Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)

[Tom Lendacky]

On 4/27/20 12:37 PM, Andy Lutomirski wrote:
> On Sat, Apr 25, 2020 at 3:10 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > On Sat, Apr 25, 2020 at 1:23 PM Joerg Roedel <joro@xxxxxxxxxx> wrote:
> > > On Sat, Apr 25, 2020 at 12:47:31PM -0700, Andy Lutomirski wrote:
> > > > I assume the race you mean is:
> > > >
> > > > #VC
> > > > Immediate NMI before IST gets shifted
> > > > #VC
> > > >
> > > > Kaboom.
> > > >
> > > > How are you dealing with this?  Ultimately, I think that NMI will need
> > > > to turn off IST before engaging in any funny business. Let me ponder
> > > > this a bit.
> > >
> > > Right, I dealt with that by unconditionally shifting/unshifting the #VC IST entry
> > > in do_nmi() (thanks to Davin Kaplan for the idea). It might cause
> > > one of the IST stacks to be unused during nesting, but that is fine. The
> > > stack memory for #VC is only allocated when SEV-ES is active (in an
> > > SEV-ES VM).
> >
> > Blech.  It probably works, but still, yuck.  It's a bit sad that we
> > seem to be growing more and more poorly designed happens-anywhere
> > exception types at an alarming rate.  We seem to have #NM, #MC, #VC,
> > #HV, and #DB.  This doesn't really scale.
>
> I have a somewhat serious question: should we use IST for #VC at all?
> As I understand it, Rome and Naples make it mandatory for hypervisors
> to intercept #DB, which means that, due to the MOV SS mess, it's sort
> of mandatory to use IST for #VC.  But Milan fixes the #DB issue, so,
> if we're running under a sufficiently sensible hypervisor, we don't
> need IST for #VC.
>
> So I think we have two choices:
>
> 1. Use IST for #VC and deal with all the mess that entails.
>
> 2. Say that we SEV-ES client support on Rome and Naples is for
> development only and do a quick boot-time check for whether #DB is
> intercepted.  (Just set TF and see what vector we get.)  If #DB is
> intercepted, print a very loud warning and refuse to boot unless some
> special sev_es.insecure_development_mode or similar option is set.
>
> #2 results in simpler and more robust entry code.  #1 is more secure.
>
> So my question is: will anyone actually use SEV-ES in production on
> Rome or Naples?  As I understand it, it's not really ready for prime
> time on those chips.  And do we care if the combination of a malicious

Naples was limited in the number of encryption keys available for guests 
(15), but Rome increased that significantly (509). SEV-ES is ready on 
those chips - Rome more so with the increased key count given the 
requirement that SEV and SEV-ES guests have non-overlapping ASID ranges 
(which corresponds to key usage).

Thanks,
Tom

> hypervisor and malicious guest userspace on Milan can compromise the
> guest kernel?  I don't think SEV-ES is really mean to resist a
> concerted effort by the hypervisor to compromise the guest.
>
> --Andy