[PATCH -next] nbd: get config_lock before sock_shutdown

[Zhong Jinghua <zhongjinghua@xxxxxxxxxxxxxxx>]

Config->socks in sock_shutdown may trigger a UAF problem.
The reason is that sock_shutdown does not hold the config_lock,
so that nbd_ioctl can release config->socks at this time.

T0: NBD_SET_SOCK
T1: NBD_DO_IT

T0						T1

nbd_ioctl
  mutex_lock(&nbd->config_lock)
  // get lock
  __nbd_ioctl
    nbd_start_device_ioctl
      nbd_start_device
       mutex_unlock(&nbd->config_lock)
         // relase lock
         wait_event_interruptible
         (kill, enter sock_shutdown)
         sock_shutdown
					nbd_ioctl
					  mutex_lock(&nbd->config_lock)
					  // get lock
					  __nbd_ioctl
					    nbd_add_socket
					      krealloc
						kfree(p)
					        //config->socks is NULL
           nbd_sock *nsock = config->socks // error

Fix it by moving config_lock up before sock_shutdown.

Signed-off-by: Zhong Jinghua <zhongjinghua@xxxxxxxxxxxxxxx>
---
 drivers/block/nbd.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index c410cf29fb0c..accbe99ebb7e 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1428,13 +1428,18 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd)
 	mutex_unlock(&nbd->config_lock);
 	ret = wait_event_interruptible(config->recv_wq,
 					 atomic_read(&config->recv_threads) == 0);
+
+	/*
+	 * recv_work in flush_workqueue will not get this lock, because nbd_open
+	 * will hold nbd->config_refs
+	 */
+	mutex_lock(&nbd->config_lock);
 	if (ret) {
 		sock_shutdown(nbd);
 		nbd_clear_que(nbd);
 	}
 
 	flush_workqueue(nbd->recv_workq);
-	mutex_lock(&nbd->config_lock);
 	nbd_bdev_reset(nbd);
 	/* user requested, ignore socket errors */
 	if (test_bit(NBD_RT_DISCONNECT_REQUESTED, &config->runtime_flags))
-- 
2.31.1