Friday, 12 October 2007

SELinux: unusable from a newbie perspective

Thanks Russell for the explanations on the execmem bits.

Now I am trying to go further and set up my system to really work with SELinux enabled because, although the promise of the targeted policy is to allow you to do your job mostly as you did before, that "mostly" has a really wide meaning, more than you'd think you bargained for.

Examples from my laptop: hald does not start by default (various denials), resolvconf is denied some getattr operations on tmpfs, hald-addon-dell-backlight is denied access to some character device I just know it should have access to, etc. Of course, this means that automatic mounting does not work anymore and there is a decrease in usability just because of the "mostly" part.

I am sure that some of the denials are correct (see the execstack stuff or the memexec), but there are cases where this "mostly" is stretched way too much. IMHO, desktop installs should suffer no restrictions when using the targeted policy (and I mean "no restrictions that would make my system less than it was before enabling and enforcing SELinux").

But let me tell a reason why SELinux sucks without any help from others:

The interface sucks big time.

Probably there is are good reasons, but if you need to create and use a new policy based on the denials found in the logs, you need to use no less than 4 (four) different tools with the right incantation (although you can use just 2, if you don't want to customize the rules[1]):
  • audit2allow -m local -l
  • checkmodule -M -m -o local.mod local.te
  • semodule_package -o local.pp -m local.mod
  • semodule -i local.pp
In the end, you still hit the bad interface:

bounty:~# semodule -i local.pp
libsepol.check_assertion_helper: assertion on line 0 violated by allow hald_t memory_device_t:chr_file { read };
libsepol.check_assertions: 1 assertion violations occured
libsemanage.semanage_expand_sandbox: Expand module failed
semodule: Failed!

Now what? That error message doesn't tell me much, except that some assertion failed. But where does the assertion come from? Why is it an assertion? What is bad about that line?
To me that really looks like an internal error or an inconsistency in what the SELinux tools generate.

[1] still, why advertise the longer path as the Fedora FAQ does? I would have done it the other way around.

1 comment:

n4th4nr1ch said...

Agreed. Disable. 99.9999999999999% of all systems don't need selinux. Ever.