ipc/sem.c: fix incorrect sem_lock pairing

Based on the syzcaller test case from dvyukov:

  https://gist.githubusercontent.com/dvyukov/d0e5efefe4d7d6daed829f5c3ca26a40/raw/08d0a261fe3c987bed04fbf267e08ba04bd533ea/gistfile1.txt

The slow (i.e.: failure to acquire) syscall exit from semtimedop()
incorrectly assumed that the the same lock is acquired as it was at the
initial syscall entry.

This is wrong:
 - thread A: single semop semop(), sleeps
 - thread B: multi semop semop(), sleeps
 - thread A: woken up by signal/timeout

With this sequence, the initial sem_lock() call locks the per-semaphore
spinlock, and it is unlocked with sem_unlock().  The call at the syscall
return locks the global spinlock.  Because locknum is not updated, the
following sem_unlock() call unlocks the per-semaphore spinlock, which is
actually not locked.

The fix is trivial: Use the return value from sem_lock.

Fixes: 370b262c89 ("ipc/sem: avoid idr tree lookup for interrupted semop")
Link: http://lkml.kernel.org/r/1482215645-22328-1-git-send-email-manfred@colorfullife.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Johanna Abrahamsson <johanna@mjao.org>
Tested-by: Johanna Abrahamsson <johanna@mjao.org>
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/sem.c

@@ -1977,7 +1977,7 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
 		}
 
 		rcu_read_lock();
-		sem_lock(sma, sops, nsops);
+		locknum = sem_lock(sma, sops, nsops);
 
 		if (!ipc_valid_object(&sma->sem_perm))
 			goto out_unlock_free;