linux-kernel - Re: [PATCH v6 01/10] x86/retpoline: Add initial retpoline support

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Message-ID: <20180108135305.awviy2ae3gxh4ybl@treble>
Date:   Mon, 8 Jan 2018 07:53:05 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v6 01/10] x86/retpoline: Add initial retpoline support

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'.

-- 
Josh