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CIL – Part2: Module priorities

July 31, 2015

In my previous blog, I talked about CIL performance improvements. In this blog post, I would like to introduce another cool feature called module priorities. If you check the link, you can see a nice blog post published by Petr Lautrbach about this new feature.

With new SELinux userspace, we are able to use priorities for SELinux policy modules. It means you can ship own ipa policy module, which is based on distribution policy module, with additional rules and load it with higher priority. No more different names for policy modules and higher priority wins.

# semodule --list-modules=full | grep ipa
400 ipa pp
100 ipa pp

Of course, you can always say you want to use distro policy module and add just additional fixes. Yes, it works fine for some cases when you add just minor fixes which you are not able to get to distro policy for some reasons. Actually you can also package this local policy how Lukas Vrabec wrote in his blog.

Another way how to deal with this case is a fact you can ship SELinux policy for your application at all and don’t be a part of distro policy. Yes, we can see these cases.

For example

# semodule --list-modules=full | grep docker
400 docker pp

But what are disadvantages with this way?

* you need to know how to write SELinux policy
* you need to maintain this policy and reflect the latest distro policy changes
* you need to do “hacks” in your policies if you need to have some interfaces for types shipped by distro policy
* you would get your policy to upstream and check if there is no conflict with distribution policy if they do a merge with the same upstream

From Fedora/RHEL point of view, this was always a problem how to deal with policies for big projects like Cluster, Gluster, OpenShift and so on. We tried to get these policies out of distro policy but it was really hard to do a correct rpm handling and then we faced my above mentioned points.

So is there any easy way how to deal with it? Yes, it is. We ships a policy for a project in our distribution policy and this project takes this policy, adds additional fixes, creates pull requests against distribution policy and if there will be different timelines then it will be shipped by this project. And that’s it! It can be done easily using module priorities.

For example, we have Gluster policy in Fedora by default.

# semodule --list-modules=full | grep gluster
100 glusterd pp

And now, Gluster team needs to do a new release but it causes some SELinux issues. Gluster folks can take distribution policy, add additional rules and package it.

Then we will see something like

# semodule --list-modules=full | grep gluster
100 glusterd pp
400 glusterd pp

In the mean time, Gluster folks can do pull requests with all changes against disitribution policy and they can still ship the same policy. The Gluster policy is a part of distribution policy, it can be easily usptream-able and moreover, it can be disabled in distribution policy by default.

# semodule --list-modules=full | grep gluster
400 gluster cil
100 glusterd pp disabled

$ matchpathcon /usr/sbin/glusterfsd
/usr/sbin/glusterfsd system_u:object_r:glusterd_exec_t:s0

This model is really fantastic and give us answers for lot of issues.

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