linux-kernel - [PATCH v2] ipc/sem.c: fix incorrect sem

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Date:   Tue, 20 Dec 2016 07:34:05 +0100
From:   Manfred Spraul <manfred@...orfullife.com>
To:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Davidlohr Bueso <dave@...olabs.net>
Cc:     1vier1@....de, dvyukov@...gle.com, mingo@...nel.org,
        peterz@...radead.org, fabf@...net.be, kernel@...p.com,
        syzkaller@...glegroups.com,
        Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH v2] 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.

Reported-by: dvyukov@...gle.com
Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
Fixes: 370b262c896e ("ipc/sem: avoid idr tree lookup for interrupted semop")
Cc: dave@...olabs.net

---

Patch V2:
- subject line updated
- documentation slightly updated

 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