Re: [PATCH RFC] KVM: x86: tell guests if the exposed SMT topology is trustworthy

[Liran Alon]

> On 7 Nov 2019, at 16:00, Christophe de Dinechin <christophe.de.dinechin@xxxxxxxxx> wrote:
> 
> 
> 
>> On 6 Nov 2019, at 00:25, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>> 
>> On Tue, Nov 05, 2019 at 11:37:50AM -0800, Sean Christopherson wrote:
>>> On Tue, Nov 05, 2019 at 05:17:37PM +0100, Vitaly Kuznetsov wrote:
>>>> Virtualized guests may pick a different strategy to mitigate hardware
>>>> vulnerabilities when it comes to hyper-threading: disable SMT completely,
>>>> use core scheduling, or, for example, opt in for STIBP. Making the
>>>> decision, however, requires an extra bit of information which is currently
>>>> missing: does the topology the guest see match hardware or if it is 'fake'
>>>> and two vCPUs which look like different cores from guest's perspective can
>>>> actually be scheduled on the same physical core. Disabling SMT or doing
>>>> core scheduling only makes sense when the topology is trustworthy.
>>>> 
>>>> Add two feature bits to KVM: KVM_FEATURE_TRUSTWORTHY_SMT with the meaning
>>>> that KVM_HINTS_TRUSTWORTHY_SMT bit answers the question if the exposed SMT
>>>> topology is actually trustworthy. It would, of course, be possible to get
>>>> away with a single bit (e.g. 'KVM_FEATURE_FAKE_SMT') and not lose backwards
>>>> compatibility but the current approach looks more straightforward.
>>> 
>>> I'd stay away from "trustworthy", especially if this is controlled by
>>> userspace.  Whether or not the hint is trustworthy is purely up to the
>>> guest.  Right now it doesn't really matter, but that will change as we
>>> start moving pieces of the host out of the guest's TCB.
>>> 
>>> It may make sense to split the two (or even three?) cases, e.g.
>>> KVM_FEATURE_NO_SMT and KVM_FEATURE_ACCURATE_TOPOLOGY.  KVM can easily
>>> enforce NO_SMT _today_, i.e. allow it to be set if and only if SMT is
>>> truly disabled.  Verifying that the topology exposed to the guest is legit
>>> is a completely different beast.
>> 
>> Scratch the ACCURATE_TOPOLOGY idea, I doubt there's a real use case for
>> setting ACCURATE_TOPOLOGY and not KVM_HINTS_REALTIME.  A feature flag to
>> state that SMT is disabled seems simple and useful.

A bit such as NoNonArchitecturalCoreSharing can be set even when host SMT is enabled.
For example, when host use core-scheduling to group together vCPUs that run as sibling hyperthreads.
Therefore, I wouldnât want to tie the feature-flag semantics to host SMT being enabled/disabled.
Itâs just true that this bit can be set when host SMT is disabled.

> 
> I share that concern about the naming, although I do see some
> value in exposing the cpu_smt_possible() result. I think itâs easier
> to state that something does not work than to state something does
> work.
> 
> Also, with respect to mitigation, we may want to split the two cases
> that Paolo outlined, i.e. have KVM_HINTS_REALTIME,
> KVM_HINTS_CORES_CROSSTALK and
> KVM_HINTS_CORES_LEAKING,
> where CORES_CROSSTALKS indicates there may be some
> cross-talk between what the guest thinks are isolated cores,
> and CORES_LEAKING indicates that cores may leak data
> to some other guest.
> 
> The problem with my approach is that it is shouting âdonât trust meâ
> a bit too loudly.

I donât see a value in exposing CORES_LEAKING to guest. As guest have nothing to do with it.

-Liran