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

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Date:   Mon, 8 Jan 2018 07:42:20 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Woodhouse <dwmw@...zon.co.uk>
Cc:     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>, tglx@...utronix.de,
        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 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.

-- 
Josh