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=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=no 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 B8E3BCA9EC5 for ; Wed, 30 Oct 2019 19:26:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9379420717 for ; Wed, 30 Oct 2019 19:26:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726841AbfJ3T0g (ORCPT ); Wed, 30 Oct 2019 15:26:36 -0400 Received: from out02.mta.xmission.com ([166.70.13.232]:35269 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726769AbfJ3T0g (ORCPT ); Wed, 30 Oct 2019 15:26:36 -0400 Received: from in01.mta.xmission.com ([166.70.13.51]) by out02.mta.xmission.com with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.87) (envelope-from ) id 1iPtby-000758-Jv; Wed, 30 Oct 2019 13:26:30 -0600 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1iPtbx-0007ls-C9; Wed, 30 Oct 2019 13:26:30 -0600 From: ebiederm@xmission.com (Eric W. Biederman) To: Andrea Righi Cc: Dan Carpenter , Bartlomiej Zolnierkiewicz , Daniel Vetter , Sam Ravnborg , Maarten Lankhorst , Peter Rosin , Gerd Hoffmann , dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org, security@kernel.org, Kees Cook , Julia Lawall References: <20191029182320.GA17569@mwanda> <87zhhjjryk.fsf@x220.int.ebiederm.org> <20191030074321.GD2656@xps-13> Date: Wed, 30 Oct 2019 14:26:21 -0500 In-Reply-To: <20191030074321.GD2656@xps-13> (Andrea Righi's message of "Wed, 30 Oct 2019 08:43:21 +0100") Message-ID: <87r22ujaqq.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1iPtbx-0007ls-C9;;;mid=<87r22ujaqq.fsf@x220.int.ebiederm.org>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX19FbjDNGFR7y0rdUnCUnTTvGuw2gDijzYQ= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH] fbdev: potential information leak in do_fb_ioctl() X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Andrea Righi writes: > On Tue, Oct 29, 2019 at 02:02:11PM -0500, Eric W. Biederman wrote: >> Dan Carpenter writes: >> >> > The "fix" struct has a 2 byte hole after ->ywrapstep and the >> > "fix = info->fix;" assignment doesn't necessarily clear it. It depends >> > on the compiler. >> > >> > Fixes: 1f5e31d7e55a ("fbmem: don't call copy_from/to_user() with mutex held") >> > Signed-off-by: Dan Carpenter >> > --- >> > I have 13 more similar places to patch... I'm not totally sure I >> > understand all the issues involved. >> >> What I have done in a similar situation with struct siginfo, is that >> where the structure first appears I have initialized it with memset, >> and then field by field. >> >> Then when the structure is copied I copy the structure with memcpy. >> >> That ensures all of the bytes in the original structure are initialized >> and that all of the bytes are copied. >> >> The goal is to avoid memory that has values of the previous users of >> that memory region from leaking to userspace. Which depending on who >> the previous user of that memory region is could tell userspace >> information about what the kernel is doing that it should not be allowed >> to find out. >> >> I tried to trace through where "info" and thus presumably "info->fix" is >> coming from and only made it as far as register_framebuffer. Given >> that I suspect a local memset, and then a field by field copy right >> before copy_to_user might be a sound solution. But ick. That is a lot >> of fields to copy. > > I know it might sound quite inefficient, but what about making struct > fb_fix_screeninfo __packed? > > This doesn't solve other potential similar issues, but for this > particular case it could be a reasonable and simple fix. It is part of the user space ABI. As such you can't move the fields. Eric