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

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

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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