Re: [RFC PATCH v2 02/11] KVM: x86: Advertise CPUID.7.2.EDX and RRSBA_CTRL support

[Xiaoyao Li]

On 5/16/2023 5:09 PM, Chao Gao wrote:
> On Tue, May 16, 2023 at 03:03:15PM +0800, Xiaoyao Li wrote:
> > On 5/16/2023 11:01 AM, Chao Gao wrote:
> > > On Tue, May 16, 2023 at 10:22:22AM +0800, Xiaoyao Li wrote:
> > > > > > I think we need to fix this bug at first.
> > > > >
> > > > > I have no idea how to fix the "bug" without intercepting the MSR. The
> > > > > performance penalty makes me think intercepting the MSR is not a viable
> > > > > solution.
> > > >
> > > > I thought correctness always takes higher priority over performance.
> > >
> > > It is generally true. however, there are situations where we should make
> > > trade-offs between correctness and other factors (like performance):
> > >
> > > E.g., instructions without control bits, to be 100% compliant with CPU
> > > spec, in theory, VMMs can trap/decode every instruction and inject #UD
> > > if a guest tries to use some instructions it shouldn't.
> >
> > This is the virtualization hole. IMHO, they are different things.
>
> what are the differences between?
> 1. Executing some unsupported instructions should cause #UD. But this is allowed
>     in a KVM guest.
> 2. Setting some reserved bits in SPEC_CTRL MSR should cause #GP. But this is
>     allowed in a KVM guest.

The difference is that for virtualization hole, there is no way but 
intercept and decode every instruction if we want the correctness. It's 
a disaster.

But for MSR virtualization, we do have an option and we don't need to 
trap every instruction. MSR interception is the designated mechanism to 
correctly and elegantly virtualize the MSR.

> > Pass through MSR_IA32_SPEC_CTRL was introduced in commit d28b387fb74d
> > ("KVM/VMX: Allow direct access to MSR_IA32_SPEC_CTRL"). At that time there
> > was only a few bits defined, and the changelog called out that
> >
> >   No attempt is made to handle STIBP here, intentionally. Filtering
> >   STIBP may be added in a future patch, which may require trapping all
> >   writes if we don't want to pass it through directly to the guest.
> >
> > Per my undesrstanding, it implied that we need to re-visit it when more bits
> > added instead of following the pass-through design siliently.
>
> I don't object to re-visiting the design. My point is that to prevent guests from
> setting RRSBA_CTRL/BHI_CTRL when they are not advertised isn't a strong
> justfication for intercepting the MSR. STIBP and other bits (except IBRS) have
> the same problem. And the gain of fixing this is too small.
>
> If passing through the SPEC_CTRL MSR to guests might cause security issues, I
> would agree to intercept accesses to the MSR.

I never buy it. How to interpret the security? If the user wants to hide 
one feature from guest but KVM allows it when KVM does have a reasonable 
way to hide it. Does it violate the security?