Re: [PATCH-next] block: fix null-deref in percpu_ref_put

[Yu Kuai]

Hi,

在 2022/12/07 9:05, Dennis Zhou 写道:
> Hello,
>
> On Tue, Dec 06, 2022 at 05:09:39PM +0800, Zhong Jinghua wrote:
> > A problem was find in stable 5.10 and the root cause of it like below.
> >
> > In the use of q_usage_counter of request_queue, blk_cleanup_queue using
> > "wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->q_usage_counter))"
> > to wait q_usage_counter becoming zero. however, if the q_usage_counter
> > becoming zero quickly, and percpu_ref_exit will execute and ref->data
> > will be freed, maybe another process will cause a null-defef problem
> > like below:
> >
> > 	CPU0                             CPU1
> > blk_mq_destroy_queue
> >   blk_freeze_queue
> >    blk_mq_freeze_queue_wait
> > 				scsi_end_request
> > 				 percpu_ref_get
> > 				 ...
> > 				 percpu_ref_put
> > 				  atomic_long_sub_and_test
> >   blk_put_queue
> >    kobject_put
> >     kref_put
> >      blk_release_queue
> >       percpu_ref_exit
> >        ref->data -> NULL
> >     				   ref->data->release(ref) -> null-deref
>
> I remember thinking about this a while ago. I don't think this fix works
> as nicely as it may seem. Please correct me if I'm wrong.
>
> q->q_usage_counter has the oddity that the lifetime of the percpu_ref
> object isn't managed by the release function. The freeing is handled by
> a separate path where it depends on the percpu_ref hitting 0. So here we
> have 2 concurrent paths racing to run with 1 destroying the object. We
> probably need blk_release_queue() to wait on percpu_ref's release
> finishing, not starting.
>
> I think the above works in this specific case because there is a
> call_rcu() in blk_release_queue(). If there wasn't a call_rcu(),
> then by the same logic we could delay ref->data->release(ref) further
> and that could potentially lead to a use after free.
>
> Ideally, I think fixing the race in q->q_usage_counter's pattern is
> better than masking it here as I think we're being saved by the
> call_rcu() call further down the object release path.

Agree.

BTW, Wensheng used to send a patch to fix this in block layer:

https://www.spinics.net/lists/kernel/msg4615696.html.

Thanks,
Kuai