From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934200AbeAMKqv (ORCPT + 1 other); Sat, 13 Jan 2018 05:46:51 -0500 Received: from smtp-fw-4101.amazon.com ([72.21.198.25]:37303 "EHLO smtp-fw-4101.amazon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934151AbeAMKqr (ORCPT ); Sat, 13 Jan 2018 05:46:47 -0500 X-Amazon-filename: smime.p7s X-IronPort-AV: E=Sophos;i="5.46,352,1511827200"; d="p7s'?scan'208";a="703171988" Message-ID: <1515840369.22302.523.camel@amazon.co.uk> Subject: Re: [PATCH v1] x86/retpoline: Use lfence in the retpoline/RSB filling RSB macros To: Tom Lendacky , , , "Van De Ven, Arjan" , "asit.k.mallick@intel.com" CC: Rik van Riel , Andi Kleen , "Josh Poimboeuf" , Peter Zijlstra , Linus Torvalds , Jiri Kosina , Dan Williams , Dave Hansen , Borislav Petkov , Andy Lutomirski , Kees Cook , Thomas Gleixner , Tim Chen , "Greg Kroah-Hartman" , Paul Turner In-Reply-To: <20180113010728.27928.8537.stgit@tlendack-t1.amdoffice.net> References: <20180113010728.27928.8537.stgit@tlendack-t1.amdoffice.net> Face: 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 Organization: Amazon Content-Type: multipart/signed; micalg=sha-256; protocol="application/x-pkcs7-signature"; boundary="=-cBe/5TCcwp30zs5yF6k0" Date: Sat, 13 Jan 2018 10:46:09 +0000 X-Mailer: Evolution 3.18.5.2-0ubuntu3.2 MIME-Version: 1.0 From: "Woodhouse, David" X-Evolution-Source: 1481217574.17989.4@uc8d3ff76b9bc5848a9cc Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: --=-cBe/5TCcwp30zs5yF6k0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2018-01-12 at 19:07 -0600, Tom Lendacky wrote: > The pause instruction is currently used in the retpoline and RSB filling > macros as a speculation trap.=C2=A0=C2=A0The use of pause was originally = suggested > because it showed a very, very small difference in the amount of > cycles/time used to execute the retpoline as compared to lfence.=C2=A0=C2= =A0On AMD, > the pause instruction is not a serializing instruction, so the pause/jmp > loop will use excess power as it is speculated over waiting for return > to mispredict to the correct target. >=20 > The RSB filling macro is applicable to AMD, and, if software is unable to > verify that lfence is serializing on AMD (possible when running under a > hypervisor), the generic retpoline support will be used and, so, is also > applicable to AMD.=C2=A0=C2=A0Change the use of pause to lfence. >=20 > Signed-off-by: Tom Lendacky Conditionally-Acked-by: David Woodhouse The condition being, as noted, that I'd really like to see it acked by Arjan/Asit and Paul. > --- > =C2=A0arch/x86/include/asm/nospec-branch.h |=C2=A0=C2=A0=C2=A010 +++++---= -- > =C2=A01 file changed, 5 insertions(+), 5 deletions(-) >=20 > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/= nospec-branch.h > index 402a11c..2c4a09a 100644 > --- a/arch/x86/include/asm/nospec-branch.h > +++ b/arch/x86/include/asm/nospec-branch.h > @@ -11,7 +11,7 @@ > =C2=A0 * Fill the CPU return stack buffer. > =C2=A0 * > =C2=A0 * Each entry in the RSB, if used for a speculative 'ret', contains= an > - * infinite 'pause; jmp' loop to capture speculative execution. > + * infinite 'lfence; jmp' loop to capture speculative execution. > =C2=A0 * > =C2=A0 * This is required in various cases for retpoline and IBRS-based > =C2=A0 * mitigations for the Spectre variant 2 vulnerability. Sometimes t= o > @@ -37,12 +37,12 @@ > =C2=A0771: \ > =C2=A0 call 772f; \ > =C2=A0773: /* speculation trap */ \ > - pause; \ > + lfence; \ > =C2=A0 jmp 773b; \ > =C2=A0772: \ > =C2=A0 call 774f; \ > =C2=A0775: /* speculation trap */ \ > - pause; \ > + lfence; \ > =C2=A0 jmp 775b; \ > =C2=A0774: \ > =C2=A0 dec reg; \ > @@ -72,7 +72,7 @@ > =C2=A0.macro RETPOLINE_JMP reg:req > =C2=A0 call .Ldo_rop_\@ > =C2=A0.Lspec_trap_\@: > - pause > + lfence > =C2=A0 jmp .Lspec_trap_\@ > =C2=A0.Ldo_rop_\@: > =C2=A0 mov \reg, (%_ASM_SP) > @@ -164,7 +164,7 @@ > =C2=A0 "=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0jmp=C2=A0=C2=A0=C2=A0= =C2=A0904f;\n" \ > =C2=A0 "=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0.align 16\n" \ > =C2=A0 "901: call=C2=A0=C2=A0=C2=A0903f;\n" \ > - "902: pause;\n" \ > + "902: lfence;\n" \ > =C2=A0 "=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0jmp=C2=A0=C2=A0=C2=A0= =C2=A0902b;\n" \ > =C2=A0 "=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0.align 16\n" \ > =C2=A0 "903: addl=C2=A0=C2=A0=C2=A0$4, 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