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: 45919fbfe1c4 ("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.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index 5d56ad4ce01a10d21b275d20989ef7f208a28201..eb023d26736934d5c54cee57962145f4ee2eb93e 100644 (file)
--- a/drivers/block/null_blk/main.c
+++ b/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;