When to Use SE Linux

Recently someone asked on IRC whether they should use SE Linux on a web server machine (that is being used for no other purpose) and then went on to add “since the webserver is installed as root anyway“.

If a machine is used to run a single non-root application then the potential benefits of using SE Linux are significantly reduced, the issue will be whether the application could exploit a setuid program to gain root access if SE Linux was not there to prevent it.

The interesting point in this case is that the user notes that the webserver runs as root. It was not made clear whether the entire service ran as root or whether the parent ran as root while child processes ran as a different UID (a typical Apache configuration). In the case where the child processes run as non-root it is still potentially possible for a bug in Apache to be used to exploit the parent process and assume it’s privileges. So it’s reasonable to consider that SE Linux will protect the integrity of the base OS from a web server running as root – even for the most basic configuration (without cgi-bin scripts). If a root owned process that is confined by SE Linux is compromised then as long as there is no kernel vulnerability the base OS should keep it’s integrity and the sys-admin should be able to login and discover what happened.

If the web server is more complex and runs cgi-bin scripts then there is a further benefit for system integrity in that a cgi-bin script could be compromised but the main Apache process (which runs in a different domain) would run without interruption.

When a daemon that runs as non-root is cracked on a non-SE system it will have the ability to execute setuid programs – some of which may have exploitable bugs. Also on a non-SE system every daemon has unrestricted network access in a typical configuration (there is a Net Filter module to control access by UID and GID, but it is very rarely used and won’t work in the case of multiple programs running with the same UID/GID). With SE Linux a non-root daemon will usually have no access to run setuid programs (and if it can run them it will be without a domain transition so they gain no extra privileges). Also SE Linux permits controls over which network ports an application may talk to. So the ability of a compromised server process to attack other programs is significantly reduced on a SE Linux system.

In summary the more complex your installation is and the more privileges that are required by various server processes the more potential there is to increase the security of your system by using SE Linux. But even on a simple server running only a single daemon as non-root there is potential for SE Linux to provide real benefits to system security.