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4.2. Unconfined Processes

Unconfined processes run in unconfined domains, for example, init programs run in the unconfined initrc_t domain, unconfined kernel processes run in the kernel_t domain, and unconfined Linux users run 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 4.1, “How to Verify SELinux Status” described in Section 4.1, “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 4.3. 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 command, which is discussed later. As the Linux root user, run the following command to change the type to a type used by Samba:
    ~]# chcon -t samba_share_t /var/www/html/testfile
    Run the ls -Z /var/www/html/testfile command to 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. Run the service httpd status command to confirm that the httpd process is not running:
    ~]$ service httpd status
    httpd is stopped
    If the output differs, run the service httpd stop command as the Linux root user to stop the httpd process:
    ~]# service httpd stop
    Stopping httpd:                                            [  OK  ]
  3. To make the httpd process run unconfined, run the following command as the Linux root user to change the type of /usr/sbin/httpd, to a type that does not transition to a confined domain:
    ~]# chcon -t unconfined_exec_t /usr/sbin/httpd
  4. Run the ls -Z /usr/sbin/httpd command to confirm that /usr/sbin/httpd is labeled with the unconfined_exec_t type:
    ~]$ ls -Z /usr/sbin/httpd
    -rwxr-xr-x  root root system_u:object_r:unconfined_exec_t:s0 /usr/sbin/httpd
  5. As the Linux root user, run the service httpd start command to start the httpd process. The output is as follows if httpd starts successfully:
    ~]# service httpd start
    Starting httpd:                                            [  OK  ]
  6. Run the ps -eZ | grep httpd command to view the httpd running in the unconfined_t domain:
    ~]$ ps -eZ | grep httpd
    unconfined_u:unconfined_r:unconfined_t:s0 7721 ?      00:00:00 httpd
    unconfined_u:unconfined_r:unconfined_t:s0 7723 ?      00:00:00 httpd
    unconfined_u:unconfined_r:unconfined_t:s0 7724 ?      00:00:00 httpd
    unconfined_u:unconfined_r:unconfined_t:s0 7725 ?      00:00:00 httpd
    unconfined_u:unconfined_r:unconfined_t:s0 7726 ?      00:00:00 httpd
    unconfined_u:unconfined_r:unconfined_t:s0 7727 ?      00:00:00 httpd
    unconfined_u:unconfined_r:unconfined_t:s0 7728 ?      00:00:00 httpd
    unconfined_u:unconfined_r:unconfined_t:s0 7729 ?      00:00:00 httpd
    unconfined_u:unconfined_r:unconfined_t:s0 7730 ?      00:00:00 httpd
  7. Change into a directory where your Linux user has write access to, and run the wget http://localhost/testfile 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... 127.0.0.1
    Connecting to localhost|127.0.0.1|:80... connected.
    HTTP request sent, awaiting response... 200 OK
    Length: 0 [text/plain]
    Saving to: `testfile.1'
    
    [ <=>                            ]--.-K/s   in 0s      
    	
    2009-05-07 01:41:10 (0.00 B/s) - `testfile.1' 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_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 command restores the default SELinux context for files. As the Linux root user, run the restorecon -v /usr/sbin/httpd 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
    
    Run the ls -Z /usr/sbin/httpd command to 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 the Linux root user, run the service httpd restart command to restart httpd. After restarting, run the ps -eZ | grep httpd command to confirm that httpd is running in the confined httpd_t domain:
    ~]# service httpd restart
    Stopping httpd:                                            [  OK  ]
    Starting httpd:                                            [  OK  ]
    ~]# ps -eZ | grep httpd
    unconfined_u:system_r:httpd_t:s0    8883 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0    8884 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0    8885 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0    8886 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0    8887 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0    8888 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0    8889 ?        00:00:00 httpd
  10. As the Linux root user, run the rm -i /var/www/html/testfile command to 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 the Linux root user, run the service httpd stop command to stop httpd:
    ~]# service httpd stop
    Stopping httpd:                                            [  OK  ]
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).