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    SubjectRe: [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


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