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

Tom Lendacky
2020-04-27T18:43:33Z

On 4/27/20 12:37 PM, Andy Lutomirski wrote:
> 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

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
>