linux-kernel - [PATCH linux-block/for-linus] writeback: fix possible underflow in write bandwidth calculation

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: <20150323041848.GA8991@htj.duckdns.org>
Date:	Mon, 23 Mar 2015 00:18:48 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Jan Kara <jack@...e.cz>, Wu Fengguang <fengguang.wu@...el.com>,
	Greg Thelen <gthelen@...gle.com>, linux-kernel@...r.kernel.org
Subject: [PATCH linux-block/for-linus] writeback: fix possible underflow in
 write bandwidth calculation

>From 1ebf33901ecc75d9496862dceb1ef0377980587c Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@...nel.org>
Date: Mon, 23 Mar 2015 00:08:19 -0400

2f800fbd777b ("writeback: fix dirtied pages accounting on redirty")
introduced account_page_redirty() which reverts stat updates for a
redirtied page, making BDI_DIRTIED no longer monotonically increasing.

bdi_update_write_bandwidth() uses the delta in BDI_DIRTIED as the
basis for bandwidth calculation.  While unlikely, since the above
patch, the newer value may be lower than the recorded past value and
underflow the bandwidth calculation leading to a wild result.

Fix it by subtracing min of the old and new values when calculating
delta.  AFAIK, there hasn't been any report of it happening but the
resulting erratic behavior would be non-critical and temporary, so
it's possible that the issue is happening without being reported.  The
risk of the fix is very low, so tagged for -stable.

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Jan Kara <jack@...e.cz>
Cc: Wu Fengguang <fengguang.wu@...el.com>
Cc: Greg Thelen <gthelen@...gle.com>
Fixes: 2f800fbd777b ("writeback: fix dirtied pages accounting on redirty")
Cc: stable@...r.kernel.org
---
 mm/page-writeback.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index b4fd980..644bcb6 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -857,8 +857,11 @@ static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
 	 *                   bw * elapsed + write_bandwidth * (period - elapsed)
 	 * write_bandwidth = ---------------------------------------------------
 	 *                                          period
+	 *
+	 * @written may have decreased due to account_page_redirty().
+	 * Avoid underflowing @bw calculation.
 	 */
-	bw = written - bdi->written_stamp;
+	bw = written - min(written, bdi->written_stamp);
 	bw *= HZ;
 	if (unlikely(elapsed > period)) {
 		do_div(bw, elapsed);
-- 
2.1.0

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/