A Beginner's Guide to SELinux on CentOS 7

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 DAC and MAC is how 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 access to all programs and files on a system.

A person with root access should be a trusted party. 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 two global modes, permissive and enforcing. Permissive mode allows the system to function like a DAC system, while logging every violation to SELinux. The enforcing mode applies 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. This guide provides a brief and basic introduction to commonly used commands and practices for SELinux system administration.

1. Ensure that you have followed the Getting Started and Securing Your Server guides.

   Note

   This guide is written for a non-root user. Commands that require elevated privileges are prefixed with sudo. If you’re not familiar with the sudo command, you can check our Users and Groups guide.

2. Update your system:

   sudo yum update
   

   Note

   The Linode kernel does not support SELinux by default. However, all new Linodes running CentOS 7 use the distribution provided kernel, which has SELinux enabled by default.

   If your system is running a Linode kernel, you will need to change to an upstream kernel in order to use SELinux. See the How to Change Your Linode’s Kernel for more steps. Once you’re kernel is set to the upstream kernel, continue on with the steps in this guide.

In this section, you will install various SELinux packages that will help you when creating, managing, and analyzing SELinux policies.

1. Verify which SELinux packages are installed on your system:

    sudo rpm -aq | grep selinux
   

   A newly deployed CentOS 7 Linode should have the following packages installed:

   libselinux-2.5-14.1.el7.x86_64
   selinux-policy-3.13.1-252.el7_7.6.noarch
   selinux-policy-targeted-3.13.1-252.el7_7.6.noarch
   libselinux-utils-2.5-14.1.el7.x86_64
   libselinux-python-2.5-14.1.el7.x86_64
       

2. Install the following packages and their associated dependencies:

    sudo yum install policycoreutils policycoreutils-python setools setools-console setroubleshoot
   

   • policycoreuitls and policyoreutils-python contain several management tools to administer your SELinux environment and policies.
   • setools provides command line tools for working with SELinux policies. Some of these tools include, sediff which you can use to view differences between policies, seinfo a tool to view information about the components that make up SELinux policies, and sesearch used to search through your SELinux policies. setools-console consists of sediff, seinfo, and sesearch. You can issue the --help option after any of the listed tools in order to view more information about each one.
   • setroubleshoot suite of tools help you determine why a script or file may be blocked by SELinux.

   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 SELinux’s output to human readable text.

When SELinux is installed on your system, it can be either enabled or disabled. By default, the CentOS 7 image provided by Linode has SELinux in an enabled state.

• To disable SELinux, update your SELinux configuration file using the text editor of your choice. Set the SELINUX directive to disabled as shown in the example.

  
  
  
  
  
  
    
    
  
  
  
  
      
  
          
      
   
   
  
     File: /etc/selinux/config
   
   
   
  
    
  
  
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  # 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.
  SELINUX=disabled
  # SELINUXTYPE= can take one of three values:
  #     targeted - Targeted processes are protected,
  #     minimum - Modification of targeted policy. Only selected processes are protected.
  #     mls - Multi Level Security protection.
  SELINUXTYPE=targeted
        
  Note
  You can update the SELINUX directive with any of the available SELinux
  states or
  modes.

• Reboot your Linode for the changes to take effect:

    sudo reboot
  

• Connect to your Linode via SSH (replace 192.0.2.0 with your own Linode’s IP address) and verify your SELinux installation’s status:

    ssh example_user@192.0.2.0
    sudo sestatus
  

  Its output should display disabled

  SELinux status:                 disabled
      

When SELinux is enabled, it can run in either enforcing or permissive modes.

Note

If SELinux is currently disabled, update your SELinux configuration file with the SELINUX directive set to enabled, then reboot your system, and SSH back into your Linode. These steps are outlined in the SELinux States section of the guide.

• In enforcing mode, SELinux enforces its policies on your system and denies access based on those policies. Use the following command to view SELinux policy modules currently loaded into memory:

   sudo semodule -l
  

• Permissive mode does not enforce any of your SELinux policies, instead, it logs any actions that would have been denied to your /var/log/audit/audit.log file.

• You can check which mode your system is running by issuing the following command:

    sudo getenforce
  

• To place SELinux in permissive mode, use the following command:

    sudo setenforce 0
  

  Permissive mode is useful when configuring your system, because you and your system’s components can interact with your files, scripts, and programs without restriction. However, you can use audit logs and system messages to understand what would be restricted in enforcing mode. This will help you better construct the necessary policies for your system’s user’s and programs.

• Use the sealert utility to generate a report from your audit log. The log will include information about what SELinux is preventing and how to allow the action, if desired.

    sudo sealert -a /var/log/audit/audit.log
  

  The output resembles the example, however, it varies depending on the programs and configurations on your system. The example was generated using a Linode running the Apache webserver with a virtual hosts configuration.

  SELinux is preventing /usr/sbin/httpd from write access on the directory logs.
  
  *****  Plugin httpd_write_content (92.2 confidence) suggests   ***************
  
  If you want to allow httpd to have write access on the logs directory
  Then you need to change the label on 'logs'
  Do
  # semanage fcontext -a -t httpd_sys_rw_content_t 'logs'
  # restorecon -v 'logs'
      

• To allow /usr/sbin/httpd write access to the directory logs, as shown in the output, you can execute the suggested commands, semanage fcontext -a -t httpd_sys_rw_content_t 'logs' and restorecon -v 'logs'.

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

1. Create a directory in your home folder:

    mkdir ~/example_dir
   

2. Print the SELinux security context of your home folder’s directories and files:

    ls -Z ~/
   

   The output is similar to:

   drwxrwxr-x. example_user example_user unconfined_u:object_r:user_home_t:s0 example_dir
       

   The SELinux specific information is contained in the unconfined_u:object_r:user_home_t:s0 portion, which follows the following syntax: user:role:type:level. To learn more about users, roles, and related access control, see the CentOS SELinux documentation.

An SELinux Boolean is a variable that can be toggled on and off without needing to reload or recompile an SELinux policy.

1. You can view the list of Boolean variables using the getsebool -a command. Pipe the command through grep to narrow down your results.

    sudo getsebool -a | grep "httpd_can"
   

   You will see a similar output:

   httpd_can_check_spam --> off
   httpd_can_connect_ftp --> off
   httpd_can_connect_ldap --> off
   httpd_can_connect_mythtv --> off
   httpd_can_connect_zabbix --> off
   httpd_can_network_connect --> off
   httpd_can_network_connect_cobbler --> off
   httpd_can_network_connect_db --> off
   httpd_can_network_memcache --> off
   httpd_can_network_relay --> off
   httpd_can_sendmail --> 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, for example, you want to allow HTTPD scripts and modules to connect to the network, update the corresponding boolean variable.

    sudo setsebool -P httpd_can_network_connect ON
   

   When viewing a list of your boolean variables, you should now see that it is set to ON.

    sudo getsebool -a | grep "httpd_can"
   

   httpd_can_check_spam --> off
   httpd_can_connect_ftp --> off
   httpd_can_connect_ldap --> off
   httpd_can_connect_mythtv --> off
   httpd_can_connect_zabbix --> off
   httpd_can_network_connect --> on
   httpd_can_network_connect_cobbler --> off
   httpd_can_network_connect_db --> off
   httpd_can_network_memcache --> off
   httpd_can_network_relay --> off
   httpd_can_sendmail --> off
         

This guide provides a brief and basic introduction to SELinux administration. You can now take a deeper dive into SELinux by consulting some of the resources include in the More Information section of this guide.

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.

• Graphical Guide to Policies
• SELinux User Resources
• CentOS SELinux Wiki