Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752758AbdIBRTr (ORCPT ); Sat, 2 Sep 2017 13:19:47 -0400 Received: from mga11.intel.com ([192.55.52.93]:62718 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752737AbdIBRTp (ORCPT ); Sat, 2 Sep 2017 13:19:45 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.41,464,1498546800"; d="scan'208";a="1190901145" Message-ID: <1504372784.19040.1.camel@ranerica-desktop> Subject: Re: [PATCH v8 06/28] x86/mpx: Do not use SIB.index if its value is 100b and ModRM.mod is not 11b From: Ricardo Neri To: Borislav Petkov 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, Adam Buchbinder , Colin Ian King , Lorenzo Stoakes , Qiaowei Ren , Nathan Howard , Adan Hawthorn , Joe Perches Date: Sat, 02 Sep 2017 10:19:44 -0700 In-Reply-To: <20170831193824.volcxn4i2fmpdtiq@pd.tnic> References: <20170819002809.111312-1-ricardo.neri-calderon@linux.intel.com> <20170819002809.111312-7-ricardo.neri-calderon@linux.intel.com> <20170831193824.volcxn4i2fmpdtiq@pd.tnic> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.10.4-0ubuntu2 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2629 Lines: 57 On Thu, 2017-08-31 at 21:38 +0200, Borislav Petkov wrote: > 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 Thanks for your review!