From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kees Cook Subject: Re: [PATCH RESEND 1/4] uaccess: Add user_read_access_begin/end and user_write_access_begin/end Date: Thu, 2 Apr 2020 10:38:32 -0700 Message-ID: <202004021035.573BBBE9AA@keescook> References: <27106d62fdbd4ffb47796236050e418131cb837f.1585811416.git.christophe.leroy@c-s.fr> <20200402162942.GG23230@ZenIV.linux.org.uk> <67e21b65-0e2d-7ca5-7518-cec1b7abc46c@c-s.fr> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail-pj1-f66.google.com ([209.85.216.66]:36332 "EHLO mail-pj1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389561AbgDBRig (ORCPT ); Thu, 2 Apr 2020 13:38:36 -0400 Received: by mail-pj1-f66.google.com with SMTP id nu11so1786568pjb.1 for ; Thu, 02 Apr 2020 10:38:35 -0700 (PDT) Content-Disposition: inline In-Reply-To: <67e21b65-0e2d-7ca5-7518-cec1b7abc46c@c-s.fr> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Christophe Leroy Cc: Al Viro , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , airlied@linux.ie, daniel@ffwll.ch, torvalds@linux-foundation.org, akpm@linux-foundation.org, hpa@zytor.com, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, Russell King , Christian Borntraeger On Thu, Apr 02, 2020 at 07:03:28PM +0200, Christophe Leroy wrote: > > What should we do about arm and s390? There we want a cookie passed > > from beginning of block to its end; should that be a return value? > > That was the way I implemented it in January, see > https://patchwork.ozlabs.org/patch/1227926/ > > There was some discussion around that and most noticeable was: > > H. Peter (hpa) said about it: "I have *deep* concern with carrying state in > a "key" variable: it's a direct attack vector for a crowbar attack, > especially since it is by definition live inside a user access region." I share this concern -- we want to keep user/kernel access as static as possible. It should be provable with static analysis, etc (e.g. objtool does this already for x86). Since this doesn't disrupt existing R+W access, I'd prefer the design of this series as-is. -- Kees Cook