[PATCH] nbd: don't allow reconnect after disconnect

[Yu Kuai]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

Following process can cause nbd_config UAF:

1) grab nbd_config temporarily;

2) nbd_genl_disconnect() flush all recv_work() and release the
initial reference:

  nbd_genl_disconnect
   nbd_disconnect_and_put
    nbd_disconnect
     flush_workqueue(nbd->recv_workq)
    if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF, ...))
     nbd_config_put
     -> due to step 1), reference is still not zero

3) nbd_genl_reconfigure() queue recv_work() again;

  nbd_genl_reconfigure
   config = nbd_get_config_unlocked(nbd)
   if (!config)
   -> succeed
   if (!test_bit(NBD_RT_BOUND, ...))
   -> succeed
   nbd_reconnect_socket
    queue_work(nbd->recv_workq, &args->work)

4) step 1) release the reference;

5) Finially, recv_work() will trigger UAF:

  recv_work
   nbd_config_put(nbd)
   -> nbd_config is freed
   atomic_dec(&config->recv_threads)
   -> UAF

Fix the problem by clearing NBD_RT_BOUND in nbd_genl_disconnect(), so
that nbd_genl_reconfigure() will fail.

Fixes: b7aa3d39385d ("nbd: add a reconfigure netlink command")
Reported-by: syzbot+6b0df248918b92c33e6a@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/675bfb65.050a0220.1a2d0d.0006.GAE@xxxxxxxxxx/
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 drivers/block/nbd.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index b1a5af69a66d..259bd57fc529 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -2179,6 +2179,7 @@ static void nbd_disconnect_and_put(struct nbd_device *nbd)
 	flush_workqueue(nbd->recv_workq);
 	nbd_clear_que(nbd);
 	nbd->task_setup = NULL;
+	clear_bit(NBD_RT_BOUND, &nbd->config->runtime_flags);
 	mutex_unlock(&nbd->config_lock);
 
 	if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF,
-- 
2.39.2