SELinux Policies

Understanding SELinux policies

  • Check the active policy on the system using sestatus:
 # sestatus | grep
"Loaded policy name" 
  • Get
    an overview of SELinux booleans using the semanage command with the boolean
# semanage boolean -l |
grep policyload
secure_mode_policyload   (off, off)
  • Use getsebool for same purpose:
# getsebool
secure_mode_policyload --> off
  • If
    the name of the boolean is not exactly known, get booleans using filter :
# getsebool -a | grep
secure_mode_policyload --> off
  • Use
    the sepolicy
    booleans command:
 # sepolicy booleans -b secure_mode_policyload

This command does
not show the current value of the boolean.

  • Navigate
    /sys/fs/selinux file system to fetch
    the value of a boolean:
# cat /sys/fs/selinux/booleans/secure_mode_policyload
  • Change the value of a boolean using the setsebool command:
# setsebool
httpd_can_sendmail on
  • In order to keep the changes permanently, add the -P option to setsebool:
# setsebool -P httpd_can_sendmail
  • Another
    way to change and persist the boolean settings is to use semanage boolean:
# semanage boolean -m --on

Inspecting the impact of a boolean

  • To show information in detail, we use the -b option (for the boolean), -A option (show allow rules), and -C option (to show conditional rules):
[root@localhost ~]#
sesearch -b httpd_can_sendmail -AC
Found 50 semantic av rules:
DT allow httpd_sys_script_t bin_t : lnk_file { read
getattr } ; [ httpd_can_sendmail ]
DT allow mta_user_agent httpd_suexec_t : fd use ; [
httpd_can_sendmail ]

DT - state of the boolean in the policy
(first character) and when the SELinux rule is enabled (second character).

  • Check the rules applicable between the web server domain (httpd_t) and user content type (user_home_t):
[root@localhost ~]# sesearch -s httpd_t -t user_home_t
Found 9 semantic av rules:   
allow daemon user_home_t : file { getattr
append } ;   
allow httpd_t file_type : filesystem getattr
DT allow httpd_t user_home_type : dir {
getattr search open } ; [ httpd_read_user_content ] 

  • List currently loaded SELinux policy modules:
# semodule -l
abrt       1.4.1
accountsd  1.1.0
  • Modules can be loaded with a higher priority, overriding previous modules, or with lower priority:
# semodule
400 also        pp
400 android     pp
  • Generate SELinux policy allow rules by piping the denials through the audit2allow application (investigate audit.log to find a policy denial before attempting this command. This is sample output):
# grep setkey
/var/log/audit/audit.log | audit2allow
#============= setkey_t ==============
allow setkey_t newrole_t:fd use;
allow setkey_t var_t:dir search;
  • Based on the denials, two allow rules are prepared. We can also ask audit2allow to create a SELinux module:
# grep setkey
/var/log/audit/audit.log | audit2allow -M localpolicy
********** IMPORTANT **********
To make this policy package active, execute:
semodule -i localpolicy.pp
September 21, 2019

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