linux-kernel - Re: [PATCH v4 01/13] x86/retpoline: Add initial retpoline support

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Date:   Fri, 5 Jan 2018 21:53:49 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>
Cc:     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 v4 01/13] x86/retpoline: Add initial retpoline support

On 01/04/18 18:00, David Woodhouse wrote:
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index d4fc98c..1009d1a 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -429,6 +429,19 @@ config GOLDFISH
>         def_bool y
>         depends on X86_GOLDFISH
>  
> +config RETPOLINE
> +	bool "Avoid speculative indirect branches in kernel"
> +	default y
> +	help
> +	  Compile kernel with the retpoline compiler options to guard against
> +	  kernel to user data leaks by avoiding speculative indirect

On first reading, I encountered a parse err^W^W <what the heck did that say>
on "kernel to user data".  I get it after rereading it, but
	kernel-to-user data leaks
would be better. (IMHO)

> +	  branches. Requires a compiler with -mindirect-branch=thunk-extern
> +	  support for full protection. The kernel may run slower.
> +
> +	  Without compiler support, at least indirect branches in assembler
> +	  code are eliminated. Since this includes the syscall entry path,
> +	  it is not entirely pointless.


-- 
~Randy