Re: [PATCH v3 5/8] x86/sme: Replace occurrences of sme_active() with cc_platform_has()

[Tom Lendacky]

On 9/24/21 4:51 AM, Borislav Petkov wrote:
> On Fri, Sep 24, 2021 at 12:41:32PM +0300, Kirill A. Shutemov wrote:
> > On Thu, Sep 23, 2021 at 08:21:03PM +0200, Borislav Petkov wrote:
> > > On Thu, Sep 23, 2021 at 12:05:58AM +0300, Kirill A. Shutemov wrote:
> > > > Unless we find other way to guarantee RIP-relative access, we must use
> > > > fixup_pointer() to access any global variables.
> > >
> > > Yah, I've asked compiler folks about any guarantees we have wrt
> > > rip-relative addresses but it doesn't look good. Worst case, we'd have
> > > to do the fixup_pointer() thing.
> > >
> > > In the meantime, Tom and I did some more poking at this and here's a
> > > diff ontop.
> > >
> > > The direction being that we'll stick both the AMD and Intel
> > > *cc_platform_has() call into cc_platform.c for which instrumentation
> > > will be disabled so no issues with that.
> > >
> > > And that will keep all that querying all together in a single file.
> >
> > And still do cc_platform_has() calls in __startup_64() codepath?
> >
> > It's broken.
> >
> > Intel detection in cc_platform_has() relies on boot_cpu_data.x86_vendor
> > which is not initialized until early_cpu_init() in setup_arch(). Given
> > that X86_VENDOR_INTEL is 0 it leads to false-positive.
>
> Yeah, Tom, I had the same question yesterday.
>
> /me looks in his direction.

Yup, all the things we talked about.

But we also know that cc_platform_has() gets called a few other times 
before boot_cpu_data is initialized - so more false-positives. For 
cc_platform_has() to work properly, given the desire to consolidate the 
calls, IMO, Intel will have to come up with some early setting that can be 
enabled and checked in place of boot_cpu_data or else you live with 
false-positives.

Thanks,
Tom

> :-)