Install SELinux on Ubuntu 22.04

According to the official Security Enhanced Linux project page, SELinux is a security enhancement to Linux. Linux-based security-sensitive projects largely standardize on it. Ubuntu 22.04 is compatible with SELinux and these instructions make it available on your Ubuntu 22.04 host. The steps in this guide appear as command line instructions. Both physical and virtual machines can be configured for SELinux, but it is not possible to enable SELinux in a Docker container without additional steps not covered here.

1. If you have not already done so, create a Linode account and Compute Instance. See our Getting Started with Linode and Creating a Compute Instance guides.

2. Follow our Setting Up and Securing a Compute Instance guide to update your system. You may also wish to set the timezone, configure your hostname, create a limited user account, and harden SSH access.

Note

The steps in this guide require root privileges. Be sure to run the steps below as root or with the sudo prefix. For more information on privileges, see our Users and Groups guide.

SELinux’s technical basis is access control, meaning how different users can and cannot read, write, update, remove, or otherwise change different resources, and how administrators manage those differences. Over twenty years ago, SELinux introduced tools to enhance conventional Linux so these administrative chores are now less complex and more reliable. SELinux is currently implemented as a Mandatory Access Control (MAC) module within the kernel.

A standard modern Ubuntu distribution includes AppArmor, a Linux application security system which emphasizes ease-of-use and routine reliability. Both AppArmor and SELinux work through the Linux Security Module (LSM) interface. Since Linux only permits a single LSM to be active, the first step in an SELinux installation is to deactivate AppArmor.

Caution

SELinux alters parts of Linux profoundly. An error in its installation can easily render an entire host unresponsive. Make backups, be prepared to dispose or recycle a particular instance, and work with care. Start your SELinux experiments in permissive mode, and make backups again before any switch to enforcing mode. The simplest SELinux installations are somewhat time-consuming, as they affect the entire filesystem. Each reboot takes a while, since SELinux methodically confirms the state of all filesystems and other resources.

1. It’s good practice to begin any Linux installation work by ensuring a consistent package state with:

   sudo apt update && sudo apt upgrade -y
   

2. Your Ubuntu 22.04 installation probably runs AppArmor by default. To verify its status, request:

   systemctl status apparmor
   

   There are several lines of output, including “… enabled … SUCCESS …”.

3. Press Q to close the status info.

   Note

   A few Ubuntu 22.04 variants for embedded computing do not run AppArmor. Therefore, if you see “… apparmor.service could not be found …” you can safely skip the next paragraphs and go immediately to the “Install SELinux” section below.

4. In the more common case, where AppArmor is running, stop it:

   sudo systemctl stop apparmor
   

5. Now disable AppArmor to prevent it from re-enabling:

   sudo systemctl disable apparmor
   

It is not necessary, or even desirable, to remove AppArmor. Most administrators leave it installed, but stopped and disabled.

1. Install SELinux:

   sudo apt install policycoreutils selinux-basics selinux-utils -y
   

2. Enable SELinux:

   sudo selinux-activate
   

   Now you see:

   SELinux is activated. You may need to reboot now.

Do not reboot immediately! First, review the current state of your new SELinux host.

1. To confirm the current status of your installation, use the command:

   getenforce
   

   This shows the one-word response:

   Disabled

This means that your SELinux is ready to work. It’s “active” but not yet turned on.

While getenforce provides the current state of SELinux, sestatus is a different command that provides more details.

2. When you enter:

   sestatus
   

   You see:

   SELinux status:		disabled

While this output is similar to getenforce, once SELinux is enabled, sestatus reports more fully on the configuration, as detailed later.

SELinux can be enabled in one of two states: permissive or enforcing. Your current SELinux installation remains disabled.

Caution

If you connect to your host via SSH, access will be lost once SELinux is enabled. If you’re using a Linode host, you can still login via the LISH console.

1. Reboot, and the Ubuntu 22.04 host likely comes up with SELinux “on”:

   sudo reboot
   

   Note

   The first reboot with SELinux enabled begins a relabelling process that could take a long time, so be patient.

2. Verify this through examination of /etc/selinux/config:

   cat /etc/selinux/config
   

   It should include the line:

   SELINUX=permissive

The presence of /etc/selinux/config is a sign that the host is ready for configuration and the reboot needed to make most configurations effective:

• Installation creates /etc/selinux/config.
• Configuration updates /etc/selinux/config.
• A reboot puts SELinux into action.

SELinux has several options beyond the scope of this guide. Configuration is commonly achieved through configuration files rather than graphical user interface (GUI) or command line applications.

Basic configuration of an SELinux installation starts with the SELINUX attribute in /etc/selinux/config.

1. Open /etc/selinux/config:

   sudo nano /etc/selinux/config
   

2. Enable SELinux by changing the line:

   SELINUX=permissive
   

   to

   SELINUX=enforcing
   

3. Press CTRL+X to exit nano.

4. Press Y to confirm.

5. Press ENTER to save.

6. The next time the host is rebooted, it comes up as an enforcing SELinux instance:

   sudo reboot
   

7. Verify this again via /etc/selinux/config:

   cat /etc/selinux/config
   

   It should include the line:

   SELINUX=enforcing

/etc/selinux/config controls how an SELinux instance launches. However, it’s possible to adjust the action of SELinux between reboots with setenforce.

1. Temporarily put SELinux in permissive mode with:

   sudo setenforce 0
   

2. Check the current status of SELinux with:

   getenforce
   

   This should now display:

   Permissive

3. Now check the status of SELinux again using sestatus

   sestatus
   

   You not only see the current SELinux mode, but also the config file (boot) mode, and other information:

   SELinux status:                 enabled
   SELinuxfs mount:                /sys/fs/selinux
   SELinux root directory:         /etc/selinux
   Loaded policy name:             default
   Current mode:                   permissive
   Mode from config file:          enforcing
   Policy MLS status:              enabled
   Policy deny_unknown status:     allowed
   Memory protection checking:     requested (insecure)
   Max kernel policy version:      33

4. Switch SELinux back from permissive to enforcing mode with:

   sudo setenforce 1
   

5. Check the current status of SELinux again:

   getenforce
   

   This should now display:

   Enforcing

6. Confirm the status again using sestatus

   sestatus
   

   You can see that setenforce only changes the current mode:

   SELinux status:                 enabled
   SELinuxfs mount:                /sys/fs/selinux
   SELinux root directory:         /etc/selinux
   Loaded policy name:             default
   Current mode:                   enforcing
   Mode from config file:          enforcing
   Policy MLS status:              enabled
   Policy deny_unknown status:     allowed
   Memory protection checking:     requested (insecure)
   Max kernel policy version:      33

Other possible values for getenforce output are Permissive and Disabled. These values are capitalized, unlike the values permitted within /etc/selinux/config.

There are two distinct ways to disable SELinux:

• Have SELinux immediately stop its enforcement of resource permissions, and continue in the same way until its next reboot, at which point SELinux returns to enforcement.
• Stop SELinux enforcement permanently, so that SELinux is not in effect after each boot operation.

It’s common to speak of SELinux’s permissive operation as “disabled”. This first of the two descriptions may be confusing, because, although SELinux is disabled, and users can’t see its action, it remains active. When SELinux runs as Permissive, it logs access violations, but doesn’t enforce them. When SELinux is enabled as enforcing, the only effective way to disable it fully so that it doesn’t log its observations or otherwise act in the background, requires updates of /etc/selinux/config and sudo reboot.

Fewer than a couple dozen commands are required to install, enable, and activate SELinux on Ubuntu 22.04. You can quickly have an enforcing SELinux, but refer to other references for details on how to configure your SELinux to achieve specific useful requirements.