Re: [PATCH v2] blk-mq: fix out-of-bounds read in blk_mq_free_rqs

[Krystian Kaniewski <krystianmkaniewski@gmail.com>]

On 10/14/26 19:05, Krystian Kaniewski wrote:
> 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.
> 
> 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.
> 
> 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@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=e90526cab23b9efcd03c
> Link: https://syzkaller.appspot.com/ai_job?id=827b5760-86a0-4f44-9c69-430b572eac12
> Reviewed-by: Nilay Shorff <nilay@linux.ibm.com>
> Cc: stable@vger.kernel.org # 6.17+
> Signed-off-by: Krystian Kaniewski <krystianmkaniewski@gmail.com>
> ---
> Changes since v1:
> - Drop the additional blk_mq_alloc_rqs() claim, as that path is bounded by
>   set->nr_hw_queues.
> - Add Nilay's Reviewed-by tag.
> - Rebase onto current mainline and refresh the recipient list.
> 
> v1: https://lore.kernel.org/all/c92b0c7e-17fb-44c8-99d9-59d45004d05a@mail.kernel.org/
> 
>  block/blk-mq.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 025a799f35b92..259a5705efb79 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -3484,8 +3484,10 @@ 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;
> 
> base-commit: 0fcb05b72344f40806cbe810de8872f1698bf053
Hi Jens,

A gentle ping on this fix. The patch has Nilay's Reviewed-by, and
syzbot CI tested v2 without finding any issues.

Please let me know if any changes are needed.

Thanks,
-- 
Krystian Kaniewski