[PATCH] futex: futex_find_get_task remove credentails check

[Michal Hocko]

Hi stable team,
could you consider including the following patch (Linus tree commit:
7a0ea09ad5352efce8fe79ed853150449903b9f5).

The original discussion which led to this commit can be found at
http://lkml.org/lkml/2010/6/23/52.

In short:
The original pi locking implementation (since it got to the kernel)
contains a credential check (in futex_find_get_task) if we want to
create a PI state for already held lock. This test fails if the lock
owner has a different (e)uid than the process for which we want to
create the state.
The lock operation then fails with ESRCH which is the error code 
which is returned if the process holding a lock doesn't exist. 
The userspace (glibc) doesn't expect this behavior for shared robust PI
futexes and fails with an assert or hang the task in the end-less loop.

The test case is attached in the referenced thread.

The credential test, which is removed by this patch, doesn't look
correct and it limits the functionality without any good reason. There
are no security consequences as well because the only thing that should
matter in the shared futex-es is accessibility to the shared memory.

The patch applies as is on top of Vanilla 2.6.32, but let me know if you
want to base it on top of the any of the stable trees.


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