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

[Ming Lei <ming.lei@redhat.com>]

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,
Ming