Confining user applications


Sven Vermeulen Sun 16 January 2011

Ever since I started using SELinux, I'm getting more and more fond of what it can do for (security) administrators. Lately, I've started confining user applications (like skype) in the idea that I do not want any application connecting to the Internet or working with content received from untrusted sources to work inside the main user domain (user_t or staff_t in my case). This particular exercise has been quite interesting, not only to learn more on SELinux, not only to get acquainted with the reference policy which Gentoo basis its policies upon. No, it's been interesting because you learn how applications work underneith...

Take the skype application for example. Little did I know it read stuff from my firefox configuration (like the sec8.db and prefs.js file), most likely to see if the skype firefox plugin is installed. With SELinux, I saw that it did all that - and also denied it. But it isn't easy to find out why an application behaves as it does. After all, these aren't questions that average joe asks. It also isn't easy to deduce if you want to allow it or not. If it was purely for my own system, I wouldn't hesitate for long, but the idea is that the modules should work for the majority of people - and who knows, perhaps even be included in the reference policy in the future.

Perhaps Gentoo Hardened can write up some rules on the SELinux policies and how they should be made for the distribution. Do we want to deny as much as possible, only allowing those things developers can safely verify need to be allowed? Or do we want to allow everything that the application already does (but nothing more) so that no AVC denials are shown anymore? And if Gentoo Hardened chooses "deny as much as possible", do we configure the policy to not audit those things we don't think we need (hiding it) or do we expect the security administrator to manage his own dontaudit rules? Well, guess I'll ask the hardened folks and see what they think ;-)

During the quest, I'll try to update the Gentoo Hardened SELinux handbook draft. It's far from finished, but should be usable for most interested parties. If you're interested in SELinux and want to give it a try with Gentoo Hardened, this might be the document you are looking for.