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3.3. 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. This Linux user mapping is seen by running 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       *
system_u             system_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
The following procedure demonstrates how to add a new Linux user to the system and how to map that user to the SELinux unconfined_u user. It assumes that the root user is running unconfined, as it does by default in Red Hat Enterprise Linux:

Procedure 3.4. Mapping a New Linux User to the SELinux unconfined_u User

  1. As root, enter the following command to create a new Linux user named newuser: 
    ~]# useradd newuser
  2. To assign a password to the Linux newuser user. Enter the following command as root: 
    ~]# passwd newuser
Changing password for user newuser.
New UNIX password: Enter a password
Retype new UNIX password: Enter the same password again
passwd: all authentication tokens updated successfully.
  3. Log out of your current session, and log in as the Linux newuser 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. Enter the following command to view the context of a Linux user: 
    [newuser@localhost ~]$ id -Z
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

    Note

    If you no longer need the newuser user on your system, log out of the Linux newuser's session, log in with your account, and run the userdel -r newuser command as root. It will remove newuser along with their home directory.
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
   sysadm_u
   system_u
   xguest_u
   root
   guest_u
   staff_u
   user_u
   unconfined_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 Linux 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 Linux 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. In the table below, you can see examples of basic confined domains for Linux users in Red Hat Enterprise Linux:

Table 3.1. SELinux User Capabilities

User Role Domain X Window System su or sudo Execute in home directory and /tmp (default) Networking
sysadm_u sysadm_r sysadm_t yes su and sudo yes yes
staff_u staff_r staff_t yes only sudo yes yes
user_u user_r user_t yes no yes yes
guest_u guest_r guest_t no no yes no
xguest_u xguest_r xguest_t yes no yes Firefox only
  • 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_contentbooleans to off. This helps prevent flawed or malicious applications from modifying users' files.
    See Section 6.6, “Booleans for Users Executing Applications” for information about allowing and preventing users from executing applications in their home directories and /tmp.
  • The only network access Linux users in the xguest_t domain have is Firefox connecting to web pages.
Note that system_u is a special user identity for system processes and objects. It must never be associated to a Linux user. Also, unconfined_u and root are unconfined users. For these reasons, they are not included in the aforementioned table of SELinux user capabilities.
Alongside with the already mentioned SELinux users, there are special roles, that can be mapped to those users. These roles determine what SELinux allows the user to do:
  • webadm_r can only administrate SELinux types related to the Apache HTTP Server. See Section 13.2, “Types” for further information.
  • dbadm_r can only administrate SELinux types related to the MariaDB database and the PostgreSQL database management system. See Section 20.2, “Types” and Section 21.2, “Types” for further information.
  • 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 following command: 
~]$ seinfo -r
As mentioned before, the seinfo command is provided by the setools-console package, which is not installed by default.

3.3.1. The sudo Transition and SELinux Roles

In certain cases, confined users need to perform an administrative task that require root privileges. To do so, such a confined user has to gain a confined administrator SELinux role using the sudo command. The sudo command is used to give trusted users administrative access. When users precede an administrative command with sudo, they are prompted for their own password. Then, when they have been authenticated and assuming that the command is permitted, the administrative command is executed as if they were the root user.
As shown in Table 3.1, “SELinux User Capabilities”, only the staff_u and sysadm_u SELinux confined users are permitted to use sudo by default. When such users execute a command with sudo, their role can be changed based on the rules specified in the /etc/sudoers configuration file or in a respective file in the /etc/sudoers.d/ directory if such a file exists.
For more information about sudo, see the Gaining Privileges section in the Red Hat Enterprise Linux 7 System Administrator's Guide.

Procedure 3.5. Configuring the sudo Transition

This procedure shows how to set up sudo to transition a newly-created SELinux_user_u confined user from a default_role_r to an administrator_r administrator role.

Note

To configure a confined administrator role for an already existing SELinux user, skip the first two steps.
  1. Create a new SELinux user and specify the default SELinux role and a supplementary confined administrator role for this user: 
    ~]# semanage user -a -r s0-s0:c0.c1023 -R "default_role_r administrator_r" SELinux_user_u
  2. Set up the default SElinux policy context file. For example, to have the same SELinux rules as the staff_u SELinux user, copy the staff_u context file: 
    ~]# cp /etc/selinux/targeted/contexts/users/staff_u /etc/selinux/targeted/contexts/users/SELinux_user_u
  3. Map the newly-created SELinux user to an existing Linux user: 
    semanage login -a -s SELinux_user_u -rs0:c0.c1023 linux_user
  4. Create a new configuration file with the same name as your Linux user in the /etc/sudoers.d/ directory and add the following string to it: 
    ~]# echo "linux_user ALL=(ALL) TYPE=administrator_t ROLE=administrator_r /bin/bash " > /etc/sudoers.d/linux_user
  5. Use the restorecon utility to relabel the linux_user home directory: 
    ~]# restorecon -FR -v /home/linux_user
  6. Log in to the system as the newly-created Linux user and check that the user is labeled with the default SELinux role: 
    ~]$ id -Z
SELinux_user_u:default_role_r:SELinux_user_t:s0:c0.c1023
  7. Run sudo to change the user's SELinux context to the supplementary SELinux role as specified in /etc/sudoers.d/linux_user. The -i option used with sudo causes that an interactive shell is executed: 
    ~]$ sudo -i
~]# id -Z
SELinux_user_u:administrator_r:administrator_t:s0:c0.c1023
To better understand the placeholders, such as default_role_r or administrator_r, see the following example.

Example 3.1. Configuring the sudo Transition

This example creates a new SELinux user confined_u with default assigned role staff_r and with sudo configured to change the role of confined_u from staff_r to webadm_r.
  • Enter all the following commands as the root user in the sysadm_r or unconfined_r role. 
    ~]# semanage user -a -r s0-s0:c0.c1023 -R "staff_r webadm_r" confined_u
~]# cp /etc/selinux/targeted/contexts/users/staff_u /etc/selinux/targeted/contexts/users/confined_u
~]# semanage login -a -s confined_u -rs0:c0.c1023 linux_user
~]# restorecon -FR -v /home/linux_user
~]# echo "linux_user ALL=(ALL) ROLE=webadm_r TYPE=webadm_t /bin/bash " > /etc/sudoers.d/linux_user
  • Log in to the system as the newly-created Linux user and check that the user is labeled with the default SELinux role: 
    ~]$ id -Z
confined_u:staff_r:staff_t:s0:c0.c1023
~]$ sudo -i
~]# id -Z
confined_u:webadm_r:webadm_t:s0:c0.c1023