Updated readthedocs for SELinux policy

Signed-off-by: Michael Engel <mengel@redhat.com>

Makefile

@@ -5,7 +5,7 @@ BUILDDIR=builddir
 
 CODESPELL_PARAMS=\
 	-S Makefile,imgtype,copy,AUTHORS,bin,.git,CHANGELOG.md,changelog.txt,.cirrus.yml,"*.xz,*.gz,*.tar,*.tgz,*ico,*.png,*.1,*.5,*.orig,*.rej,*.xml,*xsl",build.ninja,intro-targets.json,./tests/tests/tier0/proxy-service-fails-on-typo-in-file/systemd/simple.service,tags,./builddir,./subprojects,\
-	-L keypair,flate,uint,iff,od,ERRO,crate\
+	-L keypair,flate,uint,iff,od,ERRO,crate,te \
 	--ignore-regex=".*codespell-ignore$$"
 
 build:

doc/docs/security/selinux.md

@@ -1,4 +1,5 @@
 <!-- markdownlint-disable-file MD013-->
+
 # BlueChi's SELinux policy
 
 BlueChi provides a custom SELinux policy, limiting access of the `bluechi-controller` and `bluechi-agent`. It can be installed via
@@ -42,3 +43,55 @@ semanage permissive -a bluechi_t
 # add the permissive property to bluechi-agent
 semanage permissive -a bluechi_agent_t
 ```
+
+## Allowing access to restricted units
+
+The SELinux policy of BlueChi allows it to manage all systemd units.
+
+However, when installing some applications and their respective systemd units, e.g. via `dnf`, there might also be additional SELinux Policies installed which prevent BlueChi from managing these. For example, when installing `httpd` on Fedora, it installs also the systemd unit `httpd.service` and the [policy module for apache](https://github.com/fedora-selinux/selinux-policy/blob/rawhide/policy/modules/contrib/apache.te). When trying to start the service via `bluechictl`, the policy will prevent certain operations:
+
+```bash
+# The apache policy prevents BlueChi from managing the httpd service
+$ bluechictl stop <node-name> httpd.service
+Failed to issue method call: SELinux policy denies access: Permission denied
+
+# However, using systemctl works as this is enabled by default in the apache policy
+$ systemctl stop httpd.service
+```
+
+In order to allow BlueChi to manage the `httpd.service`, the necessary allow rule(s) need to be added. The [audit2allow](https://man7.org/linux/man-pages/man1/audit2allow.1.html) tool can be used to generate these rules based on AVCs.
+
+First, run all operations with BlueChi that should be allowed. These will fail and create the AVCs used by `audit2allow`. Then use `audit2allow` to generate the allow rules and create the policy package (.pp) which can be installed via `semodule`. The following snippet shows an example for the httpd.service:
+
+```bash
+# generate AVC for the status operation
+$ bluechictl status <node-name> httpd.service
+Failed to issue method call: SELinux policy denies access: Permission denied
+
+# generate AVC for the stop operation
+$ bluechictl stop <node-name> httpd.service
+Failed to issue method call: SELinux policy denies access: Permission denied
+
+# view the type enforcement rule that allows the denied operations
+$ audit2allow -a
+#============= bluechi_agent_t ==============
+
+allow bluechi_agent_t httpd_unit_file_t:service { status stop };
+
+# create the policy package (.pp) and type enforcement file (.te)
+$ audit2allow -a -M httpd-allow
+******************** IMPORTANT ***********************
+To make this policy package active, execute:
+
+semodule -i httpd-allow.pp
+
+$ ls
+httpd-allow.pp httpd-allow.te
+
+# install policy package and run the previously prevented operation
+$ semodule -i httpd-allow.pp
+$ bluechictl status <node-name> httpd.service
+UNIT		    | LOADED	| ACTIVE	| SUBSTATE	| FREEZERSTATE	| ENABLED	|
+---------------------------------------------------------------------------------
+httpd.service	| loaded	| inactive	| dead		| running	    | disabled	|
+```