Using CUSTOM_BUILDOPT in refpolicy for USE flag-alike functionality?


Sven Vermeulen Fri 16 August 2013

As you are probably aware, Gentoo uses the reference policy as its base for SELinux policies. Yes, we do customize it and not everything is already pushed upstream (for instance, our approach to use xdg_*_home_t customizable types to further restrict user application access has been sent up for comments a few times but we still need to iron it out further) but all in all, we're pretty close to the upstream releases. This is also visible when there are changes upstream as we very easily integrate them back in our repository.

But there are still a few things that I want to implement further, and one of these things is perhaps too specific for Gentoo but would benefit us (security-wise) in great detail: enabling domain privileges based on USE flags. Allow me to quickly use an example to make this more tangible.

Consider the MPlayer application. As a media application, it of course offers support for ALSA and PulseAudio (amongst other things). In the SELinux policy, support for (and thus privileges related to) ALSA and PulseAudio is handled through optional_policy statements:


This means that the rules defined in pulseaudio_tmpfs_content are executed if the dependencies match:

                attribute pulseaudio_tmpfsfile;

        typeattribute $1 pulseaudio_tmpfsfile;

If the pulseaudio_tmpfsfile attribute exists, then the mplayer_tmpfs_t type gets the pulseaudio_tmpfsfile attribute assigned to it.

This is flexible, because if the server/workstation does not use PulseAudio, then in Gentoo, no pulseaudio SELinux module will be loaded and thus the attribute will not exist. However, Gentoo tries to be a bit more flexible in this - it is very well possible to have PulseAudio installed, but disable PulseAudio support in MPlayer (build mplayer with USE="-pulseaudio"). The current definitions in the policy do not support this flexibility: if the pulseaudio module is loaded, then the privileges become active.

One way SELinux supports a more flexible approach is to use conditionals in the policy. One could create booleans that administrators can toggle to enable / disable SELinux rules. For instance, in the mplayer policy:

        allow mencoder_t self:process { execmem execstack };

If an administrator toggles the allow_mplayer_execstack boolean to "on", then the mentioned allow rule becomes active.

Sadly, this approach is not fully usable for USE driven decisions. Not all rules can be enclosed in tunable_policy statements, and assigning attributes to a type is one of them (cfr our pulseaudio example). A recent discussion on the reference policy mailinglist gave me two ideas to investigate:

  • See if we can support CIL (a new language for SELinux policy definitions) where such an approach would be easier
  • Use build-time decisions

In this post, I want to go through the build-time decisions idea. The reference policy supports build-time options using ifdef constructs. These look at parameters provided by the build system (M4/Makefile based) to see if rules need to be activated or not. For type attribute declarations, this is a valid approach. So one idea would be to transform USE flags, if they are set, into use_${USEFLAG}, and make decisions based on this in the policy code:


We can add in the USE flags, if set, through the CUSTOM_BUILDOPT parameter that the reference policy provides. So introducing this is not that difficult. The only thing I'm currently a bit weary about is the impact on the policy files themselves (which is why I haven't done this already) and the fact that USE flags on the "real" package are not know to policy packages. In other words, if a user explicitly marks USE="-pulseaudio" on mplayer, but has USE="pulseaudio" set as general value, then the selinux-mplayer package will still have pulseaudio enabled.

Still, I do like the idea. It would make it more consistent with what Gentoo aims to do: if you do not want a certain support/feature in the code, then why would the policy still have to allow it? With the proper documentation towards administrators, I think this would be a good approach.