nbd: Fix memory leak in nbd_add_socket

When adding first socket to nbd, if nsock's allocation failed, the data
structure member "config->socks" was reallocated, but the data structure
member "config->num_connections" was not updated. A memory leak will occur
then because the function "nbd_config_put" will free "config->socks" only
when "config->num_connections" is not zero.

Fixes: 03bf73c3 ("nbd: prevent memory leak")
Reported-by: syzbot+934037347002901b8d2a@syzkaller.appspotmail.com
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -1033,25 +1033,26 @@ static int nbd_add_socket(struct nbd_device *nbd, unsigned long arg,
 	     test_bit(NBD_RT_BOUND, &config->runtime_flags))) {
 		dev_err(disk_to_dev(nbd->disk),
 			"Device being setup by another task");
-		sockfd_put(sock);
-		return -EBUSY;
+		err = -EBUSY;
+		goto put_socket;
+	}
+
+	nsock = kzalloc(sizeof(*nsock), GFP_KERNEL);
+	if (!nsock) {
+		err = -ENOMEM;
+		goto put_socket;
 	}
 
 	socks = krealloc(config->socks, (config->num_connections + 1) *
 			 sizeof(struct nbd_sock *), GFP_KERNEL);
 	if (!socks) {
-		sockfd_put(sock);
-		return -ENOMEM;
+		kfree(nsock);
+		err = -ENOMEM;
+		goto put_socket;
 	}
 
 	config->socks = socks;
 
-	nsock = kzalloc(sizeof(struct nbd_sock), GFP_KERNEL);
-	if (!nsock) {
-		sockfd_put(sock);
-		return -ENOMEM;
-	}
-
 	nsock->fallback_index = -1;
 	nsock->dead = false;
 	mutex_init(&nsock->tx_lock);
@@ -1063,6 +1064,10 @@ static int nbd_add_socket(struct nbd_device *nbd, unsigned long arg,
 	atomic_inc(&config->live_connections);
 
 	return 0;
+
+put_socket:
+	sockfd_put(sock);
+	return err;
 }
 
 static int nbd_reconnect_socket(struct nbd_device *nbd, unsigned long arg)