linux-kernel - [PATCH 6.6.y v2 1/1] block: Fix bounce check logic in blk_queue_may

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <20250814063655.1902688-1-quic_hardshar@quicinc.com>
Date: Thu, 14 Aug 2025 12:06:55 +0530
From: Hardeep Sharma <quic_hardshar@...cinc.com>
To: Jens Axboe <axboe@...nel.dk>, Hannes Reinecke <hare@...e.de>,
        "Martin K . Petersen" <martin.petersen@...cle.com>
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Hardeep Sharma <quic_hardshar@...cinc.com>
Subject: [PATCH 6.6.y v2 1/1] block: Fix bounce check logic in blk_queue_may_bounce()

Buffer bouncing is needed only when memory exists above the lowmem region,
i.e., when max_low_pfn < max_pfn. The previous check (max_low_pfn >=
max_pfn) was inverted and prevented bouncing when it could actually be
required.

Note that bouncing depends on CONFIG_HIGHMEM, which is typically enabled
on 32-bit ARM where not all memory is permanently mapped into the kernel’s
lowmem region.

Branch-Specific Note:

This fix is specific to this branch (6.6.y) only.
In the upstream “tip” kernel, bounce buffer support for highmem pages
was completely removed after kernel version 6.12. Therefore, this
modification is not possible or relevant in the tip branch.

Fixes: 9bb33f24abbd0 ("block: refactor the bounce buffering code")
Cc: stable@...r.kernel.org
Signed-off-by: Hardeep Sharma <quic_hardshar@...cinc.com>
---
 block/blk.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk.h b/block/blk.h
index 67915b04b3c1..f8a1d64be5a2 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -383,7 +383,7 @@ static inline bool blk_queue_may_bounce(struct request_queue *q)
 {
 	return IS_ENABLED(CONFIG_BOUNCE) &&
 		q->limits.bounce == BLK_BOUNCE_HIGH &&
-		max_low_pfn >= max_pfn;
+		max_low_pfn < max_pfn;
 }

 static inline struct bio *blk_queue_bounce(struct bio *bio,
-- 
2.25.1