Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751910AbdHaWGr (ORCPT ); Thu, 31 Aug 2017 18:06:47 -0400 Received: from mx2.suse.de ([195.135.220.15]:38508 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751816AbdHaWGo (ORCPT ); Thu, 31 Aug 2017 18:06:44 -0400 Date: Thu, 31 Aug 2017 21:38:24 +0200 From: Borislav Petkov To: Ricardo Neri Cc: Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Andrew Morton , Brian Gerst , Chris Metcalf , Dave Hansen , Paolo Bonzini , Masami Hiramatsu , Huang Rui , Jiri Slaby , Jonathan Corbet , "Michael S. Tsirkin" , Paul Gortmaker , Vlastimil Babka , Chen Yucong , "Ravi V. Shankar" , Shuah Khan , linux-kernel@vger.kernel.org, x86@kernel.org, ricardo.neri@intel.com, Adam Buchbinder , Colin Ian King , Lorenzo Stoakes , Qiaowei Ren , Nathan Howard , Adan Hawthorn , Joe Perches Subject: Re: [PATCH v8 06/28] x86/mpx: Do not use SIB.index if its value is 100b and ModRM.mod is not 11b Message-ID: <20170831193824.volcxn4i2fmpdtiq@pd.tnic> References: <20170819002809.111312-1-ricardo.neri-calderon@linux.intel.com> <20170819002809.111312-7-ricardo.neri-calderon@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20170819002809.111312-7-ricardo.neri-calderon@linux.intel.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2571 Lines: 60 On Fri, Aug 18, 2017 at 05:27:47PM -0700, Ricardo Neri wrote: > Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software > Developer's Manual volume 2A states that when ModRM.mod !=11b and > ModRM.rm = 100b indexed register-indirect addressing is used. In other > words, a SIB byte follows the ModRM byte. In the specific case of > SIB.index = 100b, the scale*index portion of the computation of the > effective address is null. To signal callers of this particular situation, > get_reg_offset() can return -EDOM (-EINVAL continues to indicate that an > error when decoding the SIB byte). > > An example of this situation can be the following instruction: > > 8b 4c 23 80 mov -0x80(%rbx,%riz,1),%rcx > ModRM: 0x4c [mod:1b][reg:1b][rm:100b] > SIB: 0x23 [scale:0b][index:100b][base:11b] > Displacement: 0x80 (1-byte, as per ModRM.mod = 1b) > > The %riz 'register' indicates a null index. > > In long mode, a REX prefix may be used. When a REX prefix is present, > REX.X adds a fourth bit to the register selection of SIB.index. This gives > the ability to refer to all the 16 general purpose registers. When REX.X is > 1b and SIB.index is 100b, the index is indicated in %r12. In our example, > this would look like: > > 42 8b 4c 23 80 mov -0x80(%rbx,%r12,1),%rcx > REX: 0x42 [W:0b][R:0b][X:1b][B:0b] > ModRM: 0x4c [mod:1b][reg:1b][rm:100b] > SIB: 0x23 [scale:0b][.X: 1b, index:100b][.B:0b, base:11b] > Displacement: 0x80 (1-byte, as per ModRM.mod = 1b) > > %r12 is a valid register to use in the scale*index part of the effective > address computation. > > Cc: Borislav Petkov > Cc: Andy Lutomirski > Cc: Dave Hansen > Cc: Adam Buchbinder > Cc: Colin Ian King > Cc: Lorenzo Stoakes > Cc: Qiaowei Ren > Cc: Peter Zijlstra > Cc: Nathan Howard > Cc: Adan Hawthorn > Cc: Joe Perches > Cc: Ravi V. Shankar > Cc: x86@kernel.org > Signed-off-by: Ricardo Neri > --- > arch/x86/mm/mpx.c | 22 ++++++++++++++++++++-- > 1 file changed, 20 insertions(+), 2 deletions(-) Reviewed-by: Borislav Petkov -- Regards/Gruss, Boris. SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) --