linux-kernel - Re: [tip:x86/pti] x86/retpoline: Fill return stack buffer on vmexit

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Message-ID: <20180125173224.ynvqjriah73cfk7q@treble>
Date:   Thu, 25 Jan 2018 11:32:24 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Borislav Petkov <bp@...en8.de>, tim.c.chen@...ux.intel.com,
        pjt@...gle.com, jikos@...nel.org, gregkh@...ux-foundation.org,
        dave.hansen@...el.com, mingo@...nel.org, riel@...hat.com,
        luto@...capital.net, torvalds@...ux-foundation.org,
        ak@...ux.intel.com, keescook@...gle.com, peterz@...radead.org,
        tglx@...utronix.de, hpa@...or.com, linux-kernel@...r.kernel.org,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/pti] x86/retpoline: Fill return stack buffer on vmexit

On Thu, Jan 25, 2018 at 05:00:39PM +0000, David Woodhouse wrote:
> On Thu, 2018-01-25 at 10:56 -0600, Josh Poimboeuf wrote:
> > On Thu, Jan 25, 2018 at 04:03:18PM +0000, David Woodhouse wrote:
> > > On Thu, 2018-01-25 at 16:51 +0100, Borislav Petkov wrote:
> > > > 
> > > > > And the seg fault is objtool's way of telling you you need a
> > > > > ANNOTATE_NOSPEC_ALTERNATIVE above the alternative ;-)
> > > > 
> > > > Except that it blew up when I did this which doesn't have ALTERNATIVE
> > > > (it's the diff I saved :-))
> > > 
> > > Yeah, ANNOTATE_NOSPEC_ALTERNATIVE just tells objtool "don't look at the
> > > alternative; you're not going to like it".
> > > 
> > > If you start putting a __fill_rsb() function out of line somewhere and
> > > only *calling* it from alternatives, then objtool is going to shit
> > > itself when it sees that function, regardless.
> > 
> > Right, if you *really* want it always inline, the short term solution is
> > to just patch it in with X86_FEATURE_ALWAYS.
> 
> And the whole problem here is that patching it in with alternatives is
> painful on kernels < 4.1 because back then, we didn't cope with
> oldinstr and altinstr being different lengths.

We just manually added the nops for the in-line path, that should be
good enough.

-- 
Josh