Re: [PATCH] null_blk: serialize configfs attribute updates with device setup

[Zizhi Wo <wozizhi@huaweicloud.com>]

Hi Niklas,

在 2026/8/13 22:14, Niklas Cassel 写道:
> The attribute store methods generated with NULLB_DEVICE_ATTR() refuse to
> change the configuration of a live device by testing
> NULLB_DEV_FL_CONFIGURED, but that flag is only set by
> nullb_device_power_store() after null_add_dev() has returned, and the
> store methods take no lock at all. configfs only serializes writes to
> the same open file (buffer->mutex), so a write to any attribute can run
> concurrently with null_add_dev() and change the device configuration
> while it is being used.
> 
> null_add_dev() reads the configuration several times, e.g. dev->zoned is
> read once to set up the queue limits and once to initialize the zone
> resources:
> 
>    CPU0: echo 1 > nullb0/power         CPU1: echo 1 > nullb0/zoned
>    nullb_device_power_store()
>      mutex_lock(&lock)
>      null_add_dev()
>        if (dev->zoned) -> false
>          /* no BLK_FEAT_ZONED */       nullb_device_zoned_store()
>                                          test_bit(FL_CONFIGURED) -> 0
>                                          dev->zoned = true
>        blk_mq_alloc_disk()
>          /* queue is not zoned */
>        if (nullb->dev->zoned) -> true
>          null_register_zoned_dev()
>            blk_revalidate_disk_zones()
> 
> blk_revalidate_disk_zones() is then called for a queue that does not
> have BLK_FEAT_ZONED set, which triggers its WARN_ON_ONCE() and fails the
> device setup with -EIO:
> 
>    WARNING: CPU: 2 PID: 322 at block/blk-zoned.c:2357 blk_revalidate_disk_zones+0x4c/0x560
> 
> Clearing dev->zoned in the same window is worse: the queue is created
> with BLK_FEAT_ZONED but the zone resources are never initialized, so
> add_disk() succeeds for a zoned disk that has no zones. And a store that
> lands after the last dev->zoned test leaves dev->zoned set while
> dev->zones is still NULL, which null_process_zoned_cmd() dereferences on
> the first write.
> 
> Fix this by taking the global lock, which nullb_device_power_store()
> already holds across null_add_dev() and null_del_dev(), around both the
> NULLB_DEV_FL_CONFIGURED test and the update of the device configuration.
> The submit_queues and poll_queues apply callbacks are now called with
> that lock held, so remove the locking they did themselves.
> 
> Since the store methods can run as soon as configfs_register_subsystem()
> returns, that is, before null_init() gets to mutex_init(&lock), also
> initialize the lock statically with DEFINE_MUTEX().
> 
> Fixes: 3bf2bd20734e ("nullb: add configfs interface")
> Reported-by: syzbot+643a6dd130546afdf1fb@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/linux-block/6a7d0b3f.ac361c09.22ff0a.004c.GAE@google.com/
> Signed-off-by: Niklas Cassel <cassel@kernel.org>
> ---
>   drivers/block/null_blk/main.c | 39 ++++++++++++++++-------------------
>   1 file changed, 18 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
> index f8c0fd57e041..2e8f99873956 100644
> --- a/drivers/block/null_blk/main.c
> +++ b/drivers/block/null_blk/main.c
> @@ -66,7 +66,7 @@ struct nullb_page {
>   #define NULLB_PAGE_FREE (MAP_SZ - 2)
>   
>   static LIST_HEAD(nullb_list);
> -static struct mutex lock;
> +static DEFINE_MUTEX(lock);
>   static int null_major;
>   static DEFINE_IDA(nullb_indexes);
>   static struct blk_mq_tag_set tag_set;
> @@ -340,7 +340,15 @@ static ssize_t nullb_device_bool_attr_store(bool *val, const char *page,
>   	return count;
>   }
>   
> -/* The following macro should only be used with TYPE = {uint, ulong, bool}. */
> +/*
> + * The following macro should only be used with TYPE = {uint, ulong, bool}.
> + *
> + * The device configuration is modified under the global lock to serialize
> + * attribute changes against null_add_dev() and null_del_dev(): without this,
> + * an attribute could be changed while null_add_dev() is running, that is,
> + * before NULLB_DEV_FL_CONFIGURED is set, which would let null_add_dev()
> + * observe inconsistent values for the device configuration.
> + */
>   #define NULLB_DEVICE_ATTR(NAME, TYPE, APPLY)				\
>   static ssize_t								\
>   nullb_device_##NAME##_show(struct config_item *item, char *page)	\
> @@ -360,13 +368,16 @@ nullb_device_##NAME##_store(struct config_item *item, const char *page,	\
>   	ret = nullb_device_##TYPE##_attr_store(&new_value, page, count);\
>   	if (ret < 0)							\
>   		return ret;						\
> +	mutex_lock(&lock);						\
>   	if (apply_fn)							\
>   		ret = apply_fn(dev, new_value);				\
>   	else if (test_bit(NULLB_DEV_FL_CONFIGURED, &dev->flags)) 	\
>   		ret = -EBUSY;						\
> +	if (ret >= 0)							\
> +		dev->NAME = new_value;					\
> +	mutex_unlock(&lock);						\
>   	if (ret < 0)							\
>   		return ret;						\
> -	dev->NAME = new_value;						\
>   	return count;							\
>   }									\
>   CONFIGFS_ATTR(nullb_device_, NAME);
> @@ -379,6 +390,8 @@ static int nullb_update_nr_hw_queues(struct nullb_device *dev,
>   	struct blk_mq_tag_set *set;
>   	int ret, nr_hw_queues;
>   
> +	lockdep_assert_held(&lock);
> +
>   	if (!dev->nullb)
>   		return 0;
>   
> @@ -421,25 +434,13 @@ 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)
>   {
> -	int ret;
> -
> -	mutex_lock(&lock);
> -	ret = nullb_update_nr_hw_queues(dev, submit_queues, dev->poll_queues);
> -	mutex_unlock(&lock);
> -
> -	return ret;
> +	return nullb_update_nr_hw_queues(dev, submit_queues, dev->poll_queues);
>   }
>   
>   static int nullb_apply_poll_queues(struct nullb_device *dev,
>   				   unsigned int poll_queues)
>   {
> -	int ret;
> -
> -	mutex_lock(&lock);
> -	ret = nullb_update_nr_hw_queues(dev, dev->submit_queues, poll_queues);
> -	mutex_unlock(&lock);
> -
> -	return ret;
> +	return nullb_update_nr_hw_queues(dev, dev->submit_queues, poll_queues);
>   }
>   
>   NULLB_DEVICE_ATTR(size, ulong, NULL);
> @@ -2166,8 +2167,6 @@ static int __init null_init(void)
>   	if (ret)
>   		return ret;
>   
> -	mutex_init(&lock);
> -
>   	null_major = register_blkdev(0, "nullb");
>   	if (null_major < 0) {
>   		ret = null_major;
> @@ -2211,8 +2210,6 @@ static void __exit null_exit(void)
>   
>   	if (tag_set.ops)
>   		blk_mq_free_tag_set(&tag_set);
> -
> -	mutex_destroy(&lock);
>   }
>   
>   module_init(null_init);

Thanks for the patch. This issue has already been addressed in my 
null_blk series posted back in July:

https://lore.kernel.org/all/20260725022509.714271-1-
wozizhi@huaweicloud.com/

This includes DEFINE_MUTEX and the serialization, but this series hasn't
been merged yet. It has already collected some Reviewed-by tags, and I'm
hoping it can be picked up for mainline soon.

Thanks,
Zizhi Wo