March 05, 2025

HPR4328: Use SELinux the easy way

24 minutes

This show has been flagged as Clean by the host.

The most basic security toggle on your Linux computer is the

setenforce command. Using just a single setenforce instruction, you can configure SELinux to allow a violation it would normally prevent. There are two states: Enabled and Permissive. By default, SELinux is Enabled (also represented as 1 when using Boolean values). To set SELinux to permissive mode:

$ sudo setenforce Permissive

When something works in Permissive mode, you've successfully identified the symptom, but you haven't fixed the problem yet. Activate Enforcing mode again:

$ sudo setenforce Enforcing

Check the status of SELinux

You can check the state of SELinux at any time using the

sestatus command:

$ sestatus

SELinux status: enabled

SELinuxfs mount: /sys/fs/selinux

SELinux root directory: /etc/selinux

Loaded policy name: targeted

Current mode: enforcing

[...]

Look at labels and contexts

If you have a running Linux system, then you have an example of what SELinux requires for normal operation. You don't have to learn about security contexts or memorize labels. For most anything you try to do on your computer, there are likely already files doing something similar. Use those files as templates.

You can look at the security labels of any file you have access to by using the

-Z (that's a capital Z) option of ls:

$ touch hellotouch hello.txt

$ ls -Z hello.txt

unconfined_u:object_r:user_home_t:s0 hello

An empty file created by a user in the user's own home directory has, as you might expect, a very specific security profile. Even with the executable bit set, that file would not be permitted to run as a systemwide service. It just doesn't have the correct security context.

If you use an

ll alias, try adding the -Z option to its option list so you get used to seeing SELinux labels. The more you see what labels exist on your system, and how they relate to various system roles, you're more likely to recognize when they're wrong.

Copy contexts

Suppose you were developing a custom SELinux service for your laptop. You've written a shell script, a service file, and you've placed them in the appropriate system locations. You're also careful to set ownership and permissions correctly. But no matter what you do, you get errors when attempting to start the service.

You suspect that SELinux might be preventing an unrecognized service from running. That would normally be appreciated, but in this case you want to make an exception.

First, confirm that the service runs successfully with SELinux in Permissive mode:

$ sudo setenforce Permissive

$ sestatus | grep Current

Current mode: permissive

$ sudo systemctl start hello.service || echo "fail"

$ sudo setenforce Enforcing

Then look at the files you've created using the

-Z and compare them with other files that you know to be working properly. Note the differences:

$ ls -Z /usr/lib/systemd/system/hello.service

unconfined_u:object_r:systemd_unit_file_t:s0

$ ls -Z /usr/lib/systemd/system/rdisc.service

system_u:object_r:rdisc_unit_file_t:s0

The working service (

rdisc.service in this example, chosen at random) features the system_u label as well as a special rdisc_unit_file_t label. Suppose you know from previous experience with ls -Z that a common SELinux label for systemd service files is systemd_unit_file_t so you ignore that difference. However, unconfined_u and system_u seem to be important.

Use the

chcon ("change context") command to change the security context of your service file:

$ sudo chcon system_u:object_r:systemd_unit_file_t:s0 \

/usr/lib/systemd/system/hello.service

$ ls -Z /usr/lib/systemd/system/hello.service

system_u:object_r:systemd_unit_file_t:s0

Your systemd service is probably triggering some executable file on your system. If you created that yourself, it probably also has the incorrect security context. Comparing it to a known working script:

$ ls -Z /usr/bin/example.sh

unconfined_u:object_r:gconf_home_t:s0

$ ls -Z /usr/bin/brltty-prologue.sh

system_u:object_r:bin_t:s0

Again, there's one obvious difference, which you can correct with

chcon:

$ chcon system_u:object_r:bin_t:s0 \

/usr/bin/example.sh

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March 05, 2025

HPR4328: Use SELinux the easy way

24 minutes

This show has been flagged as Clean by the host.

The most basic security toggle on your Linux computer is the

$ sudo setenforce Permissive

When something works in Permissive mode, you've successfully identified the symptom, but you haven't fixed the problem yet. Activate Enforcing mode again:

$ sudo setenforce Enforcing

Check the status of SELinux

You can check the state of SELinux at any time using the

sestatus command:

$ sestatus

SELinux status: enabled

SELinuxfs mount: /sys/fs/selinux

SELinux root directory: /etc/selinux

Loaded policy name: targeted

Current mode: enforcing

[...]

Look at labels and contexts

You can look at the security labels of any file you have access to by using the

-Z (that's a capital Z) option of ls:

$ touch hellotouch hello.txt

$ ls -Z hello.txt

unconfined_u:object_r:user_home_t:s0 hello

If you use an

Copy contexts

You suspect that SELinux might be preventing an unrecognized service from running. That would normally be appreciated, but in this case you want to make an exception.

First, confirm that the service runs successfully with SELinux in Permissive mode:

$ sudo setenforce Permissive

$ sestatus | grep Current

Current mode: permissive

$ sudo systemctl start hello.service || echo "fail"

$ sudo setenforce Enforcing

Then look at the files you've created using the

-Z and compare them with other files that you know to be working properly. Note the differences:

$ ls -Z /usr/lib/systemd/system/hello.service

unconfined_u:object_r:systemd_unit_file_t:s0

$ ls -Z /usr/lib/systemd/system/rdisc.service

system_u:object_r:rdisc_unit_file_t:s0