null_blk: fix null-ptr-dereference while configuring 'power' and 'submit_queues'

Writing 'power' and 'submit_queues' concurrently will trigger kernel
panic:

Test script:

modprobe null_blk nr_devices=0
mkdir -p /sys/kernel/config/nullb/nullb0
while true; do echo 1 > submit_queues; echo 4 > submit_queues; done &
while true; do echo 1 > power; echo 0 > power; done

Test result:

BUG: kernel NULL pointer dereference, address: 0000000000000148
Oops: 0000 [#1] PREEMPT SMP
RIP: 0010:__lock_acquire+0x41d/0x28f0
Call Trace:
 <TASK>
 lock_acquire+0x121/0x450
 down_write+0x5f/0x1d0
 simple_recursive_removal+0x12f/0x5c0
 blk_mq_debugfs_unregister_hctxs+0x7c/0x100
 blk_mq_update_nr_hw_queues+0x4a3/0x720
 nullb_update_nr_hw_queues+0x71/0xf0 [null_blk]
 nullb_device_submit_queues_store+0x79/0xf0 [null_blk]
 configfs_write_iter+0x119/0x1e0
 vfs_write+0x326/0x730
 ksys_write+0x74/0x150

This is because del_gendisk() can concurrent with
blk_mq_update_nr_hw_queues():

nullb_device_power_store	nullb_apply_submit_queues
 null_del_dev
 del_gendisk
				 nullb_update_nr_hw_queues
				  if (!dev->nullb)
				  // still set while gendisk is deleted
				   return 0
				  blk_mq_update_nr_hw_queues
 dev->nullb = NULL

Fix this problem by resuing the global mutex to protect
nullb_device_power_store() and nullb_update_nr_hw_queues() from configfs.

Fixes: 45919fbf ("null_blk: Enable modifying 'submit_queues' after an instance has been configured")
Reported-and-tested-by: Yi Zhang <yi.zhang@redhat.com>
Closes: https://lore.kernel.org/all/CAHj4cs9LgsHLnjg8z06LQ3Pr5cax-+Ps+xT7AP7TPnEjStuwZA@mail.gmail.com/Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Link: https://lore.kernel.org/r/20240523153934.1937851-1-yukuai1@huaweicloud.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk/main.c

@@ -413,13 +413,25 @@ static int nullb_update_nr_hw_queues(struct nullb_device *dev,
 static int nullb_apply_submit_queues(struct nullb_device *dev,
 				     unsigned int submit_queues)
 {
-	return nullb_update_nr_hw_queues(dev, submit_queues, dev->poll_queues);
+	int ret;
+
+	mutex_lock(&lock);
+	ret = nullb_update_nr_hw_queues(dev, submit_queues, dev->poll_queues);
+	mutex_unlock(&lock);
+
+	return ret;
 }
 
 static int nullb_apply_poll_queues(struct nullb_device *dev,
 				   unsigned int poll_queues)
 {
-	return nullb_update_nr_hw_queues(dev, dev->submit_queues, poll_queues);
+	int ret;
+
+	mutex_lock(&lock);
+	ret = nullb_update_nr_hw_queues(dev, dev->submit_queues, poll_queues);
+	mutex_unlock(&lock);
+
+	return ret;
 }
 
 NULLB_DEVICE_ATTR(size, ulong, NULL);
@@ -468,28 +480,31 @@ static ssize_t nullb_device_power_store(struct config_item *item,
 	if (ret < 0)
 		return ret;
 
+	ret = count;
+	mutex_lock(&lock);
 	if (!dev->power && newp) {
 		if (test_and_set_bit(NULLB_DEV_FL_UP, &dev->flags))
-			return count;
+			goto out;
+
 		ret = null_add_dev(dev);
 		if (ret) {
 			clear_bit(NULLB_DEV_FL_UP, &dev->flags);
-			return ret;
+			goto out;
 		}
 
 		set_bit(NULLB_DEV_FL_CONFIGURED, &dev->flags);
 		dev->power = newp;
 	} else if (dev->power && !newp) {
 		if (test_and_clear_bit(NULLB_DEV_FL_UP, &dev->flags)) {
-			mutex_lock(&lock);
 			dev->power = newp;
 			null_del_dev(dev->nullb);
-			mutex_unlock(&lock);
 		}
 		clear_bit(NULLB_DEV_FL_CONFIGURED, &dev->flags);
 	}
 
-	return count;
+out:
+	mutex_unlock(&lock);
+	return ret;
 }
 
 CONFIGFS_ATTR(nullb_device_, power);
@@ -1932,15 +1947,12 @@ static int null_add_dev(struct nullb_device *dev)
 	nullb->q->queuedata = nullb;
 	blk_queue_flag_set(QUEUE_FLAG_NONROT, nullb->q);
 
-	mutex_lock(&lock);
 	rv = ida_alloc(&nullb_indexes, GFP_KERNEL);
-	if (rv < 0) {
-		mutex_unlock(&lock);
+	if (rv < 0)
 		goto out_cleanup_disk;
-	}
+
 	nullb->index = rv;
 	dev->index = rv;
-	mutex_unlock(&lock);
 
 	if (config_item_name(&dev->group.cg_item)) {
 		/* Use configfs dir name as the device name */
@@ -1969,9 +1981,7 @@ static int null_add_dev(struct nullb_device *dev)
 	if (rv)
 		goto out_ida_free;
 
-	mutex_lock(&lock);
 	list_add_tail(&nullb->list, &nullb_list);
-	mutex_unlock(&lock);
 
 	pr_info("disk %s created\n", nullb->disk_name);
 
@@ -2020,7 +2030,9 @@ static int null_create_dev(void)
 	if (!dev)
 		return -ENOMEM;
 
+	mutex_lock(&lock);
 	ret = null_add_dev(dev);
+	mutex_unlock(&lock);
 	if (ret) {
 		null_free_dev(dev);
 		return ret;