3.2. Unconfined Processes
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 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: View the changes: Enter the following command to confirm that the httpd process is not running: If the output differs, enter the following command as root to stop the httpd process: 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: Confirm that /usr/sbin/httpd is labeled with the bin_t type: As root, start the httpd process and confirm, that it started successfully: Enter the following command to view httpd running in the unconfined_service_t domain: 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: 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. 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 : Confirm that /usr/sbin/httpd is labeled with the httpd_exec_t type: As root, enter the following command to restart httpd . After restarting, confirm that httpd is running in the confined httpd_t domain: As root, remove testfile : If you do not require httpd to be running, as root, enter the following command to stop httpd : 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).
~]# chcon -t samba_share_t /var/www/html/testfile ~]USD ls -Z /var/www/html/testfile -rw-r--r-- root root unconfined_u:object_r:samba_share_t:s0 /var/www/html/testfile ~]USD systemctl status httpd.service httpd.service - The Apache HTTP Server Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled) Active: inactive (dead) ~]# systemctl stop httpd.service ~]# chcon -t bin_t /usr/sbin/httpd ~]USD ls -Z /usr/sbin/httpd -rwxr-xr-x. root root system_u:object_r:bin_t:s0 /usr/sbin/httpd ~]# 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 ~]USD 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 ~]USD 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' [ <=> ]--.-K/s in 0s 2009-05-07 01:41:10 (0.00 B/s) - `testfile' saved [0/0] ~]# 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 ~]USD ls -Z /usr/sbin/httpd -rwxr-xr-x root root system_u:object_r:httpd_exec_t:s0 /usr/sbin/httpd ~]# systemctl restart httpd.service ~]USD 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 ~]# rm -i /var/www/html/testfile rm: remove regular empty file `/var/www/html/testfile'? y ~]# systemctl stop httpd.service