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

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Date:   Thu, 25 Jan 2018 17:00:39 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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, 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.

And they don't want to fix *that* because kABI...

I just stopped caring.
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