Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752692AbdGMVMs (ORCPT ); Thu, 13 Jul 2017 17:12:48 -0400 Received: from mail-pg0-f46.google.com ([74.125.83.46]:33399 "EHLO mail-pg0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751271AbdGMVMr (ORCPT ); Thu, 13 Jul 2017 17:12:47 -0400 Date: Thu, 13 Jul 2017 14:12:45 -0700 From: Matthias Kaehlcke To: Josh Poimboeuf Cc: Andrey Rybainin , Chris J Arges , Borislav Petkov , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , x86@kernel.org, linux-kernel@vger.kernel.org, Douglas Anderson , Michael Davidson , Greg Hackmann , Nick Desaulniers , Stephen Hines , Kees Cook , Arnd Bergmann , Bernhard.Rosenkranzer@linaro.org Subject: Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get_user() inline asm" Message-ID: <20170713211245.GG95735@google.com> References: <20170712212744.23660-1-mka@chromium.org> <20170712221242.puv5illztsla4nph@treble> <20170712222040.GD95735@google.com> <20170712223547.fyra43dizqooosbs@treble> <20170712223630.gb237t4vhrqeu5zd@treble> <20170712232213.GE95735@google.com> <20170713180001.mvwzdmudht56hdk5@treble> <20170713184748.GF95735@google.com> <75850bb7-a60e-057d-d88b-afa0c79e94a0@gmail.com> <20170713203416.isvijqbwbcgupgj7@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20170713203416.isvijqbwbcgupgj7@treble> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2485 Lines: 58 El Thu, Jul 13, 2017 at 03:34:16PM -0500 Josh Poimboeuf ha dit: > On Thu, Jul 13, 2017 at 11:20:04PM +0300, Andrey Rybainin wrote: > > On 07/13/2017 09:47 PM, Matthias Kaehlcke wrote: > > > > > Thanks for your analysis! > > > > > >> What happens if you try the below patch instead of the revert? Any > > >> chance the offending instruction goes away? > > >> > > >> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h > > >> index 11433f9..beac907 100644 > > >> --- a/arch/x86/include/asm/uaccess.h > > >> +++ b/arch/x86/include/asm/uaccess.h > > >> @@ -171,7 +171,7 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL)) > > >> might_fault(); \ > > >> asm volatile("call __get_user_%P4" \ > > >> : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp) \ > > >> - : "0" (ptr), "i" (sizeof(*(ptr)))); \ > > >> + : "0" (ptr), "i" (sizeof(*(ptr))), "r" (__sp)); \ > > >> (x) = (__force __typeof__(*(ptr))) __val_gu; \ > > >> __builtin_expect(__ret_gu, 0); \ > > >> }) > > > > > > The generated code is basically the same, only that now the value from > > > the stack is stored in a register and written twice to RSP: > > > > > > > AFAIR clang works much better with global named registers. > > Could you check if the patch bellow helps? > > And yet another one to try (clobbering sp) :-) > > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h > index 11433f9..21f0c39 100644 > --- a/arch/x86/include/asm/uaccess.h > +++ b/arch/x86/include/asm/uaccess.h > @@ -166,12 +166,12 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL)) > ({ \ > int __ret_gu; \ > register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX); \ > - register void *__sp asm(_ASM_SP); \ > __chk_user_ptr(ptr); \ > might_fault(); \ > - asm volatile("call __get_user_%P4" \ > - : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp) \ > - : "0" (ptr), "i" (sizeof(*(ptr)))); \ > + asm volatile("call __get_user_%P3" \ > + : "=a" (__ret_gu), "=r" (__val_gu) \ > + : "0" (ptr), "i" (sizeof(*(ptr))) \ > + : "sp"); \ > (x) = (__force __typeof__(*(ptr))) __val_gu; \ > __builtin_expect(__ret_gu, 0); \ > }) This compiles with both gcc and clang, clang does not corrupt the stack pointer. I wouldn't be able to tell though if it forces a stack frame if it doesn't already exist, as the original patch intends.