UPSTREAM: mqueue: fix a use-after-free in sys_mq_notify()

The retry logic for netlink_attachskb() inside sys_mq_notify()
is nasty and vulnerable:

1) The sock refcnt is already released when retry is needed
2) The fd is controllable by user-space because we already
   release the file refcnt

so we when retry but the fd has been just closed by user-space
during this small window, we end up calling netlink_detachskb()
on the error path which releases the sock again, later when
the user-space closes this socket a use-after-free could be
triggered.

Setting 'sock' to NULL here should be sufficient to fix it.

Reported-by: GeneBlue <geneblue.mail@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrey Ulanov <andreyu@google.com>

(cherry-picked from commit f991af3daabaecff34684fd51fac80319d1baad1)

BUG=b:63722644
TEST=kernel compiles

Change-Id: I402633e3308eb80c40ae881b13b2353eece0db8b
Reviewed-on: https://chromium-review.googlesource.com/575443
Commit-Ready: Andrey Ulanov <andreyu@google.com>
Tested-by: Andrey Ulanov <andreyu@google.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
(cherry picked from commit 1ae3ecacc8e915b702ee85a29935a1e4594e2582)
Reviewed-on: https://chromium-review.googlesource.com/577927
Commit-Queue: Andrey Ulanov <andreyu@google.com>
1 file changed