CentOS 8
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SELinux : SELinux Context2019/09/28

Access Controls to files or directories are controled by additional informations that 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.
[targeted] Policy that 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 -lZ /root

total 4
-rw-------. 1 root root system_u:object_r:admin_home_t:s0 1507 Sep 24 17:11 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     0:02 /usr/lib/systemd/syst
system_u:system_r:kernel_t:s0       2 ?        S      0:00 [kthreadd]
system_u:system_r:kernel_t:s0       3 ?        I<     0:00 [rcu_gp]
system_u:system_r:kernel_t:s0       4 ?        I<     0:00 [rcu_par_gp]
system_u:system_r:kernel_t:s0       5 ?        I      0:00 [kworker/0:0-memcg_km
system_u:system_r:tuned_t:s0      836 ?        Ssl    0:00 /usr/libexec/platform
system_u:system_r:crond_t:s0-s0:c0.c1023 842 ? Ss     0:00 /usr/sbin/atd -f
system_u:system_r:crond_t:s0-s0:c0.c1023 844 ? Ss     0:00 /usr/sbin/crond -n
system_u:system_r:getty_t:s0-s0:c0.c1023 850 tty1 Ss+   0:00 /sbin/agetty -o -p
system_u:system_r:local_login_t:s0-s0:c0.c1023 851 ? Ss   0:00 login -- root

# own ID

[root@dlp ~]#
id -Z

[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       *

* if [semanage] command does not exist, install it

[root@dlp ~]#
dnf -y install policycoreutils-python-utils
For the example above (RHEL/CentOS Default), [root] is mapped to [unconfined_u]. Other common users are mapped to [__default__] once and finally mapped to [unconfined_u].
[unconfined_u] users are assigned [unconfined_t] Role, and Processes which are started by [unconfined_u] users are run as [unconfined_t] Domain.
Processes that [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 1304 ? Ss   0:00 /usr/lib/systemd/systemd --user
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 1315 ttyS0 Ss   0:00 -bash
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 1359 ttyS0 R+   0:00 ps axZ
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 1360 ttyS0 S+   0:00 grep --color=auto unconfined_t
Matched Content