3.2. Unconfined Processes

Unconfined processes run in unconfined domains, for example, unconfined services executed by init end up running in the unconfined_service_t domain, unconfined services executed by kernel end up running in the kernel_t domain, and unconfined services executed by unconfined Linux users end up running in the unconfined_t domain. For unconfined processes, SELinux policy rules are applied, but policy rules exist that allow processes running in unconfined domains almost all access. Processes running in unconfined domains fall back to using DAC rules exclusively. If an unconfined process is compromised, SELinux does not prevent an attacker from gaining access to system resources and data, but of course, DAC rules are still used. SELinux is a security enhancement on top of DAC rules – it does not replace them.
To ensure that SELinux is enabled and the system is prepared to perform the following example, complete the Procedure 3.1, “How to Verify SELinux Status” described in Section 3.1, “Confined Processes”.
The following example demonstrates how the Apache HTTP Server (httpd) can access data intended for use by Samba, when running unconfined. Note that in Red Hat Enterprise Linux, the httpd process runs in the confined httpd_t domain by default. This is an example, and should not be used in production. It assumes that the httpd, wget, dbus and audit packages are installed, that the SELinux targeted policy is used, and that SELinux is running in enforcing mode.

Procedure 3.3. An Example of Unconfined Process

  1. The chcon command relabels files; however, such label changes do not survive when the file system is relabeled. For permanent changes that survive a file system relabel, use the semanage utility, which is discussed later. As the root user, enter the following command to change the type to a type used by Samba:
    ~]# chcon -t samba_share_t /var/www/html/testfile
    View the changes:
    ~]$ ls -Z /var/www/html/testfile
    -rw-r--r--  root root unconfined_u:object_r:samba_share_t:s0 /var/www/html/testfile
  2. Enter the following command to confirm that the httpd process is not running:
    ~]$ systemctl status httpd.service
    httpd.service - The Apache HTTP Server
       Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled)
       Active: inactive (dead)
    If the output differs, enter the following command as root to stop the httpd process:
    ~]# systemctl stop httpd.service
  3. To make the httpd process run unconfined, enter the following command as root to change the type of the /usr/sbin/httpd file, to a type that does not transition to a confined domain:
    ~]# chcon -t bin_t /usr/sbin/httpd
  4. Confirm that /usr/sbin/httpd is labeled with the bin_t type:
    ~]$ ls -Z /usr/sbin/httpd
    -rwxr-xr-x. root root system_u:object_r:bin_t:s0       /usr/sbin/httpd
  5. As root, start the httpd process and confirm, that it started successfully:
    ~]# systemctl start httpd.service
    ~]# systemctl status httpd.service
    httpd.service - The Apache HTTP Server
       Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled)
       Active: active (running) since Thu 2013-08-15 11:17:01 CEST; 5s ago
  6. Enter the following command to view httpd running in the unconfined_service_t domain:
    ~]$ ps -eZ | grep httpd
    system_u:system_r:unconfined_service_t:s0 11884 ? 00:00:00 httpd
    system_u:system_r:unconfined_service_t:s0 11885 ? 00:00:00 httpd
    system_u:system_r:unconfined_service_t:s0 11886 ? 00:00:00 httpd
    system_u:system_r:unconfined_service_t:s0 11887 ? 00:00:00 httpd
    system_u:system_r:unconfined_service_t:s0 11888 ? 00:00:00 httpd
    system_u:system_r:unconfined_service_t:s0 11889 ? 00:00:00 httpd
  7. Change into a directory where your Linux user has write access to, and enter the following command. Unless there are changes to the default configuration, this command succeeds:
    ~]$ wget http://localhost/testfile
    --2009-05-07 01:41:10--  http://localhost/testfile
    Resolving localhost...
    Connecting to localhost||:80... connected.
    HTTP request sent, awaiting response... 200 OK
    Length: 0 [text/plain]
    Saving to: `testfile'
    [ <=>                            ]--.-K/s   in 0s      
    2009-05-07 01:41:10 (0.00 B/s) - `testfile' saved [0/0]
    Although the httpd process does not have access to files labeled with the samba_share_t type, httpd is running in the unconfined unconfined_service_t domain, and falls back to using DAC rules, and as such, the wget command succeeds. Had httpd been running in the confined httpd_t domain, the wget command would have failed.
  8. The restorecon utility restores the default SELinux context for files. As root, enter the following command to restore the default SELinux context for /usr/sbin/httpd:
    ~]# restorecon -v /usr/sbin/httpd
    restorecon reset /usr/sbin/httpd context system_u:object_r:unconfined_exec_t:s0->system_u:object_r:httpd_exec_t:s0
    Confirm that /usr/sbin/httpd is labeled with the httpd_exec_t type:
    ~]$ ls -Z /usr/sbin/httpd
    -rwxr-xr-x  root root system_u:object_r:httpd_exec_t:s0 /usr/sbin/httpd
  9. As root, enter the following command to restart httpd. After restarting, confirm that httpd is running in the confined httpd_t domain:
    ~]# systemctl restart httpd.service
    ~]$ ps -eZ | grep httpd
    system_u:system_r:httpd_t:s0    8883 ?        00:00:00 httpd
    system_u:system_r:httpd_t:s0    8884 ?        00:00:00 httpd
    system_u:system_r:httpd_t:s0    8885 ?        00:00:00 httpd
    system_u:system_r:httpd_t:s0    8886 ?        00:00:00 httpd
    system_u:system_r:httpd_t:s0    8887 ?        00:00:00 httpd
    system_u:system_r:httpd_t:s0    8888 ?        00:00:00 httpd
    system_u:system_r:httpd_t:s0    8889 ?        00:00:00 httpd
  10. As root, remove testfile:
    ~]# rm -i /var/www/html/testfile
    rm: remove regular empty file `/var/www/html/testfile'? y
  11. If you do not require httpd to be running, as root, enter the following command to stop httpd:
    ~]# systemctl stop httpd.service
The examples in these sections demonstrate how data can be protected from a compromised confined-process (protected by SELinux), as well as how data is more accessible to an attacker from a compromised unconfined-process (not protected by SELinux).