linux-kernel - Re: [PATCH v2.1] x86/retpoline: Fill return stack buffer on vmexit

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Message-ID: <20180111150419.2nsqst6qpifeq7k5@treble>
Date:   Thu, 11 Jan 2018 09:04:19 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk,
        x86@...nel.org, bp@...en8.de, rga@...zon.de,
        thomas.lendacky@....com
Subject: Re: [PATCH v2.1] x86/retpoline: Fill return stack buffer on vmexit

On Thu, Jan 11, 2018 at 02:53:57PM +0000, David Woodhouse wrote:
> On Thu, 2018-01-11 at 15:32 +0100, Peter Zijlstra wrote:
> > On Thu, Jan 11, 2018 at 02:28:32PM +0000, David Woodhouse wrote:
> > > 
> > > On Thu, 2018-01-11 at 08:20 -0600, Josh Poimboeuf wrote:
> > > > 
> > > > 
> > > > This seems weird.  I liked v1 a lot better.  What's the problem with
> > > > patching in the whole thing?
> > > > 
> > > > Also, if you go back to v1, it should be an easy objtool fix, just add
> > > > ANNOTATE_NOSPEC_ALTERNATIVE in front of it.
> > > The objection was that I was patching in a fairly long set of
> > > instructions. I confess I don't actually know why that's a problem,
> >
> > You get a giant string of NOPs, a single jmp should be faster.
> 
> How about this one then (with ANNOTATE_NOSPEC_ALTERNATIVE):
> 
> -       asm volatile (ALTERNATIVE("",
> +       asm volatile (ALTERNATIVE("jmp " alt_end_marker "f",
>                                   __stringify(__FILL_RETURN_BUFFER(%0, %1, _%=)),
>                                   X86_FEATURE_RETPOLINE)
>                       : "=r" (dummy), ASM_CALL_CONSTRAINT : : "memory" );

Looks good to me.

Another IRC discussion was that Boris may eventually add a feature to
the alternatives code to automatically insert such a jump if there are a
lot of nops.

-- 
Josh