Re: [PATCH v2] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL

[Borislav Petkov]

On Mon, Oct 18, 2021 at 07:29:41PM +0000, Sean Christopherson wrote:
> This isn't correct.  When running as a guest, the intended behavior is to fully
> trust the CPUID.0x80000021 bit.

Really? Because I'm coming from an SEV-SNP mail thread where we don't
trust the HV at all and we even hand in a CPUID page into the guest...

:-P

> If bit 6 is set, yay, the hypervisor has told the kernel that it
> will only ever run on hardware without the bug. If bit 6 is clear
> and HYPERVISOR is true, then the FMS crud can't be trusted because
> the kernel _may_ run on affected hardware in the future even if the
> current underlying hardware is not affected.

Ok, I see, then the CPUID check needs to go first, makes sense.

> I agree.  If the argument for this patch is that the kernel can be migrated to
> older hardware, then it stands to reason that the kernel could also be migrated
> to a different CPU vendor entirely.  E.g. start on Intel, migrate to Zen1, kaboom.

Migration across vendors? Really, that works?

I'll believe it only when I see it with my own eyes.

:-)

-- 
Regards/Gruss,
    Boris.

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