The patch below does not apply to the 5.4-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 stable@vger.kernel.org.
Possible dependencies:
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From b52be557e24c47286738276121177a41f54e3b83 Mon Sep 17 00:00:00 2001 From: Jann Horn jannh@google.com 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).
Fixes: 370b262c896e ("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
diff --git a/ipc/sem.c b/ipc/sem.c index c8496f98b139..00f88aa01ac5 100644 --- a/ipc/sem.c +++ b/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))