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

[Yu Kuai]

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.