SELinux

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Security-Enhanced Linux (SELinux) is a Linux feature that provides a variety of security policies, including U.S. Department of Defense style Mandatory Access Control (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.

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.

Current status in Arch Linux

SELinux is not officially supported (see [1][2]). The status of unofficial support is:

Name Status Available at
SELinux enabled kernel Implemented for all officially supported kernels Available in official repositories since 4.18.8.
SELinux Userspace tools and libraries Implemented in AUR: https://aur.archlinux.org/packages/?O=0&K=selinux Work is done at https://github.com/archlinuxhardened/selinux
SELinux Policy Work in progress, using Reference Policy as upstream Upstream: https://github.com/SELinuxProject/refpolicy (since release 20170805 the policy has integrated support for systemd and single-/usr/bin directory)

Summary of changes in AUR as compared to official core packages:

Name Status and comments
linux, linux-lts, linux-zen, linux-hardened Need to set the lsm= kernel parameter
coreutils Need a rebuild with --with-selinux flag to link with libselinux
cronie Need a rebuild with --with-selinux flag
dbus Need a rebuild with --enable-libaudit and --enable-selinux flags
findutils Need a rebuild with libselinux installed to enable SELinux-specific options
iproute2 Need a rebuild with --with-selinux flag
logrotate Need a rebuild with --with-selinux flag
openssh Need a rebuild with --with-selinux flag
pam Need a rebuild with --enable-selinux flag for Linux-PAM ; Need a patch for pam_unix2, which only removes a function already implemented in a recent versions of libselinux
pambase Configuration changes to add pam_selinux.so to /etc/pam.d/system-login
psmisc Need a rebuild with --with-selinux flag
shadow Need a rebuild with --with-selinux flags
sudo Need a rebuild with --with-selinux flag
systemd Need a rebuild with --enable-audit and --enable-selinux flags
util-linux Need a rebuild with --with-selinux flag

All of the other SELinux-related packages may be included without changes nor risks.

Concepts: Mandatory Access Controls

Note: This section is meant for beginners. If you know what SELinux does and how it works, feel free to skip ahead to the installation.

Before you enable SELinux, it is worth understanding what it does. Simply and succinctly, SELinux enforces Mandatory Access Controls (MACs) on Linux. In contrast to SELinux, the traditional user/group/rwx permissions are a form of Discretionary Access Control (DAC). MACs are different from DACs because security policy and its execution are completely separated.

An example would be the use of the sudo command. When DACs are enforced, sudo allows temporary privilege escalation to root, giving the process so spawned unrestricted systemwide access. However, when using MACs, if the security administrator deems the process to have access only to a certain set of files, then no matter what the kind of privilege escalation used, unless the security policy itself is changed, the process will remain constrained to simply that set of files. So if sudo is tried on a machine with SELinux running in order for a process to gain access to files its policy does not allow, it will fail.

Another set of examples are the traditional (-rwxr-xr-x) type permissions given to files. When under DAC, these are user-modifiable. However, under MAC, a security administrator can choose to freeze the permissions of a certain file by which it would become impossible for any user to change these permissions until the policy regarding that file is changed.

As you may imagine, this is particularly useful for processes which have the potential to be compromised, i.e. web servers and the like. If DACs are used, then there is a particularly good chance of havoc being wreaked by a compromised program which has access to privilege escalation.

For further information, visit Wikipedia:Mandatory access control.

Installing SELinux

Package description

All SELinux related packages belong to the selinux group in the AUR.

SELinux aware system utilities

coreutils-selinuxAUR
Modified coreutils package compiled with SELinux support enabled. It replaces the coreutils package
cronie-selinuxAUR
Fedora fork of Vixie cron with SELinux enabled. It replaces the cronie package.
dbus-selinuxAUR
An SELinux aware version of D-Bus. It replaces the dbus package.
findutils-selinuxAUR
Patched findutils package compiled with SELinux support to make searching of files with specified security context possible. It replaces the findutils package.
iproute2-selinuxAUR
iproute2 package compiled with SELinux support; for example, it adds the -Z option to ss. It replaces the iproute2 package.
logrotate-selinuxAUR
Logrotate package compiled with SELinux support. It replaces the logrotate package.
openssh-selinuxAUR
OpenSSH package compiled with SELinux support to set security context for user sessions. It replaces the openssh package.
pam-selinuxAUR and pambase-selinuxAUR
PAM package with pam_selinux.so. and the underlying base package. They replace the pam and pambase packages respectively.
psmisc-selinuxAUR
Psmisc package compiled with SELinux support; for example, it adds the -Z option to killall. It replaces the psmisc package.
shadow-selinuxAUR
Shadow package compiled with SELinux support; contains a modified /etc/pam.d/login file to set correct security context for user after login. It replaces the shadow package.
sudo-selinuxAUR
Modified sudo package compiled with SELinux support which sets the security context correctly. It replaces the sudo package.
systemd-selinuxAUR
An SELinux aware version of Systemd. It replaces the systemd package.
util-linux-selinuxAUR
Modified util-linux package compiled with SELinux support enabled. It replaces the util-linux package.

SELinux userspace utilities

checkpolicyAUR
Tools to build SELinux policy
mcstransAUR
Daemon which is used by libselinux to translate MCS labels
libselinuxAUR
Library for security-aware applications. Python bindings needed for semanage and setools now included.
libsemanageAUR
Library for policy management. Python bindings needed for semanage and setools now included.
libsepolAUR
Library for binary policy manipulation.
policycoreutilsAUR
SELinux core utils such as newrole, setfiles, etc.
restorecondAUR
Daemon which maintains the label of some files
secilcAUR
Compiler for SELinux policies written in CIL (Common Intermediate Language)
selinux-dbus-configAUR
DBus service which allows managing SELinux configuration
selinux-guiAUR
SELinux GUI tools (system-config-selinux)
selinux-pythonAUR and selinux-python2AUR
SELinux python tools and libraries (semanage, sepolgen, sepolicy, etc.)
selinux-sandboxAUR
Sandboxing tool for SELinux
semodule-utilsAUR
Tools to handle SELinux modules when building a policy

SELinux policy packages

selinux-refpolicy-srcAUR
Reference policy sources
selinux-refpolicy-gitAUR
Reference policy git master (https://github.com/SELinuxProject/refpolicy) built with configuration specific for Arch Linux
selinux-refpolicy-archAUR
Precompiled modular Reference policy with headers and documentation but without sources. Development Arch Linux Refpolicy patches included, which fixes issues related to path labeling and systemd support. These patches are also sent to Reference Policy maintainers and their inclusion in selinux-refpolicy-archAUR is mainly a way to perform updates between Refpolicy releases.

Other SELinux tools

setoolsAUR
CLI and GUI tools to manage SELinux
selinux-alpm-hookAUR
pacman hook to label files accordingly to SELinux policy when installing and updating packages

Installation

There are two methods to install the requisite SELinux packages.

Via AUR

After all these steps, you can install a SELinux kernel (like linux) and a policy (like selinux-refpolicy-archAUR or selinux-refpolicy-gitAUR).

Using the GitHub repository

All packages are maintained at https://github.com/archlinuxhardened/selinux . This repository also contains a script named build_and_install_all.sh which builds and installs (or updates) all packages in the needed order. Here is an example of a way this script can be used in a user shell to install all packages (with downloading the GPG keys which are used to verify the source tarballs of the package): 

$ git clone https://github.com/archlinuxhardened/selinux.git
$ cd selinux
$ ./recv_gpg_keys.sh
$ ./build_and_install_all.sh

Of course, it is possible to modify the content of build_and_install_all.sh before running it, for example if you already have SELinux support in your kernel.

Enable SELinux LSM

To enable SELinux as default security model on every boot, set the following kernel parameter: 

lsm=landlock,lockdown,yama,selinux,bpf
Note: The lsm= kernel parameter sets the initialization order of Linux security modules. The kernel's configured lsm= value can be found with zgrep CONFIG_LSM= /proc/config.gz and the current value with cat /sys/kernel/security/lsm.
  • Make sure that selinux is the first "major" module in the list.[3] Examples of valid values and their order can be found in security/Kconfig.
  • capability should be omitted from lsm= as it will always get included automatically.

Custom kernel

When compiling the kernel, it is required to set at least the following options: 

CONFIG_SECURITY_SELINUX=y
CONFIG_AUDIT=y

To enable the SELinux Linux security model by default and omit the need to set kernel parameters, additionally set the CONFIG_LSM option and specify selinux as the first "major" module in the list: 

CONFIG_LSM="landlock,lockdown,yama,selinux,bpf"

Checking PAM

A correctly set-up PAM is important to get the proper security context after login. Check for the presence of 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

Installing a policy

Warning: The reference policy as given by SELinuxProject is not very good for Arch Linux. Most people submitting patches to improve the policy use an other distribution (Debian, Gentoo, RHEL, etc.), therefore the compatibility with Arch Linux packages is not perfect (for example the policy may not support the most recent features of a program).

Policies are the mainstay of SELinux. They are what govern its behaviour. The only policy currently available in the AUR is the Reference Policy. In order to install it, you should use the source files, which may be got from the package selinux-refpolicy-srcAUR or by downloading the latest release on https://github.com/SELinuxProject/refpolicy/wiki/DownloadRelease#current-release. When using the AUR package, navigate to /etc/selinux/refpolicy/src/policy and run the following commands: 

# make bare
# make conf
# make install

to install the reference policy as it is. Those who know how to write SELinux policies can tweak them to their heart's content before running the commands written above. The command takes a while to do its job and taxes one core of your system completely, so do not worry. Just sit back and let the command run for as long as it takes.

To load the reference policy run: 

# make load

Then, make the file /etc/selinux/config with the following contents (Only works if you used the defaults as mentioned above. If you decided to change the name of the policy, you need to tweak the file): 

/etc/selinux/config
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#       enforcing - SELinux security policy is enforced.
#                   Set this value once you know for sure that SELinux is configured the way you like it and that your system is ready for deployment
#       permissive - SELinux prints warnings instead of enforcing.
#                    Use this to customise your SELinux policies and booleans prior to deployment. Recommended during policy development.
#       disabled - No SELinux policy is loaded.
#                  This is not a recommended setting, for it may cause problems with file labelling
SELINUX=permissive
# SELINUXTYPE= takes the name of SELinux policy to
# be used. Current options are:
#       refpolicy (vanilla reference policy)
#       <custompolicy> - Substitute <custompolicy> with the name of any custom policy you choose to load
SELINUXTYPE=refpolicy

Now, you may reboot. After rebooting, run: 

# restorecon -r /

to label your filesystem.

Now, make a file requiredmod.te with the contents: 

requiredmod.te
module requiredmod 1.0;

require {
        type devpts_t;
        type kernel_t;
        type device_t;
        type var_run_t;
        type udev_t;
        type hugetlbfs_t;
        type udev_tbl_t;
        type tmpfs_t;
        class sock_file write;
        class unix_stream_socket { read write ioctl };
        class capability2 block_suspend;
        class dir { write add_name };
        class filesystem associate;
}

#============= devpts_t ==============
allow devpts_t device_t:filesystem associate;

#============= hugetlbfs_t ==============
allow hugetlbfs_t device_t:filesystem associate;

#============= kernel_t ==============
allow kernel_t self:capability2 block_suspend;

#============= tmpfs_t ==============
allow tmpfs_t device_t:filesystem associate;

#============= udev_t ==============
allow udev_t kernel_t:unix_stream_socket { read write ioctl };
allow udev_t udev_tbl_t:dir { write add_name };
allow udev_t var_run_t:sock_file write;

and run the following commands: 

# checkmodule -m -o requiredmod.mod requiredmod.te
# semodule_package -o requiredmod.pp -m requiredmod.mod
# semodule -i requiredmod.pp

This is required to remove a few messages from /var/log/audit/audit.log which are a nuisance to deal with in the reference policy. This is an ugly hack and it should be made very clear that the policy so installed simply patches the reference policy in order to hide the effects of incorrect labelling.

Testing in a Vagrant virtual machine

It is possible to use Vagrant to provision a virtual Arch Linux machine with SELinux configured. This is a convenient way to test an Arch Linux system running SELinux without modifying a current system. Here are commands which can be used to achieve this: 

$ git clone https://github.com/archlinuxhardened/selinux.git
$ cd selinux/_vagrant
$ vagrant up
$ vagrant ssh

Post-installation steps

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

SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             refpolicy
Current mode:                   permissive
Mode from config file:          permissive
Policy MLS status:              disabled
Policy deny_unknown status:     allowed
Max kernel policy version:      28

To maintain correct context, you can enable restorecond.service.

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

# echo 1 > /sys/fs/selinux/enforce

Swapfiles

If you have a swap file instead of a swap partition, issue the following commands in order to set the appropriate security context: 

# semanage fcontext -a -t swapfile_t "/path/to/swapfile"
# restorecon /path/to/swapfile

Working with SELinux

SELinux defines security using a different mechanism than traditional Unix access controls. The best way to understand it is by example. For example, the SELinux security context of the apache homepage looks like the following: 

$ ls -lZ /var/www/html/index.html
-rw-r--r--  username username system_u:object_r:httpd_sys_content_t /var/www/html/index.html

The first three and the last columns should be familiar to any (Arch) Linux user. The fourth column is new and has the format: 

user:role:type[:level]

To explain:

  1. User: The SELinux user identity. This can be associated to one or more roles that the SELinux user is allowed to use.
  2. Role: The SELinux role. This can be associated to one or more types the SELinux user is allowed to access.
  3. Type: When a type is associated with a process, it defines what processes (or domains) the SELinux user (the subject) can access. When a type is associated with an object, it defines what access permissions the SELinux user has to that object.
  4. Level: This optional field can also be know as a range and is only present if the policy supports MCS or MLS.

This is important in case you wish to understand how to build your own policies, for these are the basic building blocks of SELinux. However, for most purposes, there is no need to, for the reference policy is sufficiently mature. However, if you are a power user or someone with very specific needs, then it might be ideal for you to learn how to make your own SELinux policies.

This[dead link 2021-07-12 ⓘ] is a great series of articles for someone seeking to understand how to work with SELinux.

Troubleshooting

The place to look for SELinux errors is the systemd journal. In order to see SELinux messages related to the label system_u:system_r:policykit_t:s0 (for example), you would need to run: 

# journalctl _SELINUX_CONTEXT=system_u:system_r:policykit_t:s0

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
chcon
Change the context on a specific file

Reporting issues

Please report issues on GitHub: https://github.com/archlinuxhardened/selinux/issues

See also