ipc/sem: Fix dangling sem_array access in semtimedop race
[ Upstream commit b52be557e24c47286738276121177a41f54e3b83 ]
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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
diff --git a/ipc/sem.c b/ipc/sem.c
index 2bf535d..cc6af85 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -2148,6 +2148,7 @@
* 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) {
/*
@@ -2157,10 +2158,10 @@
* overwritten by the previous owner of the semaphore.
*/
smp_mb();
+ rcu_read_unlock();
goto out_free;
}
- rcu_read_lock();
locknum = sem_lock(sma, sops, nsops);
if (!ipc_valid_object(&sma->sem_perm))
