Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753633AbdDKWIm (ORCPT ); Tue, 11 Apr 2017 18:08:42 -0400 Received: from mx2.suse.de ([195.135.220.15]:35495 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753228AbdDKWIk (ORCPT ); Tue, 11 Apr 2017 18:08:40 -0400 Date: Wed, 12 Apr 2017 00:08:16 +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 , Alexandre Julliard , Stas Sergeev , Fenghua Yu , "Ravi V. Shankar" , Shuah Khan , linux-kernel@vger.kernel.org, x86@kernel.org, linux-msdos@vger.kernel.org, wine-devel@winehq.org, Adam Buchbinder , Colin Ian King , Lorenzo Stoakes , Qiaowei Ren , Nathan Howard , Adan Hawthorn , Joe Perches Subject: Re: [v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB byte with Mod = 0 Message-ID: <20170411220816.2u3o72qnwcwq7jzc@pd.tnic> References: <20170308003254.27833-1-ricardo.neri-calderon@linux.intel.com> <20170308003254.27833-4-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: <20170308003254.27833-4-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: 3425 Lines: 92 On Tue, Mar 07, 2017 at 04:32:36PM -0800, 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 a SIB byte is used and the > base of the SIB byte points to R/EBP (i.e., base = 5) and the mod part > of the ModRM byte is zero, the value of such register will not be used > as part of the address computation. To signal this, a -EDOM error is > returned to indicate callers that they should ignore the value. > > Also, for this particular case, a displacement of 32-bits should follow > the SIB byte if the mod part of ModRM is equal to zero. The instruction > decoder ensures that this is the case. > > 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 | 29 ++++++++++++++++++++++------- > 1 file changed, 22 insertions(+), 7 deletions(-) > > diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c > index d9e92d6..ef7eb67 100644 > --- a/arch/x86/mm/mpx.c > +++ b/arch/x86/mm/mpx.c > @@ -121,6 +121,17 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs, > > case REG_TYPE_BASE: > regno = X86_SIB_BASE(insn->sib.value); > + /* > + * If mod is 0 and register R/EBP (regno=5) is indicated in the > + * base part of the SIB byte, you can simply say here: "if SIB.base == 5, the base of the register-indirect addressing is 0." > the value of such register should > + * not be used in the address computation. Also, a 32-bit Not "Also" but "In this case, a 32-bit displacement..." > + * displacement is expected in this case; the instruction > + * decoder takes care of it. This is true for both R13 and > + * R/EBP as REX.B will not be decoded. You don't need that sentence as the only thing that matters is ModRM.mod being 0. > + */ > + if (regno == 5 && X86_MODRM_MOD(insn->modrm.value) == 0) The 0 test we normally do with the ! (also flip parts of if-condition): if (!X86_MODRM_MOD(insn->modrm.value) && regno == 5) > + return -EDOM; > + > if (X86_REX_B(insn->rex_prefix.value)) > regno += 8; > break; > @@ -161,16 +172,21 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs) > eff_addr = regs_get_register(regs, addr_offset); > } else { > if (insn->sib.nbytes) { > + /* > + * Negative values in the base and index offset means > + * an error when decoding the SIB byte. Except -EDOM, > + * which means that the registers should not be used > + * in the address computation. > + */ > base_offset = get_reg_offset(insn, regs, REG_TYPE_BASE); > - if (base_offset < 0) > + if (unlikely(base_offset == -EDOM)) > + base = 0; > + else if (unlikely(base_offset < 0)) Bah, unlikely's in something which is not really a hot path. They only encumber readability, no need for them. -- Regards/Gruss, Boris. SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) --