Re: [PATCH -next v2 8/9] block: fix null-pointer dereference in ioc_pd_init

Yu Kuai
2022-12-01T02:12:23Z

Hi,

在 2022/12/01 4:50, Tejun Heo 写道:
> On Wed, Nov 30, 2022 at 09:21:55PM +0800, Li Nan wrote:
>> Remove block device when iocost is initializing may cause
>> null-pointer dereference:
>>
>> CPU1 CPU2
>> ioc_qos_write
>> blkcg_conf_open_bdev
>> blkdev_get_no_open
>> kobject_get_unless_zero
>> blk_iocost_init
>> rq_qos_add
>> del_gendisk
>> rq_qos_exit
>> q->rq_qos = rqos->next
>> //iocost is removed from q->roqs
>> blkcg_activate_policy
>> pd_init_fn
>> ioc_pd_init
>> ioc = q_to_ioc(blkg->q)
>> //cant find iocost and return null
>>
>> Fix problem by moving rq_qos_exit() to disk_release(). ioc_qos_write() get
>> bd_device.kobj in blkcg_conf_open_bdev(), so disk_release will not be
>> actived until iocost initialization is complited.
>
> I think it'd be better to make del_gendisk wait for these in-flight
> operations because this might fix the above particular issue but now all the
> policies are exposed to request_queue in a state it never expected before.
>
> del_gendisk() is quiescing the queue around rq_qos_exit(), so maybe we can
> piggyback on that and update blkcg_conf_open_bdev() to provide such
> exclusion?

Let del_gendisk waiting for that sounds good, but I'm litter confused
how to do that. Following are what I think about:

1) By mentioning that "del_gendisk() is quiescing the queue", do you
suggest to add rcu_read_lock()? This seems wrong because blk_iocost_init
requires memory allocation.

2) Hold gendisk open mutex

3) Use q_usage_counter, and in the meantime, rq_qos_add() and
blkcg_activate_policy() will need refactoring to factor out freeze
queue.

4) Define a new metux

Thanks,
Kuai
>
> Thanks.
>