Re: [RFC PATCH V6 01/14] x86/sev: Add a #HV exception handler

[Peter Zijlstra]

On Tue, May 30, 2023 at 10:59:01AM -0500, Tom Lendacky wrote:
> On 5/30/23 09:35, Peter Zijlstra wrote:
> > On Tue, May 30, 2023 at 02:16:55PM +0200, Gupta, Pankaj wrote:
> > > 
> > > > > Add a #HV exception handler that uses IST stack.
> > > > > 
> > > > 
> > > > Urgh.. that is entirely insufficient. Like it doesn't even begin to
> > > > start to cover things.
> > > > 
> > > > The whole existing VC IST stack abuse is already a nightmare and you're
> > > > duplicating that.. without any explanation for why this would be needed
> > > > and how it is correct.
> > > > 
> > > > Please try again.
> > > 
> > > #HV handler handles both #NMI & #MCE in the guest and nested #HV is never
> > > raised by the hypervisor.
> > 
> > I thought all this confidental computing nonsense was about not trusting
> > the hypervisor, so how come we're now relying on the hypervisor being
> > sane?
> 
> That should really say that a nested #HV should never be raised by the
> hypervisor, but if it is, then the guest should detect that and
> self-terminate knowing that the hypervisor is possibly being malicious.

I've yet to see code that can do that reliably.