Messages in this thread | | | Subject | Re: [PATCH-next] block: fix null-deref in percpu_ref_put | From | Yu Kuai <> | Date | Wed, 7 Dec 2022 21:10:33 +0800 |
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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
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