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

[Chao Gao]

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.

>
>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.