[PATCH] ipc/sem.c: fix semop()/semop() locking failure

[Manfred Spraul <manfred@colorfullife.com>]

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, the call at the syscall return locks
the global spinlock.

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

Reported-by: dvyukov@google.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Fixes: 370b262c896e ("ipc/sem: avoid idr tree lookup for interrupted semop")
Cc: dave@stgolabs.net
---
 ipc/sem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index e08b948..3ec5742 100644
--- a/ipc/sem.c
+++ b/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;
-- 
2.7.4