In the Linux kernel, the following vulnerability has been resolved:

nbd: don't allow reconnect after disconnect

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.

https://git.kernel.org/stable/c/844b8cdc681612ff24df62cdefddeab5772fadf1

https://git.kernel.org/stable/c/9793bd5ae4bdbdb2dde401a3cab94a6bfd05e302

https://git.kernel.org/stable/c/a8ee6ecde2b7bfb58c8a3afe8a9d2b848f580739

https://git.kernel.org/stable/c/d208d2c52b652913b5eefc8ca434b0d6b757f68f

https://git.kernel.org/stable/c/e7343fa33751cb07c1c56b666bf37cfca357130e