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authorJann Horn <jannh@google.com>2022-12-05 17:59:27 +0100
committerLinus Torvalds <torvalds@linux-foundation.org>2022-12-05 10:54:44 -0800
commitb52be557e24c47286738276121177a41f54e3b83 (patch)
tree52f3f9d59b50122fb86e964d2f7d9cf167f9caca
parent76dcd734eca23168cb008912c0f69ff408905235 (diff)
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>
-rw-r--r--ipc/sem.c3
1 files changed, 2 insertions, 1 deletions
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))