Re: [PATCH] blk-mq: fix use-after-free in blk_mq_exit_sched

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

On Wed, Jun 09, 2021 at 09:59:43AM +0100, John Garry wrote:
> On 09/06/2021 07:30, Ming Lei wrote:
> 
> Thanks for the fix
> 
> > tagset can't be used after blk_cleanup_queue() is returned because
> > freeing tagset usually follows blk_clenup_queue(). Commit d97e594c5166
> > ("blk-mq: Use request queue-wide tags for tagset-wide sbitmap") adds
> > check on q->tag_set->flags in blk_mq_exit_sched(), and causes
> > use-after-free.
> > 
> > Fixes it by using hctx->flags.
> > 
> 
> The tagset is a member of the Scsi_Host structure. So it is true that this
> memory may be freed before the request_queue is exited?

Yeah, please see commit c3e2219216c9 ("block: free sched's request pool in
blk_cleanup_queue")

> 
> > Reported-by: syzbot+77ba3d171a25c56756ea@syzkaller.appspotmail.com
> > Fixes: d97e594c5166 ("blk-mq: Use request queue-wide tags for tagset-wide sbitmap")
> > Cc: John Garry <john.garry@huawei.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >   block/blk-mq-sched.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> > index a9182d2f8ad3..80273245d11a 100644
> > --- a/block/blk-mq-sched.c
> > +++ b/block/blk-mq-sched.c
> > @@ -680,6 +680,7 @@ void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
> >   {
> >   	struct blk_mq_hw_ctx *hctx;
> >   	unsigned int i;
> > +	unsigned int flags = 0;
> >   	queue_for_each_hw_ctx(q, hctx, i) {
> >   		blk_mq_debugfs_unregister_sched_hctx(hctx);
> > @@ -687,12 +688,13 @@ void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
> >   			e->type->ops.exit_hctx(hctx, i);
> >   			hctx->sched_data = NULL;
> >   		}
> > +		flags = hctx->flags;
> 
> I know the choice is limited, but it is unfortunate that we must set flags
> in a loop

Does it matter?

> 
> >   	}
> >   	blk_mq_debugfs_unregister_sched(q);
> >   	if (e->type->ops.exit_sched)
> >   		e->type->ops.exit_sched(e);
> >   	blk_mq_sched_tags_teardown(q);
> > -	if (blk_mq_is_sbitmap_shared(q->tag_set->flags))
> > +	if (blk_mq_is_sbitmap_shared(flags))
> >   		blk_mq_exit_sched_shared_sbitmap(q);
> 
> this is
> 
> blk_mq_exit_sched_shared_sbitmap(struct request_queue *queue)
> {
> 	sbitmap_queue_free(&queue->sched_bitmap_tags);
> 	..
> }
> 
> And isn't it safe to call sbitmap_queue_free() when
> sbitmap_queue_init_node() has not been called?
> 
> I'm just wondering if we can always call blk_mq_exit_sched_shared_sbitmap()?
> I know it's not an ideal choice either.

So far it may work, not sure if it can in future, I suggest to follow
the traditional alloc & free pattern.


Thanks,
Ming