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

[Andrew Cooper]

On 27/04/2020 18:37, 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
> 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.

More specifically, it is mandatory for hypervisors to intercept #DB to
defend against CVE-2015-8104, unless they're willing to trust the guest
not to tickle that corner case.

This is believed fixed with SEV-SNP to allow the encrypted guest to use
debugging functionality without posing a DoS risk to the host. In this
case, the hypervisor is expected not to intercept #DB.

If #DB is intercepted, and #VC doesn't use IST, malicious userspace can
cause problems with a movss-delayed breakpoint over SYSCALL.

The question basically whether it is worth going to the effort of making
#VC IST and all the problems that entails, to cover one corner case in
earlier hardware.

Ultimately, this depends on whether anyone plans to put SEV-ES into
production on pre SEV-SNP hardware, and if developers using pre-SEV-SNP
hardware are happy with "don't run malicious userspace" or "don't run
malicious kernels and skip the #DB intercept" as a fair tradeoff to
avoid the #VC IST fun.

~Andrew