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

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Date:   Mon, 8 Jan 2018 14:26:11 -0000
From:   "David Woodhouse" <dwmw2@...radead.org>
To:     "Josh Poimboeuf" <jpoimboe@...hat.com>
Cc:     "Thomas Gleixner" <tglx@...utronix.de>,
        "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'.

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