Re: [PATCH v4 1/4] KVM: TDX: Synchronize user-return MSRs immediately after VP.ENTER

Adrian Hunter
2025-10-21T16:36:27Z

On 21/10/2025 18:06, Sean Christopherson wrote:
> On Tue, Oct 21, 2025, Adrian Hunter wrote:
>> On 21/10/2025 01:55, Edgecombe, Rick P wrote:
>>>> + * Several of KVM's user-return MSRs are clobbered by the TDX-Module if
>>>> + * VP.ENTER succeeds, i.e. on TD-Exit. Mark those MSRs as needing an
>>>> + * update to synchronize the "current" value in KVM's cache with the
>>>> + * value in hardware (loaded by the TDX-Module).
>>>> + */
>>>
>>> I think we should be synchronizing only after a successful VP.ENTER with a real
>>> TD exit, but today instead we synchronize after any attempt to VP.ENTER.
>
> Well this is all completely @#($*#. Looking at the TDX-Module source, if the
> TDX-Module synthesizes an exit, e.g. because it suspects a zero-step attack, it
> will signal a "normal" exit but not "restore" VMM state.
>
>> If the MSR's do not get clobbered, does it matter whether or not they get
>> restored.
>
> It matters because KVM needs to know the actual value in hardware. If KVM thinks
> an MSR is 'X', but it's actually 'Y', then KVM could fail to write the correct
> value into hardware when returning to userspace and/or when running a different
> vCPU.

I don't quite follow: if an MSR does not get clobbered, where does the
incorrect value come from?