Re: [PATCH] blk-mq: fix out-of-bounds read in blk_mq_free_rqs

[Nilay Shroff]

On 9/10/26 3:49 PM, syzbot wrote:
> From: Krystian Kaniewski <krystianmkaniewski@xxxxxxxxx>
>
> A KASAN slab-out-of-bounds read can occur in blk_mq_free_rqs() when there
> is a mismatch between the number of hardware queues allocated for the IO
> scheduler (et->nr_hw_queues) and the number of hardware queues in the block
> tag set (set->nr_hw_queues).
>
> This mismatch can happen if blk_mq_update_nr_hw_queues() fails halfway
> through (e.g., due to memory pressure during
> blk_mq_prealloc_tag_set_tags()). In this error path, the elevator is
> restored with a larger number of hardware queues than the block tag set
> actually has.
>
> When the elevator is later freed, blk_mq_free_sched_tags() iterates up to
> the new et->nr_hw_queues and calls blk_mq_free_rqs(). In blk_mq_free_rqs(),
> it attempts to access set->tags[hctx_idx] to get the driver tags. Because
> set->nr_hw_queues was not updated due to the earlier failure, set->tags
> still has the old (smaller) size, leading to an out-of-bounds read.
Yes makes sense, this scenario seems feasible.

> BUG: KASAN: slab-out-of-bounds in blk_mq_free_rqs+0xde/0x680
> Read of size 8 at addr ffff88818d67fc28 by task syz-executor117/5839
> Call Trace:
>   blk_mq_free_rqs+0xde/0x680
>   blk_mq_free_map_and_rqs+0x40/0xf0
>   blk_mq_free_sched_tags
>   blk_mq_free_sched_res+0xeb/0x280
>   elevator_change_done+0x1d2/0x5c0
>   elevator_change+0x34f/0x480
>   elv_iosched_store+0x504/0x630
>   queue_attr_store+0x207/0x2b0
>
> To fix this, explicitly check if hctx_idx < set->nr_hw_queues before
> accessing set->tags[hctx_idx]. If hctx_idx >= set->nr_hw_queues, the
> hardware queue doesn't exist in the tag set, meaning there are no driver
> tags to clear mappings from. In this case, safely set drv_tags to NULL. The
> subsequent call to blk_mq_clear_rq_mapping() already handles a NULL
> drv_tags pointer and will safely return.
Fix looks reasonable.

> This fix also prevents a similar out-of-bounds read in the failure path of
> blk_mq_alloc_rqs(), where a failure during new driver tag allocation could
> lead to blk_mq_free_rqs() being called with an hctx_idx greater than or
> equal to set->nr_hw_queues.
I'm not sure if this is possible. Looking at the callers of blk_mq_alloc_rqs(),
the allocation loops appear to be bounded by set->nr_hw_queues, so an
hctx_idx >= set->nr_hw_queues should not be passed to blk_mq_alloc_rqs() in
that path. Nevertheless, the guard in blk_mq_free_rqs() looks reasonable as
a defensive check and fixes the reported failure.

> Fixes: 04225d13aef1 ("block: fix potential deadlock while running nr_hw_queue update")
> Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
> Reported-by: syzbot+e90526cab23b9efcd03c@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=e90526cab23b9efcd03c
> Link: https://syzkaller.appspot.com/ai_job?id=827b5760-86a0-4f44-9c69-430b572eac12
> Signed-off-by: Krystian Kaniewski <krystianmkaniewski@xxxxxxxxx>
>
> ---
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 2c850330a..8e6726b37 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -3473,8 +3473,10 @@ void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
>   
>   	if (blk_mq_is_shared_tags(set->flags))
>   		drv_tags = set->shared_tags;
> -	else
> +	else if (hctx_idx < set->nr_hw_queues)
>   		drv_tags = set->tags[hctx_idx];
> +	else
> +		drv_tags = NULL;
>   
>   	if (tags->static_rqs && set->ops->exit_request) {
>   		int i;
Thanks for the fix! Overall, this looks good to me.

Reviewed-by: Nilay Shorff <nilay@xxxxxxxxxxxxx>