Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1434618AbdDZCEd (ORCPT ); Tue, 25 Apr 2017 22:04:33 -0400 Received: from mga11.intel.com ([192.55.52.93]:2855 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1434602AbdDZCEW (ORCPT ); Tue, 25 Apr 2017 22:04:22 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.37,252,1488873600"; d="scan'208";a="961111957" Message-ID: <1493172260.36058.26.camel@ranerica-desktop> Subject: Re: [v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB byte with Mod = 0 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 , 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 Date: Tue, 25 Apr 2017 19:04:20 -0700 In-Reply-To: <20170411220816.2u3o72qnwcwq7jzc@pd.tnic> References: <20170308003254.27833-1-ricardo.neri-calderon@linux.intel.com> <20170308003254.27833-4-ricardo.neri-calderon@linux.intel.com> <20170411220816.2u3o72qnwcwq7jzc@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: 3859 Lines: 102 On Wed, 2017-04-12 at 00:08 +0200, Borislav Petkov wrote: > 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." This is better wording. I will change it. > > > 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..." Will change. > > > + * 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. For the specific case of ModRM.mod being 0, I feel I need to clarify that REX.B is not decoded and if SIB.base is %r13 the base is also 0. This comment adds clarity because REX.X is decoded when determining SIB.index. > > > + */ > > + 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) Will change it. > > > + 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. I will remove them. Thanks and BR, Ricardo