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

Andy Lutomirski
2020-04-27T17:37:56Z

On Sat, Apr 25, 2020 at 3:10 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> On Sat, Apr 25, 2020 at 1:23 PM Joerg Roedel <joro@8bytes.org> 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
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