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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 7A8AAC43387 for ; Fri, 11 Jan 2019 07:36:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4090720874 for ; Fri, 11 Jan 2019 07:36:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729072AbfAKHgL (ORCPT ); Fri, 11 Jan 2019 02:36:11 -0500 Received: from ipmail06.adl6.internode.on.net ([150.101.137.145]:35251 "EHLO ipmail06.adl6.internode.on.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727578AbfAKHgK (ORCPT ); Fri, 11 Jan 2019 02:36:10 -0500 Received: from ppp59-167-129-252.static.internode.on.net (HELO dastard) ([59.167.129.252]) by ipmail06.adl6.internode.on.net with ESMTP; 11 Jan 2019 18:06:07 +1030 Received: from dave by dastard with local (Exim 4.80) (envelope-from ) id 1ghrMM-0003fE-8m; Fri, 11 Jan 2019 18:36:06 +1100 Date: Fri, 11 Jan 2019 18:36:06 +1100 From: Dave Chinner To: Linus Torvalds Cc: Dominique Martinet , Jiri Kosina , Matthew Wilcox , Jann Horn , Andrew Morton , Greg KH , Peter Zijlstra , Michal Hocko , Linux-MM , kernel list , Linux API Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged Message-ID: <20190111073606.GP27534@dastard> References: <20190110004424.GH27534@dastard> <20190110070355.GJ27534@dastard> <20190110122442.GA21216@nautica> <20190111020340.GM27534@dastard> <20190111040434.GN27534@dastard> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 10, 2019 at 11:08:07PM -0800, Linus Torvalds wrote: > On Thu, Jan 10, 2019 at 8:04 PM Dave Chinner wrote: > > > > So it will only read the single page we tried to access and won't > > perturb the rest of the message encoded into subsequent pages in > > file. > > Dave, you're being intentionally obtuse, aren't you? > > It's only that single page that *matters*. That's the page that the > probe reveals the status of - but it's also the page that the probe > then *changes* the status of. It changes the state of it /after/ we've already got the information we need from it. It's not up to date, it has to come from disk, we return EAGAIN, which means it was not in the cache. i.e. if we return EAGAIN, we've leaked the inforation the attacker wants regardless of how the act of initiating readahead on the page change the state of the page. Yes, it raises the complexity bar a bit, and lowers the monitoring frequency somewhat, but that's about it. Cheers, Dave. -- Dave Chinner david@fromorbit.com