Messages in this thread | | | Date | Tue, 29 Jun 2010 09:58:15 -0700 | From | Darren Hart <> | Subject | Re: [PATCH] futex: futex_find_get_task make credentials check conditional |
| |
On 06/29/2010 09:41 AM, Linus Torvalds wrote: > On Tue, Jun 29, 2010 at 1:42 AM, Michal Hocko<mhocko@suse.cz> wrote: >> >> futex_find_get_task is currently used (through lookup_pi_state) from two >> contexts, futex_requeue and futex_lock_pi_atomic. While credentials check >> makes sense in the first code path, the second one is more problematic >> because this check requires that the PI lock holder (pid parameter) has >> the same uid and euid as the process's euid which is trying to lock the >> same futex (current). > > So exactly why does it make sense to check the credentials in the > first code path then? Shouldn't the futex issue in the end depend on > whether you have a shared page or not - and not on credentials at all? > Any two processes that share a futex in the same shared page should be > able to use that without any regard for whether they are the same > user. That's kind of the point, no?
I agree and haven't been able to come up with a need for the test either, but I wanted to hear back from Ingo as the he authored the original check.
I was trying to see if futex_lock_pi() could somehow be abused, but if so, I don't see it:
TaskUserA TaskUserB futex_lock_pi(addrA) *addrB = TID_OF(TaskUserA) futex_lock_pi(addrB)
TaskUserB would lookup the pi_state, not find it as addrB and addrA don't hash to the same key, create a new pi_state and mark TaskUserA as the owner, then block.
Once TaskUserA exits, the pi_list will contain the pi_state for the addrB futex. This is "bad", but the kernel cleans it up, releases the lock - but doesn't wake TaskUserB. That seems acceptable to me since TaskUserB is in the wrong here.
> IOW, I personally dislike these kinds of conditional checks, > especially since the discussion (at least the part I've seen) hasn't > made it clear why it should be conditional - or exist - in the first > place. > > So I'd like the patch to include an explanation of exactly why the two > cases are different.
Agreed, waiting on Ingo at the moment.
> The other thing I'd like to see is to move the whole cred checking up > a level. There's no reason to check the credentials in > futex_find_get_task() that I can see - why not do it in the caller > instead? IOW, I think futex_find_get_task() should just look something > like this instead:
/me beats head on desk, duh. Still, I'm hoping this isn't necessary and we can lose the credentials checking entirely.
Thanks,
-- Darren Hart IBM Linux Technology Center Real-Time Linux Team
| |