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: dvyu...@google.com
Signed-off-by: Manfred Spraul <manf...@colorfullife.com>
Fixes: 370b262c896e ("ipc/sem: avoid idr tree lookup for interrupted semop")
Cc: d...@stgolabs.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
