nbd: fix race between nbd_alloc_config() and module removal

When nbd module is being removing, nbd_alloc_config() may be
called concurrently by nbd_genl_connect(), although try_module_get()
will return false, but nbd_alloc_config() doesn't handle it.

The race may lead to the leak of nbd_config and its related
resources (e.g, recv_workq) and oops in nbd_read_stat() due
to the unload of nbd module as shown below:

  BUG: kernel NULL pointer dereference, address: 0000000000000040
  Oops: 0000 [#1] SMP PTI
  CPU: 5 PID: 13840 Comm: kworker/u17:33 Not tainted 5.14.0+ #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
  Workqueue: knbd16-recv recv_work [nbd]
  RIP: 0010:nbd_read_stat.cold+0x130/0x1a4 [nbd]
  Call Trace:
   recv_work+0x3b/0xb0 [nbd]
   process_one_work+0x1ed/0x390
   worker_thread+0x4a/0x3d0
   kthread+0x12a/0x150
   ret_from_fork+0x22/0x30

Fixing it by checking the return value of try_module_get()
in nbd_alloc_config(). As nbd_alloc_config() may return ERR_PTR(-ENODEV),
assign nbd->config only when nbd_alloc_config() succeeds to ensure
the value of nbd->config is binary (valid or NULL).

Also adding a debug message to check the reference counter
of nbd_config during module removal.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20220521073749.3146892-3-yukuai3@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -1518,15 +1518,20 @@ static struct nbd_config *nbd_alloc_config(void)
 {
 	struct nbd_config *config;
 
+	if (!try_module_get(THIS_MODULE))
+		return ERR_PTR(-ENODEV);
+
 	config = kzalloc(sizeof(struct nbd_config), GFP_NOFS);
-	if (!config)
+	if (!config) {
-		return NULL;
+		module_put(THIS_MODULE);
+		return ERR_PTR(-ENOMEM);
+	}
+
 	atomic_set(&config->recv_threads, 0);
 	init_waitqueue_head(&config->recv_wq);
 	init_waitqueue_head(&config->conn_wait);
 	config->blksize_bits = NBD_DEF_BLKSIZE_BITS;
 	atomic_set(&config->live_connections, 0);
-	try_module_get(THIS_MODULE);
 	return config;
 }
 
@@ -1553,12 +1558,13 @@ static int nbd_open(struct block_device *bdev, fmode_t mode)
 			mutex_unlock(&nbd->config_lock);
 			goto out;
 		}
-		config = nbd->config = nbd_alloc_config();
+		config = nbd_alloc_config();
-		if (!config) {
+		if (IS_ERR(config)) {
-			ret = -ENOMEM;
+			ret = PTR_ERR(config);
 			mutex_unlock(&nbd->config_lock);
 			goto out;
 		}
+		nbd->config = config;
 		refcount_set(&nbd->config_refs, 1);
 		refcount_inc(&nbd->refs);
 		mutex_unlock(&nbd->config_lock);
@@ -1964,13 +1970,14 @@ again:
 		nbd_put(nbd);
 		return -EINVAL;
 	}
-	config = nbd->config = nbd_alloc_config();
+	config = nbd_alloc_config();
-	if (!nbd->config) {
+	if (IS_ERR(config)) {
 		mutex_unlock(&nbd->config_lock);
 		nbd_put(nbd);
 		printk(KERN_ERR "nbd: couldn't allocate config\n");
-		return -ENOMEM;
+		return PTR_ERR(config);
 	}
+	nbd->config = config;
 	refcount_set(&nbd->config_refs, 1);
 	set_bit(NBD_RT_BOUND, &config->runtime_flags);
 
@@ -2543,6 +2550,9 @@ static void __exit nbd_cleanup(void)
 	while (!list_empty(&del_list)) {
 		nbd = list_first_entry(&del_list, struct nbd_device, list);
 		list_del_init(&nbd->list);
+		if (refcount_read(&nbd->config_refs))
+			printk(KERN_ERR "nbd: possibly leaking nbd_config (ref %d)\n",
+					refcount_read(&nbd->config_refs));
 		if (refcount_read(&nbd->refs) != 1)
 			printk(KERN_ERR "nbd: possibly leaking a device\n");
 		nbd_put(nbd);