Selinux

This document is written for RHEL based systems

helper tools#

• setools-console CLI tool collection
• setroubleshoot-server
• setools GUI collection
• setroubleshoot GUI collection
• policycoreutils-devel

switch selinux on#

# show status
$ sestatus
  ...
  SELinux status:                 enabled
  ...
  Current mode:                   permissive
  Mode from config file:          permissive

# check config file
> cat /etc/selinux/config
  ...
  SELINUX=permissive
# switch to 'enforcing' and reboot system

# check status again
$ sestatus
  ...
  SELinux status:                 enabled
  ...
  Current mode:                   enforcing
  Mode from config file:          enforcing

NOTE: When switching from disabled to enforcing, the whole filesystem needs to be relabels’d first. Reason for this is that the folders and files created in disabled mode arn’t labeled yet.

# paste an empty file
> touch /.autorelabel

# or use dedicated cmd, which will creating the file
> fixfiles -F onboot

# reboot system

policy types#

Find policy-type settings in /etc/selinux/config

...
SELINUXTYPE=targeted

Default policy on Almalinux 9 is default. To change the policy-type we need to ensure first that the target policy-file is already installed. In our case the mls - Multi Level Security protection package:

# install mls policy-file
> dnf -y install selinux-policy-mls

# this will install the 'mls' files in the selinux config folder
> ls -1t /etc/selinux/
  ...
  mls
  config
  targeted
  semanage.conf

Change SELINUXTYPE= to mls, relabel filesystem and reboot the host.

NOTE: ensure that your SELINUX= mode is set to permissive

Additional Notes

• The default RHEL/AlmaLinux policy forces MCS with only s0 sensitivity.
• MLS enforces the Bell-La Padula Mandatory Access Model.
• No desktop support when using MLS due to missing X-server support.

selinux context#

# show file context using '-Z' option
$ ls -Z /etc/os-release
  ...
  system_u:object_r:etc_t:s0    /etc/os-release

The ‘Z’ option can be set for many programs

# show process context using 'Z' option
$ ps axZ
  ...
  system_u:system_r:kernel_t:s0         2 ?        S      0:00 [kthreadd]

Some usual SELinux contexts

booleans#

By using SELinux booleans, you can fine-tune your system’s security configuration.

# show booleans, filter 'http'
$ getsebool -a | grep http
  ...
  httpd_anon_write --> off
  httpd_builtin_scripting --> on
  httpd_can_check_spam --> off
  httpd_can_connect_ftp --> off

# install 'semanage'
> dnf install policycoreutils-python-utils

# use 'semanage' to describe booleans
$ semanage boolean -l
  ...
  httpd_anon_write         (off  ,  off)  Allow httpd to anon write
  httpd_builtin_scripting  (on   ,   on)  Allow httpd to builtin scripting
  httpd_can_check_spam     (off  ,  off)  Allow httpd to can check spam
  httpd_can_connect_ftp    (off  ,  off)  Allow httpd to can connect ftp

Changing a boolean

We want to allow the boolean httpd_can_network_connect

# get current status
> getsebool httpd_can_network_connect
  ...
  httpd_can_network_connect --> off

# switch boolean off using 'setsebool'
> setsebool -P httpd_can_network_connect on

# or switch boolean off using 'semanage'
> semanage boolean -m httpd_can_network_connect --on

# get status
> getsebool selinuxuser_ping
  ...
  httpd_can_network_connect --> on

file-context handling#

First, all file context’s are stored in /etc/selinux/targeted/contexts/files/*

To change a file context we can use chcon or semanage. But using these commands requires a restorecon command as well to save the change.

Lets imagin you saved a certificate file on the server by transfering it to your user directory and move it to /etc/pki/tls/certs.

The filecontext will looks like this: unconfined_u:object_r:user_home_t:s0

This file-context is not allowed to be used as certicicate, we need to add at least the cert_t context.

chcon#

• semanage(8)

# change file context
> chcon -t cert_t /etc/pki/tls/certs/foo.crt

# save state
> restorecon /etc/pki/tls/certs/foo.crt

# change full-context
> chcon \
  -u system_u \
  -r object_r \
  -t cert_t \
  /etc/pki/tls/certs/foo.crt

# copy context from reference file
> chcon --reference=/foo /bar

semanage#

• restorecon(8)

# change file context
> semanage fcontext -a -t cert_t /etc/pki/tls/certs/foo.crt

# when changing the context a second time, switch '-a' to '-m'
> semanage fcontext -m -t cert_t /etc/pki/tls/certs/foo.crt

port handling#

# print services and ports, filter for 'http_port'
> semanage port -l | grep http_port
  ...
  http_port_t  tcp  80, 81, 443, 488, 8008, 8009, 8443, 9000

# add port to 'http_port'
> semanage port -a -t http_port_t -p tcp 7474

# print services and ports, filter for '7474'
> semanage port -l | grep 7474
  ...
  http_port_t  tcp  7474, 3000, 80, 81, 443, 488, 8008, 8009, 8443, 9000

log analysis#

Trigger AVC

AVC stands for Access Vector Cache.

In SELinux, AVC uses predefined rules called “tecs” (Type Enforcement Policies) to determine whether an access request is allowed or denied.

SELinux’s AVC checks if the request matches any of the predefined rules. If it does, the access is allowed; if not, it’s denied.

# install httpd
> dnf install httpd

# stop webserver
> systemctl stop httpd

# switch httpd listen port to 7575
> sed -i 's/Listen\ 80/Listen\ 7575/g' \
  /etc/httpd/conf/httpd.conf

# start webserver
> systemctl start httpd
  ...
  Job for httpd.service failed ...

log files#

• /var/log/audit/audit.log
• /var/log/messages

debug commands#

allow rules#

To allow denied actions we can builf our own policy to spefically allowing them.

generate policy#

To generate a rules created from the audit alerts we can use package policycoreutils-devel This package holds the sepolicy command which can help to perform the initial steps.

See more details in the RHEL documentation:

• Quick start to write a custom SELinux policy

manually build#

terms

.te type enforcement file: A plain text file contains SELinux policy rules.

.mod module file: A binary modul generated from the .te file.

.pp policy package file: The final policy to apply generated from the .mod file.

# define the target eg. `ssh`
> p=ssh

# View AVC denials and pipe them through `audit2allow -l`
> grep $p /var/log/audit/audit.log |\
    audit2allow -l

# Same as above but generating the `.te` type enforcement file.
> grep $p /var/log/audit/audit.log |\
    audit2allow -l -v -m local_$p > local_$p.te

# Build a binary module `.mod` from the type enforcement file.
> checkmodule -M -m -o local_$p.mod local_$p.te

# Build the `.pp` policy package from the `.mod` file
> semodule_package -o local_$p.pp -m local_$p.mod

# Install the new `.pp` policy module
> semodule -v -i local_$p.pp

Sometimes multiple iterations of starting the application and running the policy commands are required to get the final result.

There is already a script available.

policy tools#

• sediff Compare two policies to find differences.
• seinfo List policy components.
• sesearch Search rules (allow, type_transition, etc.)

command explanation#

disable selinux#

beside of switching the mode in /etc/selinux/config to disable its also possible to modify boot parameters:

# disbale selinux and reboot
> grubby --update-kernel ALL --args selinux=0


# re-enable selinux and reboot
> grubby --update-kernel ALL --remove-args selinux