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

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Message-ID: <1515680912.22302.351.camel@infradead.org>
Date:   Thu, 11 Jan 2018 14:28:32 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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,
        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, 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, but
once I looked at it I realised the alignment was broken again. Again,
alignment in the altinstr section doesn't necessarily mean alignment
when it's copied into place over the oldinstr.

I took a quick look at doing it out-of-line and calling it... and
exporting it... and defining it to take *one* register rather than
being a macro... and ditched that approach then ended up with what's in
v2.


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