security_task_getsecid() and subjective vs objective task creds
paul at paul-moore.com
Thu Feb 18 19:34:30 UTC 2021
When looking into a problem I noticed that audit was recording the
wrong subject label for a process. Doing a bit of digging I realized
this was caused by the SELinux security_task_getsecid() implementation
returning the objective security label (taken from task->real_cred),
and not the subjective security label (taken from task->cred).
Looking around at the other LSMs which implement this hook, Smack and
AppArmor, it appears they both do the same thing: return the objective
security ID for the process. Looking quickly at the various non-LSM
callers of the security_task_getsecid() hook, it unfortunately looks
like all of them expect the subjective security ID to be returned.
The only caller I'm not 100% confident in is binder, but from what I
can tell it looks like they are expecting the subjective ID too.
At least we are consistently wrong :)
How do we want to fix this? The obvious fix is to change the SELinux,
AppArmor, and Smack security_task_getsecid() implementations to return
the subjective security ID (->cred), and likely make a note in
lsm_hooks.h, but if someone can see a case where we would need both
the subjective and objective security labels speak up and we can
introduce a new hook for the subjective label, and likely add a "_obj"
to the end of the existing hook to help make it more clear. If
neither of those options are acceptable, we could convert all of the
existing callers to use something like the line below (assumes
current), but that is the least appealing option as far as I'm
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