Re: [PATCH V7 0/4] blk-mq: fix request UAF related with iterating over tagset requests

["yukuai (C)" <yukuai3@huawei.com>]

On 2021/08/06 12:12, Ming Lei wrote:
> On Fri, Aug 06, 2021 at 11:40:25AM +0800, yukuai (C) wrote:
>> On 2021/05/11 23:22, Ming Lei wrote:
>>> Hi Jens,
>>>
>>> This patchset fixes the request UAF issue by one simple approach,
>>> without clearing ->rqs[] in fast path, please consider it for 5.13.
>>>
>>> 1) grab request's ref before calling ->fn in blk_mq_tagset_busy_iter,
>>> and release it after calling ->fn, so ->fn won't be called for one
>>> request if its queue is frozen, done in 2st patch
>>>
>>> 2) clearing any stale request referred in ->rqs[] before freeing the
>>> request pool, one per-tags spinlock is added for protecting
>>> grabbing request ref vs. clearing ->rqs[tag], so UAF by refcount_inc_not_zero
>>> in bt_tags_iter() is avoided, done in 3rd patch.
>>>
>>> V7:
>>> 	- fix one null-ptr-deref during updating nr_hw_queues, because
>>> 	blk_mq_clear_flush_rq_mapping() may touch non-mapped hw queue's
>>> 	tags, only patch 4/4 is modified, reported and verified by
>>> 	Shinichiro Kawasaki
>>> 	- run blktests and not see regression
>>>
>>> V6:
>>> 	- hold spin lock when reading rq via ->rq[tag_bit], the issue is
>>> 	  added in V5
>>> 	- make blk_mq_find_and_get_req() more clean, as suggested by Bart
>>> 	- not hold tags->lock when clearing ->rqs[] for avoiding to disable
>>> 	interrupt a bit long, as suggested by Bart
>>> 	- code style change, as suggested by Christoph
>>>
>>> V5:
>>> 	- cover bt_iter() by same approach taken in bt_tags_iter(), and pass
>>> 	John's invasive test
>>> 	- add tested-by/reviewed-by tag
>>>
>>> V4:
>>> 	- remove hctx->fq-flush_rq from tags->rqs[] before freeing hw queue,
>>> 	patch 4/4 is added, which is based on David's patch.
>>>
>>> V3:
>>> 	- drop patches for completing requests started in iterator ->fn,
>>> 	  because blk-mq guarantees that valid request is passed to ->fn,
>>> 	  and it is driver's responsibility for avoiding double completion.
>>> 	  And drivers works well for not completing rq twice.
>>> 	- add one patch for avoiding double accounting of flush rq
>>>
>>> V2:
>>> 	- take Bart's suggestion to not add blk-mq helper for completing
>>> 	  requests when it is being iterated
>>> 	- don't grab rq->ref if the iterator is over static rqs because
>>> 	the use case do require to iterate over all requests no matter if
>>> 	the request is initialized or not
>>>
>>>
>>> Ming Lei (4):
>>>     block: avoid double io accounting for flush request
>>>     blk-mq: grab rq->refcount before calling ->fn in
>>>       blk_mq_tagset_busy_iter
>>>     blk-mq: clear stale request in tags->rq[] before freeing one request
>>>       pool
>>>     blk-mq: clearing flush request reference in tags->rqs[]
>>>
>>>    block/blk-flush.c  |  3 +-
>>>    block/blk-mq-tag.c | 49 ++++++++++++++++++------
>>>    block/blk-mq-tag.h |  6 +++
>>>    block/blk-mq.c     | 95 ++++++++++++++++++++++++++++++++++++++++------
>>>    block/blk-mq.h     |  1 +
>>>    5 files changed, 130 insertions(+), 24 deletions(-)
>>>
>>
>> Hi, ming
>>
>> I see this patchset was applied to fix the problem while iterating over
>> tagset, however blk_mq_tag_to_rq() is still untouched, and this
>> interface might still return freed request.
>>
>> Any reason why this interface didn't use the same solution ?
>> (hold tags->lock and return null if ref is 0)
> 
> It is driver's responsibility to cover race between normal completion
> and timeout/error handing, so driver has the knowledge if one tag is valid
> or not. In short, it is driver's responsibility to guarantee that valid 'tag'
> is passed to blk_mq_tag_to_rq().
> 

Thanks for the explanation.

Best regards,
Kuai