fuse: fix uring race condition for null dereference of fc

There is a race condition leading to a kernel crash from a null
dereference when attemping to access fc->lock in
fuse_uring_create_queue(). fc may be NULL in the case where another
thread is creating the uring in fuse_uring_create() and has set
fc->ring but has not yet set ring->fc when fuse_uring_create_queue()
reads ring->fc. There is another race condition as well where in
fuse_uring_register(), ring->nr_queues may still be 0 and not yet set
to the new value when we compare qid against it.

This fix sets fc->ring only after ring->fc and ring->nr_queues have been
set, which guarantees now that ring->fc is a proper pointer when any
queues are created and ring->nr_queues reflects the right number of
queues if ring is not NULL. We must use smp_store_release() and
smp_load_acquire() semantics to ensure the ordering will remain correct
where fc->ring is assigned only after ring->fc and ring->nr_queues have
been assigned.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://lore.kernel.org/r/20250318003028.3330599-1-joannelkoong@gmail.com
Fixes: 24fe962 ("fuse: {io-uring} Handle SQEs - register commands")
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: joannekoong

fs/fuse/dev_uring.c

@@ -208,11 +208,11 @@ static struct fuse_ring *fuse_uring_create(struct fuse_conn *fc)
 
 	init_waitqueue_head(&ring->stop_waitq);
 
-	fc->ring = ring;
 	ring->nr_queues = nr_queues;
 	ring->fc = fc;
 	ring->max_payload_sz = max_payload_size;
 	atomic_set(&ring->queue_refs, 0);
+	smp_store_release(&fc->ring, ring);
 
 	spin_unlock(&fc->lock);
 	return ring;
@@ -1041,7 +1041,7 @@ static int fuse_uring_register(struct io_uring_cmd *cmd,
 			       unsigned int issue_flags, struct fuse_conn *fc)
 {
 	const struct fuse_uring_cmd_req *cmd_req = io_uring_sqe_cmd(cmd->sqe);
-	struct fuse_ring *ring = fc->ring;
+	struct fuse_ring *ring = smp_load_acquire(&fc->ring);
 	struct fuse_ring_queue *queue;
 	struct fuse_ring_ent *ent;
 	int err;