SELinux

Security-Enhanced Linux (SELinux) is a Linux feature that provides a variety of security policies, including U.S. Department of Defense style mandatory access controls (MAC), through the use of Linux Security Modules (LSM) in the Linux kernel. It is not a Linux distribution, but rather a set of modifications that can be applied to Unix-like operating systems, such as Linux and BSD. Its architecture strives to streamline the volume of software charged with security policy enforcement, which is closely aligned with the Trusted Computer System Evaluation Criteria (TCSEC, referred to as Orange Book) requirement for trusted computing base (TCB) minimization (applicable to evaluation classes B3 and A1) but is quite unrelated to the least privilege requirement (B2, B3, A1) as is often claimed. The germinal concepts underlying SELinux can be traced to several earlier projects by the U.S. National Security Agency (NSA). [1]

Running SELinux under a Linux distribution requires three things: An SELinux enabled kernel, SELinux Userspace tools and libraries, and SELinux Policies (mostly based on the Reference Policy). Some common Linux programs will also need to be patched/compiled with SELinux features.

Prerequisites

Only ext2, ext3, ext4, JFS and XFS filesystems are supported to use SELinux.

Note: This is probably not needed anymore:

XFS users should use 512 byte inodes (the default is 256). SELinux uses extended attributes for storing security labels in files. XFS stores this in the inode, and if the inode is too small, an extra block has to be used, which wastes a lot of space and incurs performance penalties.

 # mkfs.xfs -i size=512 /dev/sda1  (for example)

Installing needed packages

You should install at least linux-selinux^AUR, selinux-pam^AUR, selinux-usr-policycoreutils^AUR and selinux-refpolicy-src^AUR from the AUR. Installing all SELinux-related packages is recommended.

When installing from the AUR, you can use an AUR helper or download tarballs from the AUR manually and build with makepkg. Especially when installing for the first time, take extreme caution when replacing the pam and coreutils packages, as they are vital to your system. Having the Arch Linux live CD or live USB drive ready to use is strongly encouraged.

Warning: Do not remove pam via sudo, as PAM is what takes care of authentication, and you just removed it. Instead first su to root and then do:

pacman -Rdd pam
pacman -U selinux-pam

Doing pacman -Rdd coreutils, pacman -U selinux-coreutils may also cause you troubles, so maybe the best way is to install the selinux-* packages from a live CD chroot to your system.

Warning: Do not install selinux-sysvinit^AUR package unless everything is set up, as you may end up with an unbootable system. Or, do not reboot unless you have everything set up.

Package description

All SELinux related packages belong to the selinux group. Group selinux-system-utilities is used for modified packages from the [core] repository. Group selinux-userspace contains packages from SELinux Userspace project. Security policies belong to selinux-policies group. Other packages are in selinux-extras group.

SELinux aware system utils

linux-selinux^AUR

SELinux enabled kernel. Compiling custom modules like virtualbox works.

selinux-coreutils^AUR

Modified coreutils package compiled with SELinux support enabled.

selinux-flex^AUR

Flex version needed only to build checkpolicy. Current flex has error causing failure in checkmodule command.

selinux-pam^AUR

PAM package with pam_selinux.so.

selinux-sysvinit^AUR

Sysvinit which loads policy at startup. Be careful; it fails if SELinux policy cannot be loaded!

selinux-util-linux^AUR

Modified util-linux package compiled with SELinux support enabled.

selinux-udev^AUR made obsolete by selinux-sysvinit^AUR

Modified udev package compiled with SELinux support enabled for labeling of files in /dev to work correctly. The udev functionality is now provided by selinux-sysvinit-tools [1] which is part of the selinux-sysvinit^AUR splitpackage.

selinux-findutils^AUR

Patched findutils package compiled with SELinux support to make searching of files with specified security context possible.

selinux-sudo^AUR

Modified sudo package compiled with SELinux support which sets security context correctly.

selinux-procps^AUR

Procps package with SELinux patch based on some Fedora patches.

selinux-psmisc^AUR

Psmisc package compiled with SELinux support; for example, it adds the -Z option to killall.

selinux-shadow^AUR

Shadow package compiled with SELinux support; contains a modified /etc/pam.d/login file to set correct security context for user after login.

selinux-cronie^AUR

Fedora fork of Vixie cron with SELinux enabled.

selinux-logrotate^AUR

Logrotate package compiled with SELinux support.

selinux-openssh^AUR

OpenSSH package compiled with SELinux support to set security context for user sessions.

SELinux userspace

selinux-usr-checkpolicy^AUR

Tools to build SELinux policy

selinux-usr-libselinux^AUR

Library for security-aware applications. Python bindings needed for semanage and setools now included.

selinux-usr-libsemanage^AUR

Library for policy management. Python bindings needed for semanage and setools now included.

selinux-usr-libsepol^AUR

Library for binary policy manipulation.

selinux-usr-policycoreutils^AUR

SELinux core utils such as newrole, setfiles, etc.

selinux-usr-sepolgen^AUR

A Python library for parsing and modifying policy source.

SELinux policy

selinux-refpolicy^AUR

Precompiled modular-otherways-vanilla Reference policy with headers and documentation but without sources.

selinux-refpolicy-src^AUR

Reference policy sources

selinux-refpolicy-arch^AUR

Precompiled modular Reference policy with headers and documentation but without sources. Development Arch Linux Refpolicy patch included, but for now [February 2011] it only fixes some issues with /etc/rc.d/* labeling.

Other SELinux tools

selinux-setools^AUR

CLI and GUI tools to manage SELinux

audit^AUR

User space utilities for storing and searching the audit records generated by the audit subsystem in the Linux kernel. SELinux (AVC) will log all denials using audit. Very useful in troubleshooting SELinux. Also audit2allow use log from this program.

Note: If using proprietary drivers, such as NVIDIA graphics drivers, you may need to rebuild them for custom kernels.

Configuration

After the installation of needed packages, you have to set up a few things so that SELinux can be used.

Changing boot loader configuration

You have to manually change GRUB's /boot/grub/menu.lst so that the custom kernel is booted, e.g.:

# (1) Arch Linux
title  Arch Linux (SELinux)
root   (hd0,4)
kernel /boot/vmlinuz-linux-selinux root=/dev/sda5 ro vga=775
initrd /boot/initramfs-linux-selinux.img

Mounting selinuxfs

Add following to /etc/fstab:

none   /selinux   selinuxfs   noauto   0   0

Do not forget to create the mount point:

mkdir /selinux

Main SELinux configuration file

Main SELinux configuration file (/etc/selinux/config) is part of the selinux-refpolicy^AUR package currently in the AUR. It has default contents as follows:

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#       enforcing - SELinux security policy is enforced.
#       permissive - SELinux prints warnings 
#                       instead of enforcing.
#       disabled - No SELinux policy is loaded.
SELINUX=permissive
# SELINUXTYPE= takes the name of SELinux policy to
# be used. Current options are:
#       refpolicy (vanilla reference policy)
#       refpolicy-arch (reference policy with 
#       Arch Linux patch)
SELINUXTYPE=refpolicy

Note: Option SELINUX=permissive is suitable only for testing. It gives no security. When everything is set up and working, you should change it to SELINUX=enforcing. Option SELINUXTYPE=refpolicy specifies the name of used policy. Change it if you choose another name for your policy. If you plan to compile policy from source, you have to create the file yourself.

Set up PAM

Correctly set-up PAM is important to get a proper security context after login. There should be the following lines in /etc/pam.d/system-login:

# pam_selinux.so close should be the first session rule
session         required        pam_selinux.so close
# pam_selinux.so open should only be followed by sessions to be executed in the user context
session         required        pam_selinux.so open

Similarly for logging in via SSH in /etc/pam.d/sshd, which is part of selinux-openssh^AUR package.

If you want to use SELinux with GUI, you should add the aforementioned lines to other files such as /etc/pam.d/kde, /etc/pam.d/kde-np, ... depending on your login manager.

Note: Running SELinux with GUI applications in Arch Linux is not much supported at the time being.

Reference policy

There are currently two possible ways of installing reference policy: From a pre-compiled package (selinux-refpolicy^AUR) or from a source package (selinux-refpolicy-src^AUR).

Note: It is possible to have both the source and the binary package installed. If you plan to build from source in that case, you should probably change the name of policy in build.conf to avoid overwriting of selinux-refpolicy package files.

Installing a precompiled refpolicy

Install selinux-refpolicy^AUR from AUR. This is a modular-otherways-vanilla refpolicy. This package includes policy headers (you can therefore compile your own modules), policy documentation and an install script which will load the policy for you and relabel your filesystem (which will likely take some time). It does not include the sources though.

This package also includes the main SELinux configuration file (/etc/selinux/config) defaulting to refpolicy and permissive SELinux enforcement for testing purposes.

You should verify that the policy was correctly loaded, that is if the file /etc/selinux/refpolicy/policy/policy.24 has non-zero size. If so and if you have installed selinux-sysvinit^AUR and other needed packages, you are ready to reboot and make sure that everything works.

Warning: On newer kernels (eg. 3.0) policy in file /etc/selinux/refpolicy/policy/policy.24 has zero bytes size, because it is used new version of policy from file: /etc/selinux/refpolicy/policy/policy.26

In case the policy was not correctly loaded you can as root use the following command inside of the /usr/share/selinux/refpolicy directory to do so:

/bin/ls *.pp | /bin/grep -Ev "base.pp|enableaudit.pp" | /usr/bin/xargs /usr/sbin/semodule -s refpolicy -b base.pp -i

To manually relabel your filesystem you can as root use:

/sbin/restorecon -r /

Installing refpolicy from a source package

Install selinux-refpolicy-src^AUR from AUR. Edit the file /etc/selinux/refpolicy/src/policy/build.conf to your liking.

Note: Build configuration file build.conf is overwritten on every selinux-refpolicy-src package upgrade, so backup your configuration.

To build, install and load policy from source do the following. (For other possibilities consult the README file located in /etc/selinux/refpolicy/src/policy/.)

cd /etc/selinux/refpolicy/src/policy
make bare
make conf 
make load

Copy or link the compiled binary policy to /etc/policy.bin for sysvinit to find and install selinux-sysvinit:

ln -s /etc/selinux/refpolicy/policy/policy.21 /etc/policy.bin

At this moment files do not have any context, so you should relabel the whole filesystem, which will take a while:

make relabel

Create the main SELinux configuration file (/etc/selinux/config) according to the example in related section.

Now you are ready to reboot and make sure that everything works.

Post-installation steps

Warning: If you did not install selinux-sysvinit, then you will see SELinux in disabled mode, and /selinux will not be mounted.

You can check that SELinux is working with sestatus. You should get something like:

SELinux status:                 enabled
SELinuxfs mount:                /selinux
Current mode:                   permissive
Mode from config file:          enforcing
Policy version:                 24
Policy from config file:        refpolicy

To maintain correct context, you can use restorecond:

touch /etc/rc.d/restorecond
chmod ugo+x /etc/rc.d/restorecond

Which should contain:

#!/bin/sh
restorecond

Note: Do not forget to add restorecond into your DAEMONS array in /etc/rc.conf.

To switch to enforcing mode without reboot, you can use:

echo 1 >/selinux/enforce

Note: If setting SELINUX=enforcing in /etc/selinux/config does not work for you, create /etc/rc.d/selinux-enforce containing the preceding command similarly as with restorecond daemon.

Useful tools

There are some tools/commands that can greatly help with SELinux.

restorecon

Restores the context of a file/directory (or recursively with -R) based on any policy rules

rlpkg

Relabels any files belonging to that Gentoo package to their proper security context (if they have one)

chcon

Change the context on a specific file

audit2allow

Reads in log messages from the AVC log file and tells you what rules would fix the error. Do not just add these rules without looking at them though, they cannot detect errors in other places (e.g. the application is running in the wrong context in the first place), or sometimes things will generate error messages but may maintain functionality so it would be better to add dontaudit to just ignore the access attempts.

References

• Security Enhanced Linux
• Gentoo SELinux Handbook
• Fedora Project's SELinux Wiki
• NSA's Official SELinux Homepage
• Reference Policy Homepage
• SELinux Userspace Homepage
• SETools Homepage

See also

• AppArmor (Similar to SELinux, much easier to configure, less features.)