Extending SELinux policies with additional rules is easy. As SELinux uses a deny by default approach, all you need to do is to create a policy module that contains the additional (allow) rules, load that and you're all set. But what if you want to remove some rules?
Well, sadly, SELinux does not support deny rules. Once an allow rule is loaded in memory, it cannot be overturned anymore. Yes, you can disable the module itself that provides the rules, but you cannot selectively disable rules. So what to do?
Generally, you can disable the module that contains the rules you want
to disable, and load a custom module that defines everything the
original module did, except for those rules you don't like. For
instance, if you do not want the
skype_t domain to be able to
read/write to the video device, create your own skype-providing module
(myskype) with the exact same content (except for the module name at
the first line) as the original skype module, except for the video
dev_read_sound(skype_t) # dev_read_video_dev(skype_t) dev_write_sound(skype_t) # dev_write_video_dev(skype_t)
Load in this policy, and you now have the
skype_t domain without the
video access. You will get post-install failures when Gentoo pushes out
an update to the policy though, since it will attempt to reload the
skype.pp file (through the
selinux-skype package) and fail because
it declares types and attributes already provided (by myskype). You
can exclude the
from being updated, which works as long as no packages depend on it. Or
live with the post-install failure ;-) But there might be a simpler
Recently, I added in support for epatch_user in the policy ebuilds.
This allows users to create patches against the policy source code that
we use and put them in
/etc/portage/patches in the directory of the
right category/package. For module patches, the working directory used
is within the
policy/modules directory of the policy checkout. For
base, it is below the policy checkout (in other words, the patch will
need to use the
refpolicy/ directory base). But because of how
epatch_user works, any patch taken from the base will work as it will
start stripping directories up to the fourth one.
This approach is also needed if you want to exclude rules from
interfaces rather than from the
.te file: create a small patch and put
/etc/portage/patches for the
(as this provides the interfaces).