From patchwork Wed Jan 22 09:29:08 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Greg KH X-Patchwork-Id: 233612 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.8 required=3.0 tests=DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI, SIGNED_OFF_BY, SPF_HELO_NONE, SPF_PASS, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C4375C2D0DB for ; Wed, 22 Jan 2020 09:32:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 96E6C24673 for ; Wed, 22 Jan 2020 09:32:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579685536; bh=tAEX8k+O68fJuZfYx6mQBJTYLpBE2CrVhQuydpAimMg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=rI3n7/LID36I5gqzBN0hslxvG2heMyTOZjyB53vGOzVL8Q2PZYzY9oSqsRd5Lg3VK dMm8bQTOd4GTfFaJqm2XRuwT98peRyxuN9jcFFQXCApZB7/MLkwNBEX+42V+9Z58yL xmxeC8bi803jqO1somtLmqD3BEPzhLFH9wOIp7To= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729992AbgAVJcP (ORCPT ); Wed, 22 Jan 2020 04:32:15 -0500 Received: from mail.kernel.org ([198.145.29.99]:44846 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730012AbgAVJcN (ORCPT ); Wed, 22 Jan 2020 04:32:13 -0500 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 1A36124673; Wed, 22 Jan 2020 09:32:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579685532; bh=tAEX8k+O68fJuZfYx6mQBJTYLpBE2CrVhQuydpAimMg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=XzyTTNdyUMQ9pu9seHXKg7UbvNNHAXjSZHunlRER6UVUJ4iaNCumPi+99jcb8NfHo WWMYh22ckozYQkJGfGJDcOEJqUbm6j79yPpv1A+oOBtU/4SKyIgy4I7sQQUpGKNAF6 a43sBjuBA0RrfL78qsOuOeIkAg1OfGgCplboLkt4= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Wen Yang , Andrew Morton , Qian Cai , Tejun Heo , Jens Axboe , Linus Torvalds Subject: [PATCH 4.4 52/76] mm/page-writeback.c: avoid potential division by zero in wb_min_max_ratio() Date: Wed, 22 Jan 2020 10:29:08 +0100 Message-Id: <20200122092758.575784196@linuxfoundation.org> X-Mailer: git-send-email 2.25.0 In-Reply-To: <20200122092751.587775548@linuxfoundation.org> References: <20200122092751.587775548@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Wen Yang commit 6d9e8c651dd979aa666bee15f086745f3ea9c4b3 upstream. Patch series "use div64_ul() instead of div_u64() if the divisor is unsigned long". We were first inspired by commit b0ab99e7736a ("sched: Fix possible divide by zero in avg_atom () calculation"), then refer to the recently analyzed mm code, we found this suspicious place. 201 if (min) { 202 min *= this_bw; 203 do_div(min, tot_bw); 204 } And we also disassembled and confirmed it: /usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 201 0xffffffff811c37da <__wb_calc_thresh+234>: xor %r10d,%r10d 0xffffffff811c37dd <__wb_calc_thresh+237>: test %rax,%rax 0xffffffff811c37e0 <__wb_calc_thresh+240>: je 0xffffffff811c3800 <__wb_calc_thresh+272> /usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 202 0xffffffff811c37e2 <__wb_calc_thresh+242>: imul %r8,%rax /usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 203 0xffffffff811c37e6 <__wb_calc_thresh+246>: mov %r9d,%r10d ---> truncates it to 32 bits here 0xffffffff811c37e9 <__wb_calc_thresh+249>: xor %edx,%edx 0xffffffff811c37eb <__wb_calc_thresh+251>: div %r10 0xffffffff811c37ee <__wb_calc_thresh+254>: imul %rbx,%rax 0xffffffff811c37f2 <__wb_calc_thresh+258>: shr $0x2,%rax 0xffffffff811c37f6 <__wb_calc_thresh+262>: mul %rcx 0xffffffff811c37f9 <__wb_calc_thresh+265>: shr $0x2,%rdx 0xffffffff811c37fd <__wb_calc_thresh+269>: mov %rdx,%r10 This series uses div64_ul() instead of div_u64() if the divisor is unsigned long, to avoid truncation to 32-bit on 64-bit platforms. This patch (of 3): The variables 'min' and 'max' are unsigned long and do_div truncates them to 32 bits, which means it can test non-zero and be truncated to zero for division. Fix this issue by using div64_ul() instead. Link: http://lkml.kernel.org/r/20200102081442.8273-2-wenyang@linux.alibaba.com Fixes: 693108a8a667 ("writeback: make bdi->min/max_ratio handling cgroup writeback aware") Signed-off-by: Wen Yang Reviewed-by: Andrew Morton Cc: Qian Cai Cc: Tejun Heo Cc: Jens Axboe Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- mm/page-writeback.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -200,11 +200,11 @@ static void wb_min_max_ratio(struct bdi_ if (this_bw < tot_bw) { if (min) { min *= this_bw; - do_div(min, tot_bw); + min = div64_ul(min, tot_bw); } if (max < 100) { max *= this_bw; - do_div(max, tot_bw); + max = div64_ul(max, tot_bw); } }