Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757870AbeAHO0a (ORCPT + 1 other); Mon, 8 Jan 2018 09:26:30 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:43506 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757393AbeAHO03 (ORCPT ); Mon, 8 Jan 2018 09:26:29 -0500 Message-ID: In-Reply-To: <20180108135305.awviy2ae3gxh4ybl@treble> References: <1515363085-4219-1-git-send-email-dwmw@amazon.co.uk> <1515363085-4219-2-git-send-email-dwmw@amazon.co.uk> <20180108134220.2for3pwgqbmkkq2s@treble> <20180108135305.awviy2ae3gxh4ybl@treble> Date: Mon, 8 Jan 2018 14:26:11 -0000 Subject: Re: [PATCH v6 01/10] x86/retpoline: Add initial retpoline support From: "David Woodhouse" To: "Josh Poimboeuf" Cc: "Thomas Gleixner" , "David Woodhouse" , "Andi Kleen" , "Paul Turner" , "LKML" , "Linus Torvalds" , "Greg Kroah-Hartman" , "Tim Chen" , "Dave Hansen" , "Kees Cook" , "Rik van Riel" , "Peter Zijlstra" , "Andy Lutomirski" , "Jiri Kosina" , gnomes@lxorguk.ukuu.org.uk User-Agent: SquirrelMail/1.4.22-21.fc27 MIME-Version: 1.0 Content-Type: text/plain;charset=utf-8 Content-Transfer-Encoding: 8bit X-Priority: 3 (Normal) Importance: Normal X-SRS-Rewrite: SMTP reverse-path rewritten from by twosheds.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: > On Mon, Jan 08, 2018 at 02:46:32PM +0100, Thomas Gleixner wrote: >> On Mon, 8 Jan 2018, Josh Poimboeuf wrote: >> > On Sun, Jan 07, 2018 at 10:11:16PM +0000, David Woodhouse wrote: >> > > diff --git a/arch/x86/Makefile b/arch/x86/Makefile >> > > index a20eacd..918e550 100644 >> > > --- a/arch/x86/Makefile >> > > +++ b/arch/x86/Makefile >> > > @@ -235,6 +235,16 @@ KBUILD_CFLAGS += -Wno-sign-compare >> > > # >> > > KBUILD_CFLAGS += -fno-asynchronous-unwind-tables >> > > >> > > +# Avoid indirect branches in kernel to deal with Spectre >> > > +ifdef CONFIG_RETPOLINE >> > > + RETPOLINE_CFLAGS += $(call >> cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register) >> > > + ifneq ($(RETPOLINE_CFLAGS),) >> > > + KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE >> > > + else >> > > + $(warning Retpoline not supported in compiler. System may >> be insecure.) >> > > + endif >> > > +endif >> > >> > I wonder if an error might be more appropriate than a warning. I >> > learned from experience that a lot of people don't see these Makefile >> > warnings, and this would be a dangerous one to miss. >> > >> > Also if this were an error, you could get rid of the RETPOLINE define, >> > and that would be one less define cluttering up the already >> way-too-long >> > GCC arg list. >> >> It still allows to get the ASM part covered. If that's worth it I can't >> tell. > > If there's a makefile error above, then CONFIG_RETPOLINE would already > imply compiler support, so the ASM code with the new '%V' option could > just do 'ifdef CONFIG_RETPOLINE'. I did look at ditching the -DRETPOLINE but there is benefit in doing the sys_call_table jump even when GCC isn't updated. So I put it back. -- dwmw2