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

From: Yu Kuai
Date: Wed Nov 30 2022 - 21:12:24 EST


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.