Re: [PATCH RFC] blk-mq: fix potential uaf for 'queue_hw_ctx'

[yukuai (C)]

在 2022/02/23 22:30, Ming Lei 写道:
> On Wed, Feb 23, 2022 at 07:26:01PM +0800, Yu Kuai wrote:
> > blk_mq_realloc_hw_ctxs() will free the 'queue_hw_ctx'(e.g. undate
> > submit_queues through configfs for null_blk), while it might still be
> > used from other context(e.g. switch elevator to none):
> >
> > t1					t2
> > elevator_switch
> >   blk_mq_unquiesce_queue
> >    blk_mq_run_hw_queues
> >     queue_for_each_hw_ctx
> >      // assembly code for hctx = (q)->queue_hw_ctx[i]
> >      mov    0x48(%rbp),%rdx -> read old queue_hw_ctx
> >
> > 					__blk_mq_update_nr_hw_queues
> > 					 blk_mq_realloc_hw_ctxs
> > 					  hctxs = q->queue_hw_ctx
> > 					  q->queue_hw_ctx = new_hctxs
> > 					  kfree(hctxs)
> >      movslq %ebx,%rax
> >      mov    (%rdx,%rax,8),%rdi ->uaf
>
> Not only uaf on queue_hw_ctx, but also other similar issue on other
> structures, and I think the correct and easy fix is to quiesce request
> queue during updating nr_hw_queues, something like the following patch:
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index a05ce7725031..d8e7c3cce0dd 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -4467,8 +4467,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
>   	if (set->nr_maps == 1 && nr_hw_queues == set->nr_hw_queues)
>   		return;
>   
> -	list_for_each_entry(q, &set->tag_list, tag_set_list)
> +	list_for_each_entry(q, &set->tag_list, tag_set_list) {
>   		blk_mq_freeze_queue(q);
> +		blk_mq_quiesce_queue(q);
> +	}
>   	/*
>   	 * Switch IO scheduler to 'none', cleaning up the data associated
>   	 * with the previous scheduler. We will switch back once we are done
> @@ -4518,8 +4520,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
>   	list_for_each_entry(q, &set->tag_list, tag_set_list)
>   		blk_mq_elv_switch_back(&head, q);
>   
> -	list_for_each_entry(q, &set->tag_list, tag_set_list)
> +	list_for_each_entry(q, &set->tag_list, tag_set_list) {
> +		blk_mq_unquiesce_queue(q);
>   		blk_mq_unfreeze_queue(q);
> +	}
>   }
>   
>   void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues)
Hi, Ming

If blk_mq_quiesce_queue() is called from __blk_mq_update_nr_hw_queues()
first, and then swithing elevator to none won't trigger the problem. 
However, what if blk_mq_unquiesce_queue() from switching elevator
decrease quiesce_depth to 0 first, and then blk_mq_quiesce_queue() is
called from __blk_mq_update_nr_hw_queues(), it seems to me such
concurrent scenarios still exist.

Thanks,
Kuai
> Thanks,
> Ming
>
> .