From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751376AbaLRS1U (ORCPT ); Thu, 18 Dec 2014 13:27:20 -0500 Received: from mail-lb0-f169.google.com ([209.85.217.169]:56787 "EHLO mail-lb0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751194AbaLRS1T (ORCPT ); Thu, 18 Dec 2014 13:27:19 -0500 MIME-Version: 1.0 In-Reply-To: <549307F0.7090009@zytor.com> References: <549307F0.7090009@zytor.com> From: Andy Lutomirski Date: Thu, 18 Dec 2014 10:26:57 -0800 Message-ID: Subject: Re: [tip:x86/urgent] x86/tls: Don't validate lm in set_thread_area() after all To: "H. Peter Anvin" Cc: "linux-kernel@vger.kernel.org" , Linus Torvalds , Ingo Molnar , Thomas Gleixner , "linux-tip-commits@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 18, 2014 at 8:59 AM, H. Peter Anvin wrote: > On 12/18/2014 03:16 AM, tip-bot for Andy Lutomirski wrote: >> Commit-ID: 3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0 >> Gitweb: http://git.kernel.org/tip/3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0 >> Author: Andy Lutomirski >> AuthorDate: Wed, 17 Dec 2014 14:48:30 -0800 >> Committer: Ingo Molnar >> CommitDate: Thu, 18 Dec 2014 12:12:26 +0100 >> >> x86/tls: Don't validate lm in set_thread_area() after all >> >> It turns out that there's a lurking ABI issue. GCC, when >> compiling this in a 32-bit program: >> >> struct user_desc desc = { >> .entry_number = idx, >> .base_addr = base, >> .limit = 0xfffff, >> .seg_32bit = 1, >> .contents = 0, /* Data, grow-up */ >> .read_exec_only = 0, >> .limit_in_pages = 1, >> .seg_not_present = 0, >> .useable = 0, >> }; >> >> will leave .lm uninitialized. This means that anything in the >> kernel that reads user_desc.lm for 32-bit tasks is unreliable. >> > > No, it won't. However, if you initialize this dynamically field by > field rather than as an initializer, then you are correct. > I tried the code above in function scope. --Andy > -hpa > -- Andy Lutomirski AMA Capital Management, LLC