Upgrading SE Linux Policy

When I first packaged the SE Linux policy for Debian the only way to adjust the policy was to edit the source files and recompile. Often changes that you might desire involved changing macros so while it would have been theoretically possible to just tack a few type definitions and allow rules at the end, you often wanted to change a macro to have a change apply all through the policy. To deal with that I had the policy source installed on all machines and the package update policy would compile it into a binary form and load it into the kernel.

Now there was the issue of merging user changes with changes from a new policy package. For most configuration files on a Unix system you can just leave any files that are modified by the user, not taking the new default configuration might cause the user to miss out on some new features – but presumably they were happy with the way it worked in the past. However due to inter-dependencies this wasn’t possible for SE Linux, if one file was not ungraded due to user changes and other files related to it were then the result could be a compile failure.

Another issue was the fact that a newer version of the policy might permit operations that the sys-admin did not desire and therefore not meet their security goals, or it might not permit operations that are essential to the operation of the system and interrupt service.

To solve this I wrote a script that prompted for upgrades to policy source files and allowed the sys-admin to choose which files to upgrade. This worked reasonably well in the early days when the number of files was small. But as the policy increased in size it became increasingly painful to upgrade the policy with as many as 100 questions being asked.

The solution to this (as implemented in Fedora Core 5, Debian/Etch, and newer distributions) was to have binary policy modules that maintain their dependencies. Now there are binary policy modules which can be loaded at will (the default install for Debian only installs modules that match the installed programs) and the modules can have optional sections with dependencies. So if you remove a module that defines a domain and there are other modules which have rules to allow communication with that domain then the optional sections of policy in the other modules is disabled when the domain becomes undefined. This solves the technical issues related to module inter-dependencies but the issue of intent and interaction with the rest of the system remains.

With Red Hat distributions the solution has been to upgrade the running policy every time the policy package is upgraded and be cautious when changing policy. They do a good job of the upgrade process (including relabeling files when the file contexts change) and in terms of policy changes I have not heard complaints from users about that. Users who don’t want a newer version of the policy can always put the package on hold.

For the Debian distribution after Lenny I plan to have a policy upgrade process that relabels files and a debconf question as to whether package upgrades should upgrade the policy. But for Lenny the freeze is already in progress so it seems to late to make such changes. Instead I’m going to upload a new version of the selinux-basics package with a program named selinux-policy-upgrade that will upgrade all the policy modules that are in use. This is not the ideal solution, but I think it will keep Lenny users reasonably happy.

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