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

[John Garry <john.garry@huawei.com>]

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?

> 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

>   	}
>   	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.

Thanks,
John

>   	q->elevator = NULL;
>   }
>