Exercise 1.2 - SELinux Contexts

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Exercise Description

All processes and files are labeled with an SELinux context. The context contains information that SELinux needs to make control decisions. Contexts contain user, role, type, and level information,

Step 1: Examine a context

To get started, let’s check the context of a file in the ec2-user user’s home directory:

ls -Z ~ec2-user/.vimrc
-rw-r--r--. ec2-user ec2-user unconfined_u:object_r:user_home_t:s0 /home/ec2-user/.vimrc

Following the standard UNIX permissions, user and group information, the SELinux context attached to the file is displayed. The fields are SELinux user (unconfined_u), role (object_r), type (user_home_t), and level (s0).

The SELinix user is not the same as the UNIX user, as it solely exists to associate a UNIX user to a set of roles. This allows UNIX users to be constrained by SELinux policy. In this case the unconfined_u user means that the user is mapped to the default SELinux login. This means that the user is allowed to launch any application that standard filesystem permissions allow. However, if that application has a defined domain transition, a confined domain will still apply to it.

Step 2: Domain transitions

To demonstrate the difference between confined and unconfined processes, let’s launch an application which doesn’t have a defined domain transition:

yes >/dev/null &
ps -Z | grep yes
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 29448 pts/0 00:00:07 yes

Here, you can see that the yes process is labled with the unconfined_t domain, indicating that it still has full root privileges, and can do whatever it wishes.

On the other hand, if we launch the passwd utility, we will see a different story:

kill %1
passwd ec2-user >/dev/null &
ps -Z | grep passwd
unconfined_u:unconfined_r:passwd_t:s0-s0:c0.c1023 29498 pts/0 00:00:00 passwd

[1]+  Stopped                 passwd ec2-user > /dev/null

See that the third field is passwd_t, indicating that the passwd process has been confined by the passwd_t domain.

Let’s kill that hanging passwd process, and move on:

kill %1

This changing of the context, when an application in SELinux’s ruleset is launched, is called a domain transition.

Step 3: Useful utility #1: restorecon

If I had to suggest to a new SELinux user the one thing that they should remember (no, it’s not setenforce 0!), it’d be restorecon. Restorecon will reset the SELinux context to what is defined in the system’s context database.

To try this out, let’s purposefully set the context incorrectly on our example SELinux AVC log:

chcon -t httpd_sys_content_t ./testaudit
ls -Z ./testaudit
-rw-------. root root system_u:object_r:httpd_sys_content_t:s0 ./testaudit

And we can change it back, magically, with restorecon. Note that this would have fixed the problem reported in our test AVC, by setting the context to httpd_sys_content_t:

restorecon ./testaudit 
ls -Z ./testaudit
-rw-------. root root system_u:object_r:admin_home_t:s0 ./testaudit

I also slipped in chcon, which you can use to temporarily change the context on a file.

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