Getting Started with SELinux

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Getting Started with SELinux

SELinux is a Mandatory Access Control (MAC) system, developed by the NSA. SELinux was developed as a replacement for Discretionary Access Control (DAC) that ships with most Linux distributions.

The difference between Discretionary Access Controls and Mandatory Access Controls is the means by which users and applications gain access to machines. Traditionally, the command sudo gives a user the ability to heighten permissions to root-level. Root access on a DAC system gives the person or program with root access permission to perform as desired on a machine.

Ideally, the person with root access should be trusted with it. But if security has been compromised, so too has the system. SELinux and MACs resolve this issue by both confining privileged processes and automating security policy creation.

SELinux defaults to denying anything that is not explicitly allowed. SELinux has global modes, permissive and enforcing. Permissive mode allows the system to function like a Discretionary Access Control system, while logging every violation to SELinux. The enforcing mode enforces a strict denial of access to anything that isn’t explicitly allowed. To explicitly allow certain behavior on a machine, you, as the system administrator, have to write policies that allow it.

We do not recommend you disable SELinux. But if you wish to disable SELinux, please read our quick-answer guide on SELinux

Before You Begin

  1. This guide requires you to OWN the box you are going to use. SELinux is a security-control system; a small misconfiguration could cause your system to be compromised.
  2. Linode uses a custom kernel by default. This kernel does not support SELinux. If you are using a Linode, switch to a distribution-supplied kernel by using this guide: Run a non-custom kernel
  3. Update your system:

    yum update

Install SELinux

On CentOS 7 most of the SELinux packages are installed by default. Look to see what packages are installed:

rpm -aq | grep selinux

If you are dealing with a freshly installed CentOS 7 Linode, your output should be:

[root@centos ~]# rpm -aq | grep selinux

Install the following packages and their associated dependencies:

yum install policycoreutils policycoreutils-python selinux-policy selinux-policy-targeted libselinux-utils setools setools-console

Optionally, install setroubleshoot-server and mctrans. The setroubleshoot-server allows, among many other things, for email notifications to be sent from the server to notify you of any policy violations. The mctrans daemon translates the output of SELinux to human readable text.

SELinux Modes

SELinux has two modes: Enforcing and Permissive:

  • Enforcing: In Enforcing mode, SELinux enforces strict policies on the system. Things that are not allowed, will not be allowed to run under any circumstance.
  • Permissive: In Permissive mode, your system is not protected by SELinux; instead, SELinux just records any violation to a log file.

You can check what mode your system is in by running the getenforce command:

[root@centos ~ ]# getenforce

You can also retrieve even more information using sestatus:

[root@centos ~]# sestatus
SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             targeted
Current mode:                   enforcing
Mode from config file:          permissive
Policy MLS status:              enabled
Policy deny_unknown status:     allowed
Max kernel policy version:      28

You have to set SELinux to permissive, so that you can create policies on your system for SELinux to enforce. After changing SELinux’s mode, you have to reboot your system.

[root@centos ~]# setenforce 0
[root@centos ~]# getenforce
[root@centos ~]# reboot

Now that SELinux is set to Permissive, you can see the log of privacy violations by using:

grep "selinux" /var/log/messages

The output will look very similar to this:

[root@centos ~]# grep "selinux" /var/log/messages
Jun 26 12:27:16 li482-93 yum[4572]: Updated: selinux-policy-3.13.1-102.el7_3.16.noarch
Jun 26 12:27:38 li482-93 yum[4572]: Updated: selinux-policy-targeted-3.13.1-102.el7_3.16.noarch
Jun 26 16:38:15 li482-93 systemd: Removed slice system-selinux\x2dpolicy\x2dmigrate\x2dlocal\x2dchanges.slice.
Jun 26 16:38:15 li482-93 systemd: Stopping system-selinux\x2dpolicy\x2dmigrate\x2dlocal\x2dchanges.slice.
Jun 26 16:54:46 li482-93 systemd: Removed slice system-selinux\x2dpolicy\x2dmigrate\x2dlocal\x2dchanges.slice.
Jun 26 16:54:46 li482-93 systemd: Stopping system-selinux\x2dpolicy\x2dmigrate\x2dlocal\x2dchanges.slice.
Jun 26 16:55:45 li482-93 kernel: EVM: security.selinux
Jun 26 17:33:43 li482-93 kernel: EVM: security.selinux
Jun 26 17:36:21 li482-93 kernel: EVM: security.selinux

The file that contains the security states of the system is located at /etc/selinux/config, you can edit that file to change the state of the system.

vi /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.
#     permissive - SELinux prints warnings instead of enforcing.
#     disabled - No SELinux policy is loaded.
# SELINUXTYPE= can take one of three two values:
#     targeted - Targeted processes are protected,
#     minimum - Modification of targeted policy. Only selected processes are protected.
#     mls - Multi Level Security protection.

The uncommented lines can be changed to any state. After changing the state of SELinux, reboot the machine for the changes to take effect.

SELinux Context

Before switching to the enforce state in SELinux, you have to understand contexts, as they pertain to SELinux.

[root@centos ~]# useradd user
[root@centos ~]# su user
[user@centos ~]$ cd ~/ && mkdir test
[user@centos ~]$ ls -Z
drwxrwxr-x. user user unconfined_u:object_r:user_home_t:s0 test

The output of ls -Z may look familiar, but the -Z context flag prints out the SELinux security context of any file.

SELinux marks every single object on a machine with a context. That means every file, daemon, and process has a context, according to SELinux. The context is broken into three parts: user, role and type. In SELinux, a policy controls which users can get which roles. Each specific role places a constraint on what type of files that user can enter. When a user logs in to a system, a role is assigned, which can be seen in the ls -Z example above: the output unconfined_u is a user role.

SELinux Boolean

An SELinux Boolean is a variable that can be toggled on and off without needing to reload or recompile an SELinux policy. You can view the list of boolean variables using the getsebool -a command. It’s a long list, so you can pipe it through grep to narrow down the results:

[root@centos ~]# getsebool -a | grep xdm
xdm_bind_vnc_tcp_port --> off
xdm_exec_bootloader --> off
xdm_sysadm_login --> off
xdm_write_home --> off

You can change the value of any variable using the setsebool command. If you set the -P flag, the setting will persist through reboots. If you want to permit a service like openVPN to run unconfined in your system, you have to edit the policies boolean variable:

[root@centos ~]# getsebool -a  | grep "vpn"
openvpn_can_network_connect --> on
openvpn_enable_homedirs --> on
openvpn_run_unconfined --> off

[root@centos ~]# setsebool -P openvpn_run_unconfined ON

[root@centos ~]# getsebool -a  | grep "vpn"
openvpn_can_network_connect --> on
openvpn_enable_homedirs --> on
openvpn_run_unconfined --> on

Now, you are able to use OpenVPN unconfined or in permissive mode on your system, even if it is actively in enforcing mode. Set your system to enforce, and let SELinux protect your system.

[root@centos ~]# setenforce 1
[root@centos ~]# getenforce

Next Steps

SELinux is complicated. Please see the links under More Information to gather a deeper understanding of the subject. SELinux can play a critical role in system administration and security, especially once it is mastered.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

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This guide is published under a CC BY-ND 4.0 license.