CentOS 7
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SELinux : SELinux Context
2016/03/27
 
Access Controls to files or directories are controled by additional informations which is called SELinux Context.
SELinux Context has following syntax.
⇒ [SELinux User]:[Role]:[Type]:[Level]
SELinux User :
SELinux User Attribute
Each Linux User is mapped to an SELinux User by SELinux Policy.
Role :
RBAC (Role Based Access Control) Attribute
It defines SELinux User's Roles.
It controls Accesses which defined role can access to Domains by SELinux Policy.
Type :
TE (Type Enforcement) Attribute
It defines Domains for processes, and also defines Types for Files.
Level :
MLS (Multi Level Security) and MCS (Multi Category Security) Attribute
Level has [sensitivity]:[category] syntax.
By the way, "targeted" Policy which is the default Policy on RHEL/CentOS forces MCS and it is used only "s0" sensitivity on the Policy.
But for Category, it is supported c0-c1023.
MLS forces the Bell-La Padula Mandatory Access Model.
If you'd like to use it, it needs to install MLS Policy package on RHEL/CentOS.
But it does not support X Window System, so it's impossible to use it on Desktop Environment.
[1] For displaying SELinux Contexts for files or processes, add "Z" option to commands.
# files/directories

[root@dlp ~]#
ls -Z /root 

-rw-------. root root system_u:object_r:admin_home_t:s0     anaconda-ks.cfg
                      --------:--------:------------:------
                       User   : Role   : Type       : Level
# processes

[root@dlp ~]#
ps axZ 

LABEL                             PID TTY      STAT   TIME COMMAND
system_u:system_r:init_t:s0         1 ?        Ss     0:01 /usr/lib/systemd/syst
system_u:system_r:kernel_t:s0       2 ?        S      0:00 [kthreadd]
system_u:system_r:kernel_t:s0       3 ?        S      0:00 [ksoftirqd/0]
.....
.....
system_u:system_r:postfix_master_t:s0 916 ?    Ss     0:00 /usr/libexec/postfix/
system_u:system_r:postfix_pickup_t:s0 917 ?    S      0:00 pickup -l -t unix -u
system_u:system_r:postfix_qmgr_t:s0 918 ?      S      0:00 qmgr -l -t unix -u
system_u:system_r:kernel_t:s0     941 ?        S<     0:00 [kworker/1:1H]
system_u:system_r:kernel_t:s0     966 ?        S<     0:00 [kworker/0:1H]
system_u:system_r:kernel_t:s0    1246 ?        S<     0:00 [kworker/0:2H]
# own ID

[root@dlp ~]#
id -Z 

unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[2] Each Linux User is mapped to an SELinux User by SELinux Policy.
It's possible to show the mapping list like follows.
[root@dlp ~]#
semanage login -l 


Login Name           SELinux User         MLS/MCS Range        Service

__default__          unconfined_u         s0-s0:c0.c1023       *
root                 unconfined_u         s0-s0:c0.c1023       *
system_u             system_u             s0-s0:c0.c1023       *
* if semanage command does not exist, install like follows

[root@dlp ~]#
yum -y install policycoreutils-python
 
For the example above (RHEL/CentOS Default), "root" is mapped to "unconfined_u". System users like "bin" or "daemon" and others are mapped to "system_u". Other common users are mapped to "__default__" once and finally mapped to "unconfined_u".
"unconfined_u" users are assigned "unconfined_r" Role, and Processes which are started by "unconfined_u" users are run as "unconfined_t" Domain.
Processes which "unconfined_t" Domain are assigned are not controled by SELinux.
[root@dlp ~]#
ps axZ | grep unconfined_t 

unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 1435 ttyS0 Ss   0:00 -bash
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 1556 ttyS0 R+   0:00 ps axZ
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 1557 ttyS0 S+   0:00 grep --color=auto unconfined_t
 
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