ipc/sem: Fix dangling sem_array access in semtimedop race

When __do_semtimedop() goes to sleep because it has to wait for a
semaphore value becoming zero or becoming bigger than some threshold, it
links the on-stack sem_queue to the sem_array, then goes to sleep
without holding a reference on the sem_array.

When __do_semtimedop() comes back out of sleep, one of two things must
happen:

 a) We prove that the on-stack sem_queue has been disconnected from the
    (possibly freed) sem_array, making it safe to return from the stack
    frame that the sem_queue exists in.

 b) We stabilize our reference to the sem_array, lock the sem_array, and
    detach the sem_queue from the sem_array ourselves.

sem_array has RCU lifetime, so for case (b), the reference can be
stabilized inside an RCU read-side critical section by locklessly
checking whether the sem_queue is still connected to the sem_array.

However, the current code does the lockless check on sem_queue before
starting an RCU read-side critical section, so the result of the
lockless check immediately becomes useless.

Fix it by doing rcu_read_lock() before the lockless check.  Now RCU
ensures that if we observe the object being on our queue, the object
can't be freed until rcu_read_unlock().

This bug is only hittable on kernel builds with full preemption support
(either CONFIG_PREEMPT or PREEMPT_DYNAMIC with preempt=full).

Fixes: 370b262c89 ("ipc/sem: avoid idr tree lookup for interrupted semop")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/sem.c

@@ -2179,14 +2179,15 @@ long __do_semtimedop(int semid, struct sembuf *sops,
 		 * scenarios where we were awakened externally, during the
 		 * window between wake_q_add() and wake_up_q().
 		 */
+		rcu_read_lock();
 		error = READ_ONCE(queue.status);
 		if (error != -EINTR) {
 			/* see SEM_BARRIER_2 for purpose/pairing */
 			smp_acquire__after_ctrl_dep();
+			rcu_read_unlock();
 			goto out;
 		}
 
-		rcu_read_lock();
 		locknum = sem_lock(sma, sops, nsops);
 
 		if (!ipc_valid_object(&sma->sem_perm))