In an administrator’s quest to get stuff done, SELinux tends to get in the way. It is being perceived as a nuisance rather than a feature and this happens mostly for only one reason: setting correct SELinux labels requires the ability to figure out the appropriate SELinux security contexts. Here is how to do it.
In its default configuration, SELinux already has a firm idea on what security contexts to apply when restoring security labels on your system. The simplest way to poke around the SELinux configuration involves using grep.
Restore the default SELinux labels
The command restorecon restores the security context to the system’s default based on the default SELinux labels for each location.
Find out the default SELinux labels for NGINX
To find out the default SELinux labels for various elements of an NGINX installation, use this command:
[root@host]$ grep nginx /etc/selinux/targeted/contexts/files/file_contexts
The output of this command shows default SELinux labels for NGINX installation directories:
/etc/nginx(/.*)? system_u:object_r:httpd_config_t:s0 /var/run/nginx.* system_u:object_r:httpd_var_run_t:s0 /var/lib/nginx(/.*)? system_u:object_r:httpd_var_lib_t:s0 /var/log/nginx(/.*)? system_u:object_r:httpd_log_t:s0 /var/opt/rh/rh-nginx18/log(/.*)? system_u:object_r:httpd_log_t:s0 /etc/opt/rh/rh-nginx18/nginx(/.*)? system_u:object_r:httpd_config_t:s0 /usr/lib/systemd/system/nginx.* -- system_u:object_r:httpd_unit_file_t:s0 /var/opt/rh/rh-nginx18/lib/nginx(/.*)? system_u:object_r:httpd_var_lib_t:s0 /var/opt/rh/rh-nginx18/run/nginx(/.*)? system_u:object_r:httpd_var_run_t:s0 /usr/sbin/nginx -- system_u:object_r:httpd_exec_t:s0
The default web directory (/etc/nginx/html), by default, is unconfined:
[root@ip-10-0-0-63 fedora]# ls -laZ /etc/nginx/html/ total 16 drwxr-xr-x. 2 root root unconfined_u:object_r:etc_t:s0 4096 Jun 13 00:48 . drwxr-xr-x. 7 root root unconfined_u:object_r:etc_t:s0 4096 Jun 18 12:28 .. -rw-r--r--. 1 root root unconfined_u:object_r:etc_t:s0 537 Jun 13 00:48 50x.html -rw-r--r--. 1 root root unconfined_u:object_r:etc_t:s0 612 Jun 13 00:48 index.html
Find out the default SELinux labels for a CMS such as WordPress
To find out the default SELinux security contexts for WordPress, use this command:
[root@host]$ grep wordpress /etc/selinux/targeted/contexts/files/file_contexts /usr/share/wordpress/.*\.php -- system_u:object_r:httpd_sys_script_exec_t:s0 /usr/share/wordpress/wp-includes/.*\.php -- system_u:object_r:httpd_sys_script_exec_t:s0 /usr/share/wordpress-mu/wp-content(/.*)? system_u:object_r:httpd_sys_rw_content_t:s0 /usr/share/wordpress/wp-content/uploads(/.*)? system_u:object_r:httpd_sys_rw_content_t:s0 /usr/share/wordpress/wp-content/upgrade(/.*)? system_u:object_r:httpd_sys_rw_content_t:s0 /usr/share/wordpress-mu/wp-config\.php -- system_u:object_r:httpd_sys_script_exec_t:s0
The output may reveal some very useful information, for example (on CentOS 7):
/usr/share/wordpress-mu/wp-config\.php -- system_u:object_r:httpd_sys_script_exec_t:s0
The above line indicates that the main configuration file of WordPress named wp-config.php should receive the SELinux security context label:
system_u:object_r:httpd_sys_script_exec_t:s0
This SELinux label will only be applied, however, if the file is located in /usr/share/wordpress-mu/, which is almost certainly not the case on your system.
Adjust SELinux security contexts
Before implementing permanent changes, it is advisable to try adjusting SELinux security labels on the affected directory tree and its contents temporarily. By doing so you can verify that your assumptions work and if not, restore the defaults (with restorecon) easily.
List current SELinux security contexts on files and directories
Listing current SELinux security contexts involves the command:
ls -laZ
The resulting output for an installation of WordPress in the web server document directory www.website1.tld may look something like this:
drwxr-x---. owner1 nginx unconfined_u:object_r:httpd_sys_content_t:s0 www.website1.tld
Adjust the SELinux security contexts without changing defaults
To (temporarily) adjust the SELinux security contexts for WordPress so that it can run:
chcon -vR system_u:object_r:httpd_sys_content_t:s0 www.website1.tld
For all .php scripts inside the WordPress installation directory and its subdirectories:
chcon -R -v system_u:object_r:httpd_sys_script_exec_t:s0 */*/*/*.php
WordPress will run with these settings, but it will fail to write updates, install themes, and plugins. (It almost makes you wonder why these settings are the defaults.)
In order to allow NGINX to write to the WordPress installation, adjust the security context on the entire WordPress installation and its subdirectories using this command:
chcon -R -v system_u:object_r:httpd_sys_rw_content_t:s0 www.website1.tld/
Save new default SELinux contexts
Make the desired changes permanent:
semanage fcontext -a -t httpd_sys_rw_content_t "/var/www/www.website1.tld(/.*)?"
Verify that your local contexts are correct:
cat /etc/selinux/targeted/contexts/files/file_contexts.local
You can add or remove default SELinux contexts as you see fit. To remove a context that is faulty or no longer needed:
semanage fcontext -d "/var/www/www.domainname.tld(/.*)?"
Restore SELinux default labels
Finally, to restore SELinux labels from the defaults, run:
restorecon -RFv /var/www/
on the webserver document directory (adjust the path as needed).