Re: [PATCH v5 11/19] KVM:VMX: Emulate read and write to CET MSRs

Paolo Bonzini
2023-08-07T07:01:19Z

On 8/6/23 10:44, Yang, Weijiang wrote:
>> Similar to my suggestsion for XSS, I think we drop the waiver for
>> host_initiated
>> accesses, i.e. require the feature to be enabled and exposed to the
>> guest, even
>> for the host.
>
> I saw Paolo shares different opinion on this, so would hold on for a
> while...

It's not *so* different: the host initiated access should be allowed,
but it should only allow writing zero. So, something like:

> +static bool kvm_cet_is_msr_accessible(struct kvm_vcpu *vcpu,
> + struct msr_data *msr)
> +{

bool host_msr_reset =
msr->host_initiated && msr->data == 0;

and then below you use host_msr_reset instead of msr->host_initiated.

> + if (msr->index == MSR_KVM_GUEST_SSP)
> + return msr->host_initiated;
> +
> + return msr->host_initiated ||
> + guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);

This can be unified like this:

return
(host_msr_reset || guest_cpuid_has(vcpu, X86_FEATURE_SHSTK)) &&
(msr->index != MSR_KVM_GUEST_SSP || msr->host_initiated);

> + }
> +
> + if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
> + !kvm_cpu_cap_has(X86_FEATURE_IBT))
> + return false;
> +
> + return msr->host_initiated ||
> + guest_cpuid_has(vcpu, X86_FEATURE_IBT) ||
> + guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);

while this can simply use host_msr_reset.

Paolo