* Borislav Petkov (bp@alien8.de) wrote:

On Thu, Oct 21, 2021 at 06:12:53PM +0100, Dr. David Alan Gilbert wrote:

quoted

OK, so that bit is 8...21 Eax ext2eax bit 6 page 1-109

then 2.1.5.3 CPUID policy enforcement shows 8...21 EAX as
'bitmask'
'bits set in the GuestVal must also be set in HostVal.
This is often applied to feature fields where each bit indicates
support for a feature'

So that's right isn't it?

Yap, AFAIRC, it would fail the check if:

(GuestVal & HostVal) != GuestVal

and GuestVal is "the CPUID result value created by the hypervisor that
it wants to give to the guest". Let's say it clears bit 6 there.

                                               ^^^^^^^

Then HostVal comes in which is "the actual CPUID result value specified
in this PPR" and there the guest catches the HV lying its *ss off.

:-)

Hang on, I think it's perfectly fine for it to clear that bit - it just
gets caught if it *sets* it (i.e. claims to be a chip unaffected by the
bug).

i.e. if guestval=0 then (GustVal & whatever) == GuestVal
  fine

?

Dave

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK