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

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.

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.

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: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=e90526cab23b9efcd03c
Link: https://syzkaller.appspot.com/ai_job?id=827b5760-86a0-4f44-9c69-430b572eac12
To: "Jens Axboe" <[email protected]>
To: <[email protected]>
To: "Nilay Shroff" <[email protected]>
Cc: <[email protected]>

---
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;


base-commit: 075b74841bd0065a3bda3440873c747938e69b68
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