FAILED: patch "[PATCH] ipc/sem: Fix dangling sem_array access in semtimedop race" failed to apply to 4.14-stable tree

The patch below does not apply to the 4.14-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From b52be557e24c47286738276121177a41f54e3b83 Mon Sep 17 00:00:00 2001
From: Jann Horn <[email protected]>
Date: Mon, 5 Dec 2022 17:59:27 +0100
Subject: [PATCH] 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).

index c8496f98b139..00f88aa01ac5 100644
@@ -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))