From mboxrd@z Thu Jan 1 00:00:00 1970 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=unavailable 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 7B0C8C2D0DB for ; Wed, 22 Jan 2020 09:41:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4F1BF24687 for ; Wed, 22 Jan 2020 09:41:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579686116; bh=tAEX8k+O68fJuZfYx6mQBJTYLpBE2CrVhQuydpAimMg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=JFSet97MQjW6jY1pCAN6IVIna6qLPFP0mjuV3NM2BjU1U+T/fovR1MAsaMG/KBagR 1OzggA6hiIVvZcDAW7WX3SwqJHQt2knuP6ma3cvcBZDz3R5fOhMZCBoPEnhdaf6avm Sgz7usbQGbNmc2T0aLhZKxOSpSz82FOZ8dBBCLaA= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1733292AbgAVJlz (ORCPT ); Wed, 22 Jan 2020 04:41:55 -0500 Received: from mail.kernel.org ([198.145.29.99]:33208 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387617AbgAVJlr (ORCPT ); Wed, 22 Jan 2020 04:41:47 -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 A0EA724686; Wed, 22 Jan 2020 09:41:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579686107; bh=tAEX8k+O68fJuZfYx6mQBJTYLpBE2CrVhQuydpAimMg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=sWT1jJQSzDAVvkMK9js/fyLGwVIpD1fDoBHHYU2S256O9B8mRA5F9J8zfkV8AmFrf xfM2dgbdEfvzhZ2n9ipiWR0EE/SUhrw27+TEVUfIXchScPFmaQk0U4aEAYpId8TQd2 IUHO39VaHJYp/3kYCEG9mjtmw2qvYlM+n12DE/7k= 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 4.19 046/103] mm/page-writeback.c: avoid potential division by zero in wb_min_max_ratio() Date: Wed, 22 Jan 2020 10:29:02 +0100 Message-Id: <20200122092810.734690455@linuxfoundation.org> X-Mailer: git-send-email 2.25.0 In-Reply-To: <20200122092803.587683021@linuxfoundation.org> References: <20200122092803.587683021@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@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); } }