Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754319AbbBZPcf (ORCPT ); Thu, 26 Feb 2015 10:32:35 -0500 Received: from mail-la0-f47.google.com ([209.85.215.47]:41785 "EHLO mail-la0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753916AbbBZPcd (ORCPT ); Thu, 26 Feb 2015 10:32:33 -0500 MIME-Version: 1.0 In-Reply-To: References: From: Andy Lutomirski Date: Thu, 26 Feb 2015 07:32:12 -0800 Message-ID: Subject: Re: [PATCH 3.2 055/152] x86_64, switch_to(): Load TLS descriptors before switching DS and ES To: Brian Gerst Cc: Denys Vlasenko , Ben Hutchings , Linux Kernel Mailing List , stable , Andrew Morton , Ingo Molnar , Andi Kleen , Linus Torvalds Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1918 Lines: 43 On Tue, Feb 24, 2015 at 7:23 PM, Brian Gerst wrote: > On Tue, Feb 24, 2015 at 3:08 PM, Denys Vlasenko > wrote: >> On Tue, Feb 24, 2015 at 9:02 PM, Andy Lutomirski wrote: >>>> This currently fails in 32-bit kernels (at least in qemu): >>>> >>>> / # ./es_test >>>> Allocated GDT index 7 >>>> [FAIL] ES changed from 0x3b to 0x7b >>>> [FAIL] ES was corrupted 1000/1000 times >>>> / # uname -a >>>> Linux (none) 4.0.0-rc1 #1 SMP Tue Feb 24 16:41:58 CET 2015 i686 GNU/Linux >>> >>> Want to send a patch? I'll get it in a few days if no one beats me. >> >> I have no patch, sorry (in fact, I failed to find where is the relevant >> 32-bit counterpart). >> >> It's just security people asked me to backport this and I wondered >> maybe I should wait a bit on this one, since fix for 32-bit ought >> to appear as well. > > For 32-bit kernel, userspace DS and ES are saved at syscall/interrupt > entry time and reloaded on exit, unlike in 64-bit where they are saved > and loaded at context switch time. Therefore 32-bit is not affected > by the issue this patch addresses. > > It looks to me though, that the ES test program doesn't actually test > what the patch fixes - the segment attributes, like the base address. > It tests just the selector, which shouldn't change across a kernel > entry (with a few exceptions, like signals). If the test is failing, > then it is a different issue from what this patch addresses. It tests it indirectly. The 64-bit code sets the selector to zero if it fails to reload it. Testing the ES base is awkward because it can't be done in 64-bit code at all. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/