_content_t:s0
    /var/test_www/html  system_u:object_r:var_t:s0
  7. Change the SELinux type of the new /var/test_www/html/ content directory to the type of the default /var/www/html directory:

    # semanage fcontext -a -e /var/www /var/test_www
  8. Relabel the /var directory recursively:

    # restorecon -Rv /var/
    ...
    Relabeled /var/test_www/html from unconfined_u:object_r:var_t:s0 to unconfined_u:object_r:httpd_sys_content_t:s0
    Relabeled /var/test_www/html/index.html from unconfined_u:object_r:var_t:s0 to unconfined_u:object_r:httpd_sys_content_t:s0

Verification

  1. Check that the httpd service is running:

    # systemctl status httpd
    ...
    Active: active (running)
    ...
    systemd[1]: Started The Apache HTTP Server.
    httpd[14888]: Server configured, listening on: port 3131
    ...
  2. Verify that the content provided by the Apache HTTP server is accessible:

    # wget localhost:3131/index.html
    ...
    HTTP request sent, awaiting response... 200 OK
    Length: 0 [text/html]
    Saving to: ‘index.html’
    ...

Additional resources

  • The semanage(8), matchpathcon(8), and sealert(8) man pages.

4.2. Adjusting the policy for sharing NFS and CIFS volumes using SELinux booleans

You can change parts of SELinux policy at runtime using booleans, even without any knowledge of SELinux policy writing. This enables changes, such as allowing services access to NFS volumes, without reloading or recompiling SELinux policy. The following procedure demonstrates listing SELinux booleans and configuring them to achieve the required changes in the policy.

NFS mounts on the client side are labeled with a default context defined by a policy for NFS volumes. In RHEL, this default context uses the nfs_t type. Also, Samba shares mounted on the client side are labeled with a default context defined by the policy. This default context uses the cifs_t type. You can enable or disable booleans to control which services are allowed to access the nfs_t and cifs_t types.

To allow the Apache HTTP server service (httpd) to access and share NFS and CIFS volumes, perform the following steps:

Prerequisites

  • Optionally, install the selinux-policy-devel package to obtain clearer and more detailed descriptions of SELinux booleans in the output of the semanage boolean -l command.

Procedure

  1. Identify SELinux booleans relevant for NFS, CIFS, and Apache:

    # semanage boolean -l | grep 'nfs\|cifs' | grep httpd
    httpd_use_cifs                 (off  ,  off)  Allow httpd to access cifs file systems
    httpd_use_nfs                  (off  ,  off)  Allow httpd to access nfs file systems
  2. List the current state of the booleans:

    $ getsebool -a | grep 'nfs\|cifs' | grep httpd
    httpd_use_cifs --> off
    httpd_use_nfs --> off
  3. Enable the identified booleans:

    # setsebool httpd_use_nfs on
    # setsebool httpd_use_cifs on
    Note

    Use setsebool with the -P option to make the changes persistent across restarts. A setsebool -P command requires a rebuild of the entire policy, and it might take some time depending on your configuration.

Verification

  1. Check that the booleans are on:

    $ getsebool -a | grep 'nfs\|cifs' | grep httpd
    httpd_use_cifs --> on
    httpd_use_nfs --> on

Additional resources

  • semanage-boolean(8), sepolicy-booleans(8), getsebool(8), setsebool(8), booleans(5), and booleans(8) man pages

4.3. Additional resources

Chapter 6. Using Multi-Level Security (MLS)

The Multi-Level Security (MLS) policy uses levels of clearance as originally designed by the US defense community. MLS meets a very narrow set of security requirements based on information management in rigidly controlled environments such as the military.

Using MLS is complex and does not map well to general use-case scenarios.

6.1. Multi-Level Security (MLS)

The Multi-Level Security (MLS) technology classifies data in a hierarchical classification using information security levels, for example:

  • [lowest] Unclassified
  • [low] Confidential
  • [high] Secret
  • [highest] Top secret

By default, the MLS SELinux policy uses 16 sensitivity levels:

  • s0 is the least sensitive.
  • s15 is the most sensitive.

MLS uses specific terminology to address sensitivity levels:

  • Users and processes are called subjects, whose sensitivity level is called clearance.
  • Files, devices, and other passive components of the system are called objects, whose sensitivity level is called classification.

To implement MLS, SELinux uses the Bell-La Padula Model (BLP) model. This model specifies how information can flow within the system based on labels attached to each subject and object.

The basic principle of BLP is “No read up, no write down.” This means that users can only read files at their own sensitivity level and lower, and data can flow only from lower levels to higher levels, and never the reverse.

The MLS SELinux policy, which is the implementation of MLS on RHEL, applies a modified principle called Bell-La Padula with write equality. This means that users can read files at their own sensitivity level and lower, but can write only at exactly their own level. This prevents, for example, low-clearance users from writing content into top-secret files.

For example, by default, a user with clearance level s2:

  • Can read files with sensitivity levels s0, s1, and s2.
  • Cannot read files with sensitivity level s3 and higher.
  • Can modify files with sensitivity level of exactly s2.
  • Cannot modify files with sensitivity level different than s2.
Note

Security administrators may adjust this behavior by modifying the system’s SELinux policy. For example, they can allow users to modify files at lower levels, which increases the file’s sensitivity level to the user’s clearance level.

In practice, users are typically assigned to a range of clearance levels, for example s1-s2. A user can read files with sensitivity levels lower than the user’s maximum level, and write to any files within that range.

For example, by default, a user with a clearance range s1-s2:

  • Can read files with sensitivity levels s0 and s1.
  • Cannot read files with sensitivity level s2 and higher.
  • Can modify files with sensitivity level s1.
  • Cannot modify files with sensitivity level different than s1.
  • Can change own clearance level to s2.

The security context for a non-privileged user in an MLS environment is, for example:

user_u:user_r:user_t:s1

Where:

user_u
is the SELinux user.
user_r
is the SELinux role.
user_t
is the SELinux type.
s1
is the range of MLS sensitivity levels.

The system always combines MLS access rules with conventional file access permissions. For example, if a user with a security level of "Secret" uses Discretionary Access Control (DAC) to block access to a file by other users, even “Top Secret” users cannot access that file. A high security clearance does not automatically permit a user to browse the entire file system.

Users with top-level clearances do not automatically acquire administrative rights on multi-level systems. While they may have access to all sensitive information on the system, this is different from having administrative rights.

In addition, administrative rights do not provide access to sensitive information. For example, even when someone logs in as root, they still cannot read top-secret information.

You can further adjust access within an MLS system by using categories. With Multi-Category Security (MCS), you can define categories such as projects or departments, and users will only be allowed to access files in the categories to which they are assigned. For additional information, see Using Multi-Category Security (MCS) for data confidentiality .

6.2. SELinux roles in MLS

The SELinux policy maps each Linux user to an SELinux user. This allows Linux users to inherit the restrictions of SELinux users.

Important

The MLS policy does not contain the unconfined module, including unconfined users, types, and roles. As a result, users that would be unconfined, including root, cannot access every object and perform every action they could in the targeted policy.

You can customize the permissions for confined users in your SELinux policy according to specific needs by adjusting the booleans in the policy. You can determine the current state of these booleans by using the semanage boolean -l command.

Table 6.1. Roles of SELinux users in MLS

UserDefault roleAdditional roles

guest_u

guest_r

 

xguest_u

xguest_r

 

user_u

user_r

 

staff_u

staff_r

auditadm_r

secadm_r

sysadm_r

staff_r

sysadm_u

sysadm_r

 

root

staff_r

auditadm_r

secadm_r

sysadm_r

system_r

system_u

system_r

 

Note that system_u is a special user identity for system processes and objects, and system_r is the associated role. Administrators must never associate this system_u user and the system_r role to a Linux user. Also, unconfined_u and root are unconfined users. For these reasons, the roles associated to these SELinux users are not included in the following table Types and access of SELinux roles.

Each SELinux role corresponds to an SELinux type and provides specific access rights.

Table 6.2. Types and access of SELinux roles in MLS

RoleTypeLogin using X Window Systemsu and sudoExecute in home directory and /tmp (default)Networking

guest_r

guest_t

no

no

yes

no

xguest_r

xguest_t

yes

no

yes

web browsers only (Firefox, GNOME Web)

user_r

user_t

yes

no

yes

yes

staff_r

staff_t

yes

only sudo

yes

yes

auditadm_r

auditadm_t

 

yes

yes

yes

secadm_r

secadm_t

 

yes

yes

yes

sysadm_r

sysadm_t

only when the xdm_sysadm_login boolean is on

yes

yes

yes

  • By default, the sysadm_r role has the rights of the secadm_r role, which means a user with the sysadm_r role can manage the security policy. If this does not correspond to your use case, you can separate the two roles by disabling the sysadm_secadm module in the policy. For additional information, see Separating system administration from security administration in MLS
  • Non-login roles dbadm_r, logadm_r, and webadm_r can be used for a subset of administrative tasks. By default, these roles are not associated with any SELinux user.

6.3. Switching the SELinux policy to MLS

Use the following steps to switch the SELinux policy from targeted to Multi-Level Security (MLS).

Important

Red Hat does not recommend to use the MLS policy on a system that is running the X Window System. Furthermore, when you relabel the file system with MLS labels, the system may prevent confined domains from access, which prevents your system from starting correctly. Therefore ensure that you switch SELinux to permissive mode before you relabel the files. On most systems, you see a lot of SELinux denials after switching to MLS, and many of them are not trivial to fix.

Procedure

  1. Install the selinux-policy-mls package:

    # dnf install selinux-policy-mls
  2. Open the /etc/selinux/config file in a text editor of your choice, for example:

    # vi /etc/selinux/config
  3. Change SELinux mode from enforcing to permissive and switch from the targeted policy to MLS:

    SELINUX=permissive
    SELINUXTYPE=mls

    Save the changes, and quit the editor.

  4. Before you enable the MLS policy, you must relabel each file on the file system with an MLS label:

    # fixfiles -F onboot
    System will relabel on next boot
  5. Restart the system:

    # reboot
  6. Check for SELinux denials:

    # ausearch -m AVC,USER_AVC,SELINUX_ERR,USER_SELINUX_ERR -ts recent -i

    Because the previous command does not cover all scenarios, see Troubleshooting problems related to SELinux for guidance on identifying, analyzing, and fixing SELinux denials.

  7. After you ensure that there are no problems related to SELinux on your system, switch SELinux back to enforcing mode by changing the corresponding option in /etc/selinux/config:

    SELINUX=enforcing
  8. Restart the system:

    # reboot
Important

If your system does not start or you are not able to log in after you switch to MLS, add the enforcing=0 parameter to your kernel command line. See Changing SELinux modes at boot time for more information.

Also note that in MLS, SSH logins as the root user mapped to the sysadm_r SELinux role differ from logging in as root in staff_r. Before you start your system in MLS for the first time, consider allowing SSH logins as sysadm_r by setting the ssh_sysadm_login SELinux boolean to 1. To enable ssh_sysadm_login later, already in MLS, you must log in as root in staff_r, switch to root in sysadm_r using the newrole -r sysadm_r command, and then set the boolean to 1.

Verification

  1. Verify that SELinux runs in enforcing mode:

    # getenforce
    Enforcing
  2. Check that the status of SELinux returns the mls value:

    # sestatus | grep mls
    Loaded policy name:             mls

Additional resources

  • The fixfiles(8), setsebool(8), and ssh_selinux(8) man pages.

6.4. Establishing user clearance in MLS

After you switch SELinux policy to MLS, you must assign security clearance levels to users by mapping them to confined SELinux users. By default, a user with a given security clearance:

  • Cannot read objects that have a higher sensitivity level.
  • Cannot write to objects at a different sensitivity level.

Prerequisites

  • The SELinux policy is set to mls.
  • The SELinux mode is set to enforcing.
  • The policycoreutils-python-utils package is installed.
  • A user assigned to an SELinux confined user:

    • For a non-privileged user, assigned to user_u (example_user in the following procedure).
    • For a privileged user, assigned to staff_u (staff in the following procedure) .
Note

Make sure that the users have been created when the MLS policy was active. Users created in other SELinux policies cannot be used in MLS.

Procedure

  1. Optional: To prevent adding errors to your SELinux policy, switch to the permissive SELinux mode, which facilitates troubleshooting:

    # setenforce 0
    Important

    In permissive mode, SELinux does not enforce the active policy but only logs Access Vector Cache (AVC) messages, which can be then used for troubleshooting and debugging.

  2. Define a clearance range for the staff_u SELinux user. For example, this command sets the clearance range from s1 to s15 with s1 being the default clearance level:

    # semanage user -m -L s1 -r s1-s15 _staff_u
  3. Generate SELinux file context configuration entries for user home directories:

    # genhomedircon
  4. Restore file security contexts to default:

    # restorecon -R -F -v /home/
    Relabeled /home/staff from staff_u:object_r:user_home_dir_t:s0 to staff_u:object_r:user_home_dir_t:s1
    Relabeled /home/staff/.bash_logout from staff_u:object_r:user_home_t:s0 to staff_u:object_r:user_home_t:s1
    Relabeled /home/staff/.bash_profile from staff_u:object_r:user_home_t:s0 to staff_u:object_r:user_home_t:s1
    Relabeled /home/staff/.bashrc from staff_u:object_r:user_home_t:s0 to staff_u:object_r:user_home_t:s1
  5. Assign a clearance level to the user:

    # semanage login -m -r s1 example_user

    Where s1 is the clearance level assigned to the user.

  6. Relabel the user’s home directory to the user’s clearance level:

    # chcon -R -l s1 /home/example_user
  7. Optional: If you previously switched to the permissive SELinux mode, and after you verify that everything works as expected, switch back to the enforcing SELinux mode:

    # setenforce 1

Verification steps

  1. Verify that the user is mapped to the correct SELinux user and has the correct clearance level assigned:

    # semanage login -l
    Login Name      SELinux User         MLS/MCS Range        Service
    __default__     user_u               s0-s0                *
    example_user    user_u               s1                   *
    ...
  2. Log in as the user within MLS.
  3. Verify that the user’s security level works correctly:

    Important

    The files you use for verification should not contain any sensitive information in case the configuration is incorrect and the user actually can access the files without authorization.

    1. Verify that the user cannot read a file with a higher-level sensitivity.
    2. Verify that the user can write to a file with the same sensitivity.
    3. Verify that the user can read a file with a lower-level sensitivity.

6.5. Changing a user’s clearance level within the defined security range in MLS

As a user in Multi-Level Security (MLS), you can change your current clearance level within the range the administrator assigned to you. You can never exceed the upper limit of your range or reduce your level below the lower limit of your range. This allows you, for example, to modify lower-sensitivity files without increasing their sensitivity level to your highest clearance level.

For example, as a user assigned to range s1-s3:

  • You can switch to levels s1, s2, and s3.
  • You can switch to ranges s1-s2, and s2-s3.
  • You cannot switch to ranges s0-s3 or s1-s4.
Note

Switching to a different level opens a new shell with the different clearance. This means you cannot return to your original clearance level in the same way as decreasing it. However, you can always return to the previous shell by entering exit.

Prerequisites

  • SELinux policy is set to mls.
  • SELinux mode is set to enforcing.
  • You can log in as a user assigned to a range of MLS clearance levels.

Procedure

  1. Log in as the user from a secure terminal.

    Note

    Secure terminals are defined in the /etc/selinux/mls/contexts/securetty_types file. By default, the console is a secure terminal, but SSH is not.

  2. Check the current user’s security context:

    $ id -Z
    user_u:user_r:user_t:s0-s2

    In this example, the user is assigned to the user_u SELinux user, user_r role, user_t type, and the MLS security range s0-s2.

  3. Check the current user’s security context:

    $ id -Z
    user_u:user_r:user_t:s1-s2
  4. Switch to a different security clearance range within the user’s clearance range:

    $ newrole -l s1

    You can switch to any range whose maximum is lower or equal to your assigned range. Entering a single-level range changes the lower limit of the assigned range. For example, entering newrole -l s1 as a user with a s0-s2 range is equivalent to entering newrole -l s1-s2.

Verification

  1. Display the current user’s security context:

    $ id -Z
    user_u:user_r:user_t:s1-s2
  2. Return to the previous shell with the original range by terminating the current shell:

    $ exit

Additional resources

6.6. Increasing file sensitivity levels in MLS

By default, Multi-Level Security (MLS) users cannot increase file sensitivity levels. However, the security administrator (secadm_r) can change this default behavior to allow users to increase the sensitivity of files by adding the local module mlsfilewrite to the system’s SELinux policy. Then, users assigned to the SELinux type defined in the policy module can increase file classification levels by modifying the file. Any time a user modifies a file, the file’s sensitivity level increases to the lower value of the user’s current security range.

Note

The security administrator, when logged in as a user assigned to the secadm_r role, can change the security levels of files by using the chcon -l s0 /path/to/file command. For more information, see Section 6.7, “Changing file sensitivity in MLS”

Prerequisites

  • The SELinux policy is set to mls.
  • The SELinux mode is set to enforcing.
  • The policycoreutils-python-utils package is installed.
  • The mlsfilewrite local module is installed in the SELinux MLS policy.
  • You are logged in as a user in MLS which is:

    • Assigned to a defined security range. This example shows a user with a security range s0-s2.
    • Assigned to the same SELinux type defined in the mlsfilewrite module. This example requires the (typeattributeset mlsfilewrite (user_t)) module.

Procedure

  1. Optional: Display the security context of the current user:

    $ id -Z
    user_u:user_r:user_t:s0-s2
  2. Change the lower level of the user’s MLS clearance range to the level which you want to assign to the file:

    $ newrole -l s1-s2
  3. Optional: Display the security context of the current user:

    $ id -Z
    user_u:user_r:user_t:s1-s2
  4. Optional: Display the security context of the file:

    $ ls -Z /path/to/file
    user_u:object_r:user_home_t:s0 /path/to/file
  5. Change the file’s sensitivity level to the lower level of the user’s clearance range by modifying the file:

    $ touch /path/to/file
    Important

    The classification level reverts to the default value if the restorecon command is used on the system.

  6. Optional: Exit the shell to return to the user’s previous security range:

    $ exit

Verification

  • Display the security context of the file:

    $ ls -Z /path/to/file
    user_u:object_r:user_home_t:s1 /path/to/file

6.7. Changing file sensitivity in MLS

In the MLS SELinux policy, users can only modify files at their own sensitivity level. This is intended to prevent any highly sensitive information to be exposed to users at lower clearance levels, and also prevent low-clearance users creating high-sensitivity documents. Administrators, however, can manually increase a file’s classification, for example for the file to be processed at the higher level.

Prerequisites

  • SELinux policy is set to mls.
  • SELinux mode is set to enforcing.
  • You have security administration rights, which means that you are assigned to either:

    • The secadm_r role.
    • If the sysadm_secadm module is enabled, to the sysadm_r role. The sysadm_secadm module is enabled by default.
  • The policycoreutils-python-utils package is installed.
  • A user assigned to any clearance level. For additional information, see Establishing user clearance levels in MLS .

    In this example, User1 has clearance level s1.

  • A file with a classification level assigned and to which you have access.

    In this example, /path/to/file has classification level s1.

Procedure

  1. Check the file’s classification level:

    # ls -lZ /path/to/file
    -rw-r-----. 1 User1 User1 user_u:object_r:user_home_t:s1 0 12. Feb 10:43 /path/to/file
  2. Change the file’s default classification level:

    # semanage fcontext -a -r s2 /path/to/file
  3. Force the relabeling of the file’s SELinux context:

    # restorecon -F -v /path/to/file
    Relabeled /path/to/file from user_u:object_r:user_home_t:s1 to user_u:object_r:user_home_t:s2

Verification

  1. Check the file’s classification level:

    # ls -lZ /path/to/file
    -rw-r-----. 1 User1 User1 user_u:object_r:user_home_t:s2 0 12. Feb 10:53 /path/to/file
  2. Optional: Verify that the lower-clearance user cannot read the file:

    $ cat /path/to/file
    cat: file: Permission denied

6.8. Separating system administration from security administration in MLS

By default, the sysadm_r role has the rights of the secadm_r role, which means a user with the sysadm_r role can manage the security policy. If you need more control over security authorizations, you can separate system administration from security administration by assigning a Linux user to the secadm_r role and disabling the sysadm_secadm module in the SELinux policy.

Prerequisites

  • The SELinux policy is set to mls.
  • The SELinux mode is set to enforcing.
  • The policycoreutils-python-utils package is installed.
  • A Linux user which will be assigned to the secadm_r role:

    • The user is assigned to the staff_u SELinux user
    • A password for this user has been defined.
    Warning

    Make sure you can log in as the user which will be assigned to the secadm role. If not, you can prevent any future modifications of the system’s SELinux policy.

Procedure

  1. Create a new sudoers file in the /etc/sudoers.d directory for the user:

    # visudo -f /etc/sudoers.d/<sec_adm_user>

    To keep the sudoers files organized, replace <sec_adm_user> with the Linux user which will be assigned to the secadm role.

  2. Add the following content into the /etc/sudoers.d/<sec_adm_user> file:

    <sec_adm_user> ALL=(ALL) TYPE=secadm_t ROLE=secadm_r ALL

    This line authorizes <secadmuser> on all hosts to perform all commands, and maps the user to the secadm SELinux type and role by default.

  3. Log in as the <sec_adm_user> user:

    Note

    To make sure that the SELinux context (which consists of SELinux user, role, and type) is changed, log in using ssh, the console, or xdm. Other ways, such as su and sudo, cannot change the entire SELinux context.

  4. Verify the user’s security context:

    $ id
    uid=1000(<sec_adm_user>) gid=1000(<sec_adm_user>) groups=1000(<sec_adm_user>) context=staff_u:staff_r:staff_t:s0-s15:c0.c1023
  5. Run the interactive shell for the root user:

    $ sudo -i
    [sudo] password for <sec_adm_user>:
  6. Verify the current user’s security context:

    # id
    uid=0(root) gid=0(root) groups=0(root) context=staff_u:secadm_r:secadm_t:s0-s15:c0.c1023
  7. Disable the sysadm_secadm module from the policy:

    # semodule -d sysadm_secadm
    Important

    Use the semodule -d command instead of removing the system policy module by using the semodule -r command. The semodule -r command deletes the module from your system’s storage, which means it cannot be loaded again without reinstalling the selinux-policy-mls package.

Verification

  1. As the user assigned to the secadm role, and in the interactive shell for the root user, verify that you can access the security policy data:

    # seinfo -xt secadm_t
    
    Types: 1
       type secadm_t, can_relabelto_shadow_passwords, (...) userdomain;
  2. Log out from the root shell:

    # logout
  3. Log out from the <sec_adm_user> user:

    $ logout
    Connection to localhost closed.
  4. Display the current security context:

    # id
    uid=0(root) gid=0(root) groups=0(root) context=root:sysadm_r:sysadm_t:s0-s15:c0.c1023
  5. Attempt to enable the sysadm_secadm module. The command should fail:

    # semodule -e sysadm_secadm
    SELinux:  Could not load policy file /etc/selinux/mls/policy/policy.31:  Permission denied
    /sbin/load_policy:  Can't load policy:  Permission denied
    libsemanage.semanage_reload_policy: load_policy returned error code 2. (No such file or directory).
    SELinux:  Could not load policy file /etc/selinux/mls/policy/policy.31:  Permission denied
    /sbin/load_policy:  Can't load policy:  Permission denied
    libsemanage.semanage_reload_policy: load_policy returned error code 2. (No such file or directory).
    semodule:  Failed!
  6. Attempt to display the details about the sysadm_t SELinux type. The command should fail:

    # seinfo -xt sysadm_t
    [Errno 13] Permission denied: '/sys/fs/selinux/policy'

6.9. Defining a secure terminal in MLS

The SELinux policy checks the type of the terminal from which a user is connected, and allows running of certain SELinux applications, for example newrole, only from secure terminals. Attempting this from a non-secure terminal produces an error: Error: you are not allowed to change levels on a non secure terminal;.

The /etc/selinux/mls/contexts/securetty_types file defines secure terminals for the Multi-Level Security (MLS) policy.

Default contents of the file:

console_device_t
sysadm_tty_device_t
user_tty_device_t
staff_tty_device_t
auditadm_tty_device_t
secureadm_tty_device_t
Warning

Adding terminal types to the list of secure terminals can expose your system to security risks.

Prerequisites

  • SELinux policy is set to mls.
  • You are connected from an already secure terminal, or SELinux is in permissive mode.
  • You have security administration rights, which means that you are assigned to either:

    • The secadm_r role.
    • If the sysadm_secadm module is enabled, to the sysadm_r role. The sysadm_secadm module is enabled by default.
  • The policycoreutils-python-utils package is installed.

Procedure

  1. Determine the current terminal type:

    # ls -Z `tty`
    root:object_r:user_devpts_t:s0 /dev/pts/0

    In this example output, user_devpts_t is the current terminal type.

  2. Add the relevant SELinux type on a new line in the /etc/selinux/mls/contexts/securetty_types file.
  3. Optional: Switch SELinux to enforcing mode:

    # setenforce 1

Verification

  • Log in from the previously insecure terminal you have added to the /etc/selinux/mls/contexts/securetty_types file.

Additional resources

  • securetty_types(5) man page

6.10. Allowing MLS users to edit files on lower levels

By default, MLS users cannot write to files which have a sensitivity level below the lower value of the clearance range. If your scenario requires allowing users to edit files on lower levels, you can do so by creating a local SELinux module. However, writing to a file will increase its sensitivity level to the lower value of the user’s current range.

Prerequisites

  • The SELinux policy is set to mls.
  • The SELinux mode is set to enforcing.
  • The policycoreutils-python-utils package is installed.
  • The setools-console and audit packages for verification.

Procedure

  1. Optional: Switch to permissive mode for easier troubleshooting.

    # setenforce 0
  2. Open a new .cil file with a text editor, for example ~/local_mlsfilewrite.cil, and insert the following custom rule:

    (typeattributeset mlsfilewrite (_staff_t_))

    You can replace staff_t with a different SELinux type. By specifying SELinux type here, you can control which SELinux roles can edit lower-level files.

    To keep your local modules better organized, use the local_ prefix in the names of local SELinux policy modules.

  3. Install the policy module:

    # semodule -i ~/local_mlsfilewrite.cil
    Note

    To remove the local policy module, use semodule -r ~/local_mlsfilewrite. Note that you must refer to the module name without the .cil suffix.

  4. Optional: If you previously switched back to permissive mode, return to enforcing mode:

    # setenforce 1

Verification

  1. Find the local module in the list of installed SELinux modules:

    # semodule -lfull | grep "local_mls"
    400 local_mlsfilewrite  cil

    Because local modules have priority 400, you can list them also by using the semodule -lfull | grep -v ^100 command.

  2. Log in as a user assigned to the type defined in the custom rule, for example, staff_t.
  3. Attempt to write to a file with a lower sensitivity level. This increases the file’s classification level to the user’s clearance level.

    Important

    The files you use for verification should not contain any sensitive information in case the configuration is incorrect and the user actually can access the files without authorization.

Chapter 7. Using Multi-Category Security (MCS) for data confidentiality

You can use MCS to enhance the data confidentiality of your system by categorizing data, and then granting certain processes and users access to specific categories

7.1. Multi-Category Security (MCS)

Multi-Category Security (MCS) is an access control mechanism that uses categories assigned to processes and files. Files can then be accessed only by processes that are assigned to the same categories. The purpose of MCS is to maintain data confidentiality on your system.

MCS categories are defined by the values c0 to c1023, but you can also define a text label for each category or combination of categories, such as “Personnel”, “ProjectX”, or “ProjectX.Personnel”. The MCS Translation service (mcstrans) then replaces the category values with the appropriate labels in system inputs and outputs, so that users can use these labels instead of the category values.

When users are assigned to categories, they can label any of their files with any of the categories to which they have been assigned.

MCS works on a simple principle: to access a file, a user must be assigned to all of the categories that have been assigned to the file. The MCS check is applied after normal Linux Discretionary Access Control (DAC) and SELinux Type Enforcement (TE) rules, so it can only further restrict existing security configuration.

MCS within Multi-Level Security

You can use MCS on its own as a non-hierarchical system, or you can use it in combination with Multi-Level Security (MLS) as a non-hierarchical layer within a hierarchical system.

An example of MCS within MLS could be a secretive research organization, where files are classified like this:

Table 7.1. Example of combinations of security levels and categories

Security level

Category

Not specified

Project X

Project Y

Project Z

Unclassified

s0

s0:c0

s0:c1

s0:c2

Confidential

s1

s1:c0

s1:c1

s1:c2

Secret

s2

s2:c0

s2:c1

s2:c2

Top secret

s3

s3:c0

s3:c1

s3:c2

Note

A user with a range s0:c0.1023 would be able to access all files assigned to all categories on level s0, unless the access is prohibited by other security mechanisms, such as DAC or type enforcement policy rules.

The resulting security context of a file or process is a combination of:

  • SELinux user
  • SELinux role
  • SELinux type
  • MLS sensitivity level
  • MCS category

For example, a non-privileged user with access to sensitivity level 1 and category 2 in an MLS/MCS environment could have the following SELinux context:

user_u:user_r:user_t:s1:c2

Additional resources

7.2. Configuring Multi-Category Security for data confidentiality

By default, MCS is active in the targeted and mls SELinux policies but is not configured for users. In the targeted policy, MCS is configured only for:

  • OpenShift
  • virt
  • sandbox
  • network labeling
  • containers (container-selinux)

You can configure MCS to categorize users by creating a local SELinux module with a rule that constrains the user_t SELinux type by MCS rules in addition to type enforcement.

Warning

Changing the categories of certain files may render some services non-operational. If you are not an expert, contact your Red Hat sales representative and request consulting services.

Prerequisites

  • The SELinux mode is set to enforcing.
  • The SELinux policy is set to targeted or mls.
  • The policycoreutils-python-utils and setools-console packages are installed.

Procedure

  1. Create a new file named, for example, local_mcs_user.cil:

    # vim local_mcs_user.cil
  2. Insert the following rule:

    (typeattributeset mcs_constrained_type (user_t))
  3. Install the policy module:

    # semodule -i local_mcs_user.cil

Verification

  • For each user domain, display additional details for all the components:

    # seinfo -xt user_t
    
    Types: 1
    type user_t, application_domain_type, nsswitch_domain, corenet_unlabeled_type, domain, kernel_system_state_reader, mcs_constrained_type, netlabel_peer_type, privfd, process_user_target, scsi_generic_read, scsi_generic_write, syslog_client_type, pcmcia_typeattr_1, user_usertype, login_userdomain, userdomain, unpriv_userdomain, userdom_home_reader_type, userdom_filetrans_type, xdmhomewriter, x_userdomain, x_domain, dridomain, xdrawable_type, xcolormap_type;

7.3. Defining category labels in MCS

You can manage and maintain labels for MCS categories, or combinations of MCS categories with MLS levels, on your system by editing the setrans.conf file. In this file, SELinux maintains a mapping between internal sensitivity and category levels and their human-readable labels.

Note

Category labels only make it easier for users to use the categories. MCS works the same whether you define labels or not.

Prerequisites

  • The SELinux mode is set to enforcing.
  • The SELinux policy is set to targeted or mls.
  • The policycoreutils-python-utils and mcstrans packages are installed.

Procedure

  1. Modify existing categories or create new categories by editing the /etc/selinux/<selinuxpolicy>/setrans.conf file in a text editor. Replace <selinuxpolicy> with targeted or mls depending on the SELinux policy you use. For example:

    # vi /etc/selinux/targeted/setrans.conf
  2. In the setrans.conf file for your policy, define the combinations of categories required by your scenario using the syntax s_<security level>_:c_<category number>_=<category.name>, for example:

    s0:c0=Marketing
    s0:c1=Finance
    s0:c2=Payroll
    s0:c3=Personnel
    • You can use category numbers from c0 to c1023.
    • In the targeted policy, use the s0 security level.
    • In the mls policy, you can label each combination of sensitivity levels and categories.
  3. Optional: In the setrans.conf file, you can also label the MLS sensitivity levels.
  4. Save and exit the file.
  5. To make the changes effective, restart the MCS translation service:

    # systemctl restart mcstrans

Verification

  • Display the current categories:

    # chcat -L

    The example above produces the following output:

    s0:c0                          Marketing
    s0:c1                          Finance
    s0:c2                          Payroll
    s0:c3                          Personnel
    s0
    s0-s0:c0.c1023                 SystemLow-SystemHigh
    s0:c0.c1023                    SystemHigh

Additional resources

  • The setrans.conf(5) man page.

7.4. Assigning categories to users in MCS

You can define user authorizations by assigning categories to Linux users. A user with assigned categories can access and modify files that have a subset of the user’s categories. Users can also assign files they own to categories they have been assigned to.

A Linux user cannot be assigned to a category that is outside of the security range defined for the relevant SELinux user.

Note

Category access is assigned during login. Consequently, users do not have access to newly assigned categories until they log in again. Similarly, if you revoke a user’s access to a category, this is effective only after the user logs in again.

Prerequisites

  • The SELinux mode is set to enforcing.
  • The SELinux policy is set to targeted or mls.
  • The policycoreutils-python-utils package is installed.
  • Linux users are assigned to SELinux confined users:

    • Non-privileged users are assigned to user_u.
    • Privileged users are assigned to staff_u.

Procedure

  1. Define the security range for the SELinux user.

    # semanage user -m -rs0:c0,c1-s0:c0.c9 <user_u>

    Use category numbers c0 to c1023 or category labels as defined in the setrans.conf file. For additional information, see Defining category labels in MCS .

  2. Assign MCS categories to a Linux user. You can specify only a range within the range defined to the relevant SELinux user:

    # semanage login -m -rs0:c1 <Linux.user1>
    Note

    You can add or remove categories from Linux users by using the chcat command. The following example adds <category1> and removes <category2> from <Linux.user1> and <Linux.user2>:

    # chcat -l -- +<category1>,-<category2> <Linux.user1>,<Linux.user2>

    Note that you must specify -- on the command line before using the -<category> syntax. Otherwise, the chcat command misinterprets the category removal as a command option.

Verification

  • List the categories assigned to Linux users:

    # chcat -L -l <Linux.user1>,<Linux.user2>
    <Linux.user1>: <category1>,<category2>
    <Linux.user2>: <category1>,<category2>

Additional resources

  • The chcat(8) man page.

7.5. Assigning categories to files in MCS

You need administrative privileges to assign categories to users. Users can then assign categories to files. To modify the categories of a file, users must have access rights to that file. Users can only assign a file to a category that is assigned to them.

Note

The system combines category access rules with conventional file access permissions. For example, if a user with a category of bigfoot uses Discretionary Access Control (DAC) to block access to a file by other users, other bigfoot users cannot access that file. A user assigned to all available categories still may not be able to access the entire file system.

Prerequisites

  • The SELinux mode is set to enforcing.
  • The SELinux policy is set to targeted or mls.
  • The policycoreutils-python-utils package is installed.
  • Access and permissions to a Linux user that is:

  • Access and permissions to the file you want to add to the category.
  • For verification purposes: Access and permissions to a Linux user not assigned to this category

Procedure

  • Add categories to a file:

    $ chcat -- +<category1>,+<category2> <path/to/file1>

    Use category numbers c0 to c1023 or category labels as defined in the setrans.conf file. For additional information, see Defining category labels in MCS .

    You can remove categories from a file by using the same syntax:

    $ chcat -- -<category1>,-<category2> <path/to/file1>
    Note

    When removing a category, you must specify -- on the command line before using the -<category> syntax. Otherwise, the chcat command could misinterpret the category removal as a command option.

Verification

  1. Display the security context of the file to verify that it has the correct categories:

    $ ls -lZ <path/to/file>
    -rw-r--r--  <LinuxUser1> <Group1> root:object_r:user_home_t:_<sensitivity>_:_<category>_ <path/to/file>

    The specific security context of the file may differ.

  2. Optional: Attempt to access the file when logged in as a Linux user not assigned to the same category as the file:

    $ cat <path/to/file>
    cat: <path/to/file>: Permission Denied

Additional resources

  • The semanage(8) man page.
  • The chcat(8) man page.

Chapter 8. Writing a custom SELinux policy

This section guides you on how to write and use a custom policy that enables you to run your applications confined by SELinux.

8.2. Creating and enforcing an SELinux policy for a custom application

This example procedure provides steps for confining a simple daemon by SELinux. Replace the daemon with your custom application and modify the example rule according to the requirements of that application and your security policy.

Prerequisites

  • The policycoreutils-devel package and its dependencies are installed on your system.

Procedure

  1. For this example procedure, prepare a simple daemon that opens the /var/log/messages file for writing:

    1. Create a new file, and open it in a text editor of your choice:

      $ vi mydaemon.c
    2. Insert the following code:

      #include <unistd.h>
      #include <stdio.h>
      
      FILE *f;
      
      int main(void)
      {
      while(1) {
      f = fopen("/var/log/messages","w");
              sleep(5);
              fclose(f);
          }
      }
    3. Compile the file:

      $ gcc -o mydaemon mydaemon.c
    4. Create a systemd unit file for your daemon:

      $ vi mydaemon.service
      [Unit]
      Description=Simple testing daemon
      
      [Service]
      Type=simple
      ExecStart=/usr/local/bin/mydaemon
      
      [Install]
      WantedBy=multi-user.target
    5. Install and start the daemon:

      # cp mydaemon /usr/local/bin/
      # cp mydaemon.service /usr/lib/systemd/system
      # systemctl start mydaemon
      # systemctl status mydaemon
      ● mydaemon.service - Simple testing daemon
         Loaded: loaded (/usr/lib/systemd/system/mydaemon.service; disabled; vendor preset: disabled)
         Active: active (running) since Sat 2020-05-23 16:56:01 CEST; 19s ago
       Main PID: 4117 (mydaemon)
          Tasks: 1
         Memory: 148.0K
         CGroup: /system.slice/mydaemon.service
                 └─4117 /usr/local/bin/mydaemon
      
      May 23 16:56:01 localhost.localdomain systemd[1]: Started Simple testing daemon.
    6. Check that the new daemon is not confined by SELinux:

      $ ps -efZ | grep mydaemon
      system_u:system_r:unconfined_service_t:s0 root 4117    1  0 16:56 ?        00:00:00 /usr/local/bin/mydaemon
  2. Generate a custom policy for the daemon:

    $ sepolicy generate --init /usr/local/bin/mydaemon
    Created the following files:
    /home/example.user/mysepol/mydaemon.te # Type Enforcement file
    /home/example.user/mysepol/mydaemon.if # Interface file
    /home/example.user/mysepol/mydaemon.fc # File Contexts file
    /home/example.user/mysepol/mydaemon_selinux.spec # Spec file
    /home/example.user/mysepol/mydaemon.sh # Setup Script
  3. Rebuild the system policy with the new policy module using the setup script created by the previous command:

    # ./mydaemon.sh
    Building and Loading Policy
    + make -f /usr/share/selinux/devel/Makefile mydaemon.pp
    Compiling targeted mydaemon module
    Creating targeted mydaemon.pp policy package
    rm tmp/mydaemon.mod.fc tmp/mydaemon.mod
    + /usr/sbin/semodule -i mydaemon.pp
    ...

    Note that the setup script relabels the corresponding part of the file system using the restorecon command:

    restorecon -v /usr/local/bin/mydaemon /usr/lib/systemd/system
  4. Restart the daemon, and check that it now runs confined by SELinux:

    # systemctl restart mydaemon
    $ ps -efZ | grep mydaemon
    system_u:system_r:mydaemon_t:s0 root        8150       1  0 17:18 ?        00:00:00 /usr/local/bin/mydaemon
  5. Because the daemon is now confined by SELinux, SELinux also prevents it from accessing /var/log/messages. Display the corresponding denial message:

    # ausearch -m AVC -ts recent
    ...
    type=AVC msg=audit(1590247112.719:5935): avc:  denied  { open } for  pid=8150 comm="mydaemon" path="/var/log/messages" dev="dm-0" ino=2430831 scontext=system_u:system_r:mydaemon_t:s0 tcontext=unconfined_u:object_r:var_log_t:s0 tclass=file permissive=1
    ...
  6. You can get additional information also using the sealert tool:

    $ sealert -l "*"
    SELinux is preventing mydaemon from open access on the file /var/log/messages.
    
     Plugin catchall (100. confidence) suggests *
    
    If you believe that mydaemon should be allowed open access on the messages file by default.
    Then you should report this as a bug.
    You can generate a local policy module to allow this access.
    Do
    allow this access for now by executing:
    # ausearch -c 'mydaemon' --raw | audit2allow -M my-mydaemon
    # semodule -X 300 -i my-mydaemon.pp
    
    Additional Information:
    Source Context                system_u:system_r:mydaemon_t:s0
    Target Context                unconfined_u:object_r:var_log_t:s0
    Target Objects                /var/log/messages [ file ]
    Source                        mydaemon
    
    ...
  7. Use the audit2allow tool to suggest changes:

    $ ausearch -m AVC -ts recent | audit2allow -R
    
    require {
    	type mydaemon_t;
    }
    
    #============= mydaemon_t ==============
    logging_write_generic_logs(mydaemon_t)
  8. Because rules suggested by audit2allow can be incorrect for certain cases, use only a part of its output to find the corresponding policy interface:

    $ grep -r "logging_write_generic_logs" /usr/share/selinux/devel/include/ | grep .if
    /usr/share/selinux/devel/include/system/logging.if:interface(`logging_write_generic_logs',`
  9. Check the definition of the interface:

    $ cat /usr/share/selinux/devel/include/system/logging.if
    ...
    interface(`logging_write_generic_logs',`
            gen_require(`
                    type var_log_t;
            ')
    
            files_search_var($1)
            allow $1 var_log_t:dir list_dir_perms;
            write_files_pattern($1, var_log_t, var_log_t)
    ')
    ...
  10. In this case, you can use the suggested interface. Add the corresponding rule to your type enforcement file:

    $ echo "logging_write_generic_logs(mydaemon_t)" >> mydaemon.te

    Alternatively, you can add this rule instead of using the interface:

    $ echo "allow mydaemon_t var_log_t:file { open write getattr };" >> mydaemon.te
  11. Reinstall the policy:

    # ./mydaemon.sh
    Building and Loading Policy
    + make -f /usr/share/selinux/devel/Makefile mydaemon.pp
    Compiling targeted mydaemon module
    Creating targeted mydaemon.pp policy package
    rm tmp/mydaemon.mod.fc tmp/mydaemon.mod
    + /usr/sbin/semodule -i mydaemon.pp
    ...

Verification

  1. Check that your application runs confined by SELinux, for example:

    $ ps -efZ | grep mydaemon
    system_u:system_r:mydaemon_t:s0 root        8150       1  0 17:18 ?        00:00:00 /usr/local/bin/mydaemon
  2. Verify that your custom application does not cause any SELinux denials:

    # ausearch -m AVC -ts recent
    <no matches>

Additional resources

  • sepolgen(8), ausearch(8), audit2allow(1), audit2why(1), sealert(8), and restorecon(8) man pages

8.3. Creating a local SELinux policy module

Adding specific SELinux policy modules to an active SELinux policy can fix certain problems with the SELinux policy. You can use this procedure to fix a specific Known Issue described in Red Hat release notes, or to implement a specific Red Hat Solution.

Warning

Use only rules provided by Red Hat. Red Hat does not support creating SELinux policy modules with custom rules, because this falls outside of the Production Support Scope of Coverage. If you are not an expert, contact your Red Hat sales representative and request consulting services.

Prerequisites

  • The setools-console and audit packages for verification.

Procedure

  1. Open a new .cil file with a text editor, for example:

    # vim <local_module>.cil

    To keep your local modules better organized, use the local_ prefix in the names of local SELinux policy modules.

  2. Insert the custom rules from a Known Issue or a Red Hat Solution.

    Important

    Do not write your own rules. Use only the rules provided in a specific Known Issue or Red Hat Solution.

    For example, to implement the SELinux denies cups-lpd read access to cups.sock in RHEL solution, insert the following rule:

    (allow cupsd_lpd_t cupsd_var_run_t (sock_file (read)))

    Note that you can use either of the two SELinux rule syntaxes, Common Intermediate Language (CIL) and m4. For example, (allow cupsd_lpd_t cupsd_var_run_t (sock_file (read))) in CIL is equivalent to the following in m4:

    module local_cupslpd-read-cupssock 1.0;
    
    require {
        type cupsd_var_run_t;
        type cupsd_lpd_t;
        class sock_file read;
    }
    
    #============= cupsd_lpd_t ==============
    allow cupsd_lpd_t cupsd_var_run_t:sock_file read;
  3. Save and close the file.
  4. Install the policy module:

    # semodule -i <local_module>.cil
    Note

    When you want to remove a local policy module which you created by using # semodule -i, refer to the module name without the .cil suffix. To remove a local policy module, use # semodule -r <local_module>.

  5. Restart any services related to the rules:

    # systemctl restart <service-name>

Verification

  1. List the local modules installed in your SELinux policy:

    # semodule -lfull | grep "local_"
    400 local_module  cil
    Note

    Because local modules have priority 400, you can filter them from the list also by using that value, for example, by using the semodule -lfull | grep -v ^100 command.

  2. Search the SELinux policy for the relevant allow rules:

    # sesearch -A --source=<SOURCENAME> --target=<TARGETNAME> --class=<CLASSNAME> --perm=<P1>,<P2>

    Where <SOURCENAME> is the source SELinux type, <TARGETNAME> is the target SELinux type, <CLASSNAME> is the security class or object class name, and <P1> and <P2> are the specific permissions of the rule.

    For example, for the SELinux denies cups-lpd read access to cups.sock in RHEL solution:

    # sesearch -A --source=cupsd_lpd_t --target=cupsd_var_run_t --class=sock_file --perm=read
    allow cupsd_lpd_t cupsd_var_run_t:sock_file { append getattr open read write };

    The last line should now include the read operation.

  3. Verify that the relevant service runs confined by SELinux:

    1. Identify the process related to the relevant service:

      $ systemctl status <service-name>
    2. Check the SELinux context of the process listed in the output of the previous command:

      $ ps -efZ | grep <process-name>
  4. Verify that the service does not cause any SELinux denials:

    # ausearch -m AVC -ts recent
    <no matches>

8.4. Additional resources

Chapter 9. Creating SELinux policies for containers

Red Hat Enterprise Linux 9 provides a tool for generating SELinux policies for containers using the udica package. With udica, you can create a tailored security policy for better control of how a container accesses host system resources, such as storage, devices, and network. This enables you to harden your container deployments against security violations and it also simplifies achieving and maintaining regulatory compliance.

9.1. Introduction to the udica SELinux policy generator

To simplify creating new SELinux policies for custom containers, RHEL 9 provides the udica utility. You can use this tool to create a policy based on an inspection of the container JavaScript Object Notation (JSON) file, which contains Linux-capabilities, mount-points, and ports definitions. The tool consequently combines rules generated using the results of the inspection with rules inherited from a specified SELinux Common Intermediate Language (CIL) block.

The process of generating SELinux policy for a container using udica has three main parts:

  1. Parsing the container spec file in the JSON format
  2. Finding suitable allow rules based on the results of the first part
  3. Generating final SELinux policy

During the parsing phase, udica looks for Linux capabilities, network ports, and mount points.

Based on the results, udica detects which Linux capabilities are required by the container and creates an SELinux rule allowing all these capabilities. If the container binds to a specific port, udica uses SELinux user-space libraries to get the correct SELinux label of a port that is used by the inspected container.

Afterward, udica detects which directories are mounted to the container file-system name space from the host.

The CIL’s block inheritance feature allows udica to create templates of SELinux allow rules focusing on a specific action, for example:

  • allow accessing home directories
  • allow accessing log files
  • allow accessing communication with Xserver.

These templates are called blocks and the final SELinux policy is created by merging the blocks.

Additional resources

9.2. Creating and using an SELinux policy for a custom container

To generate an SELinux security policy for a custom container, follow the steps in this procedure.

Prerequisites

  • The podman tool for managing containers is installed. If it is not, use the dnf install podman command.
  • A custom Linux container - ubi8 in this example.

Procedure

  1. Install the udica package:

    # dnf install -y udica

    Alternatively, install the container-tools module, which provides a set of container software packages, including udica:

    # dnf module install -y container-tools
  2. Start the ubi8 container that mounts the /home directory with read-only permissions and the /var/spool directory with permissions to read and write. The container exposes the port 21.

    # podman run --env container=podman -v /home:/home:ro -v /var/spool:/var/spool:rw -p 21:21 -it ubi8 bash

    Note that now the container runs with the container_t SELinux type. This type is a generic domain for all containers in the SELinux policy and it might be either too strict or too loose for your scenario.

  3. Open a new terminal, and enter the podman ps command to obtain the ID of the container:

    # podman ps
    CONTAINER ID   IMAGE                                   COMMAND   CREATED          STATUS              PORTS   NAMES
    37a3635afb8f   registry.access.redhat.com/ubi8:latest  bash      15 minutes ago   Up 15 minutes ago           heuristic_lewin
  4. Create a container JSON file, and use udica for creating a policy module based on the information in the JSON file:

    # podman inspect 37a3635afb8f > container.json
    # udica -j container.json my_container
    Policy my_container with container id 37a3635afb8f created!
    [...]

    Alternatively:

    # podman inspect 37a3635afb8f | udica my_container
    Policy my_container with container id 37a3635afb8f created!
    
    Please load these modules using:
    # semodule -i my_container.cil /usr/share/udica/templates/{base_container.cil,net_container.cil,home_container.cil}
    
    Restart the container with: "--security-opt label=type:my_container.process" parameter
  5. As suggested by the output of udica in the previous step, load the policy module:

    # semodule -i my_container.cil /usr/share/udica/templates/{base_container.cil,net_container.cil,home_container.cil}
  6. Stop the container and start it again with the --security-opt label=type:my_container.process option:

    # podman stop 37a3635afb8f
    # podman run --security-opt label=type:my_container.process -v /home:/home:ro -v /var/spool:/var/spool:rw -p 21:21 -it ubi8 bash

Verification

  1. Check that the container runs with the my_container.process type:

    # ps -efZ | grep my_container.process
    unconfined_u:system_r:container_runtime_t:s0-s0:c0.c1023 root 2275 434  1 13:49 pts/1 00:00:00 podman run --security-opt label=type:my_container.process -v /home:/home:ro -v /var/spool:/var/spool:rw -p 21:21 -it ubi8 bash
    system_u:system_r:my_container.process:s0:c270,c963 root 2317 2305  0 13:49 pts/0 00:00:00 bash
  2. Verify that SELinux now allows access the /home and /var/spool mount points:

    [root@37a3635afb8f /]# cd /home
    [root@37a3635afb8f home]# ls
    username
    [root@37a3635afb8f ~]# cd /var/spool/
    [root@37a3635afb8f spool]# touch test
    [root@37a3635afb8f spool]#
  3. Check that SELinux allows binding only to the port 21:

    [root@37a3635afb8f /]# dnf install nmap-ncat
    [root@37a3635afb8f /]# nc -lvp 21
    ...
    Ncat: Listening on :::21
    Ncat: Listening on 0.0.0.0:21
    ^C
    [root@37a3635afb8f /]# nc -lvp 80
    ...
    Ncat: bind to :::80: Permission denied. QUITTING.

Additional resources

9.3. Additional resources

Chapter 10. Deploying the same SELinux configuration on multiple systems

This section provides two recommended ways for deploying your verified SELinux configuration on multiple systems:

  • Using RHEL System Roles and Ansible
  • Using semanage export and import commands in your scripts

10.1. Introduction to the SELinux System Role

RHEL System Roles is a collection of Ansible roles and modules that provide a consistent configuration interface to remotely manage multiple RHEL systems. The SELinux System Role enables the following actions:

  • Cleaning local policy modifications related to SELinux booleans, file contexts, ports, and logins.
  • Setting SELinux policy booleans, file contexts, ports, and logins.
  • Restoring file contexts on specified files or directories.
  • Managing SELinux modules.

The following table provides an overview of input variables available in the SELinux System Role.

Table 10.1. SELinux System Role variables

Role variableDescriptionCLI alternative

selinux_policy

Chooses a policy protecting targeted processes or Multi Level Security protection.

SELINUXTYPE in /etc/selinux/config

selinux_state

Switches SELinux modes.

setenforce and SELINUX in /etc/selinux/config.

selinux_booleans

Enables and disables SELinux booleans.

setsebool

selinux_fcontexts

Adds or removes a SELinux file context mapping.

semanage fcontext

selinux_restore_dirs

Restores SELinux labels in the file-system tree.

restorecon -R

selinux_ports

Sets SELinux labels on ports.

semanage port

selinux_logins

Sets users to SELinux user mapping.

semanage login

selinux_modules

Installs, enables, disables, or removes SELinux modules.

semodule

The /usr/share/doc/rhel-system-roles/selinux/example-selinux-playbook.yml example playbook installed by the rhel-system-roles package demonstrates how to set the targeted policy in enforcing mode. The playbook also applies several local policy modifications and restores file contexts in the /tmp/test_dir/ directory.

For a detailed reference on SELinux role variables, install the rhel-system-roles package, and see the README.md or README.html files in the /usr/share/doc/rhel-system-roles/selinux/ directory.

Additional resources

10.2. Using the SELinux System Role to apply SELinux settings on multiple systems

Follow the steps to prepare and apply an Ansible playbook with your verified SELinux settings.

Prerequisites

  • Access and permissions to one or more managed nodes, which are systems you want to configure with the SELinux System Role.
  • Access and permissions to a control node, which is a system from which Red Hat Ansible Core configures other systems.

    On the control node:

    • The ansible-core and rhel-system-roles packages are installed.
    • An inventory file which lists the managed nodes.
Important

RHEL 8.0-8.5 provided access to a separate Ansible repository that contains Ansible Engine 2.9 for automation based on Ansible. Ansible Engine contains command-line utilities such as ansible, ansible-playbook, connectors such as docker and podman, and many plugins and modules. For information on how to obtain and install Ansible Engine, see the How to download and install Red Hat Ansible Engine Knowledgebase article.

RHEL 8.6 and 9.0 have introduced Ansible Core (provided as the ansible-core package), which contains the Ansible command-line utilities, commands, and a small set of built-in Ansible plugins. RHEL provides this package through the AppStream repository, and it has a limited scope of support. For more information, see the Scope of support for the Ansible Core package included in the RHEL 9 and RHEL 8.6 and later AppStream repositories Knowledgebase article.

  • An inventory file which lists the managed nodes.

Procedure

  1. Prepare your playbook. You can either start from the scratch or modify the example playbook installed as a part of the rhel-system-roles package:

    # cp /usr/share/doc/rhel-system-roles/selinux/example-selinux-playbook.yml my-selinux-playbook.yml
    # vi my-selinux-playbook.yml
  2. Change the content of the playbook to fit your scenario. For example, the following part ensures that the system installs and enables the selinux-local-1.pp SELinux module:

    selinux_modules:
    - { path: "selinux-local-1.pp", priority: "400" }
  3. Save the changes, and exit the text editor.
  4. Run your playbook on the host1, host2, and host3 systems:

    # ansible-playbook -i host1,host2,host3 my-selinux-playbook.yml

Additional resources

  • For more information, install the rhel-system-roles package, and see the /usr/share/doc/rhel-system-roles/selinux/ and /usr/share/ansible/roles/rhel-system-roles.selinux/ directories.

10.3. Transferring SELinux settings to another system with semanage

Use the following steps for transferring your custom and verified SELinux settings between RHEL 9-based systems.

Prerequisites

  • The policycoreutils-python-utils package is installed on your system.

Procedure

  1. Export your verified SELinux settings:

    # semanage export -f ./my-selinux-settings.mod
  2. Copy the file with the settings to the new system:

    # scp ./my-selinux-settings.mod new-system-hostname:
  3. Log in on the new system:

    $ ssh root@new-system-hostname
  4. Import the settings on the new system:

    new-system-hostname# semanage import -f ./my-selinux-settings.mod

Additional resources

  • semanage-export(8) and semanage-import(8) man pages

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