nbd: fix null-ptr-dereference while accessing 'nbd->config'

Memory reordering may occur in nbd_genl_connect(), causing config_refs
to be set to 1 while nbd->config is still empty. Opening nbd at this
time will cause null-ptr-dereference.

   T1                      T2
   nbd_open
    nbd_get_config_unlocked
                 	   nbd_genl_connect
                 	    nbd_alloc_and_init_config
                 	     //memory reordered
                  	     refcount_set(&nbd->config_refs, 1)  // 2
     nbd->config
      ->null point
			     nbd->config = config  // 1

Fix it by adding smp barrier to guarantee the execution sequence.
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20231116162316.1740402-4-linan666@huaweicloud.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -397,8 +397,16 @@ static u32 req_to_nbd_cmd_type(struct request *req)
 
 static struct nbd_config *nbd_get_config_unlocked(struct nbd_device *nbd)
 {
-	if (refcount_inc_not_zero(&nbd->config_refs))
+	if (refcount_inc_not_zero(&nbd->config_refs)) {
+		/*
+		 * Add smp_mb__after_atomic to ensure that reading nbd->config_refs
+		 * and reading nbd->config is ordered. The pair is the barrier in
+		 * nbd_alloc_and_init_config(), avoid nbd->config_refs is set
+		 * before nbd->config.
+		 */
+		smp_mb__after_atomic();
 		return nbd->config;
+	}
 
 	return NULL;
 }
@@ -1559,7 +1567,15 @@ static int nbd_alloc_and_init_config(struct nbd_device *nbd)
 	init_waitqueue_head(&config->conn_wait);
 	config->blksize_bits = NBD_DEF_BLKSIZE_BITS;
 	atomic_set(&config->live_connections, 0);
+
 	nbd->config = config;
+	/*
+	 * Order refcount_set(&nbd->config_refs, 1) and nbd->config assignment,
+	 * its pair is the barrier in nbd_get_config_unlocked().
+	 * So nbd_get_config_unlocked() won't see nbd->config as null after
+	 * refcount_inc_not_zero() succeed.
+	 */
+	smp_mb__before_atomic();
 	refcount_set(&nbd->config_refs, 1);
 
 	return 0;