Wednesday, December 26, 2007

SELinux for all?

I recently migrated my mail server from a Gentoo Linux host to a Fedora 7 host and decided to use SELinux to improve my security posture. This is a big deal and should provide a greater level of security for my mail server, provided I can configure the security policies correctly.

SELinux was developed with the NSA and released under the GPL to provide a method of Mandatory Access Control for Linux.

The great benefit of using SELinux is that processes are prevented from accessing files that are not defined as belonging to, or associated with that process. This is a great thing! If a daemon or process were to be compromised, it would be restricted to the specific files and resources configured in the SELinux policy. Any violations are logged by the setroubleshoot daemon in the /var/log/messages syslog file and allow the user to review in order to allow or continue denial of the access attempt(s).

Enabling SELinux is very simple upon installation of Fedora or Red Hat Linux and is prompted during install. I do not recommend enabling on an OS that is in production and would only recommend enabling SELinux on a recent install or non-production host until you are very comfortable with it and have tuned your policies according to your production operation.

To get your feet wet, enable SELinux in permissive mode. This mode will allow all processes to run as if SELinux were off, but it will log any violations via auditd in syslog to allow you to tune your policies. You are prompted for this method upon install, or post-install, you could configure this via the following method:

1. use the setenforce command: setenforce permissive

usage: setenforce [ Enforcing | Permissive | 1 | 0 ]

2. edit the /etc/selinux/config file:


Also, be sure to remove any mention of selinux from the /boot/grub/grub.conf file if you enable SELinux, or provide the proper flags to enable it. This is also the best way to disable SELinux if you are so inclined. An example would be:

title Fedora 7 (
root (hd0,0)
kernel /vmlinuz- ro root=/dev/vg01/f7_root rhgb quiet selinux=0
initrd /initrd-

Note that on the kernel line I have passed 'selinux=0' to the kernel which would then disable this feature.

After enabling SELinux, check syslog frequently (/var/log/messages) to look for policy violations and tune your system. A typical policy violation would look like the following:

Dec 26 08:12:50 f7-laptop setroubleshoot: SELinux is preventing the /bin/vi from using potentially mislabeled files (.exrc). For complete SELinux messages. run sealert -l 27ae2a46-19bb-47a8-b127-068a587e9494

In the above log entry, I used 'vipw' to access the /etc/passwd file and SELinux did not allow the 'vipw' binary to access my '.exrc' file (user customized file for VIM settings).

In short, SELinux can provide a GREAT measure of security to a seasoned administrator who is willing to take the time to learn how to use it. Use permissive mode and tune the policies to fit your particular environment. I do not know of anybody who uses SELinux in production or in a business due to the issues it seems to cause by lack of training and/or expertise in the configuration phase. Remember to disable SELinux and re-create the issue before bothering the application maintainer about any issues you may have.

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