Re: [PATCH v3 13/21] KVM:VMX: Emulate reads and writes to CET MSRs

Sean Christopherson
2023-06-27T14:56:22Z

On Tue, Jun 27, 2023, Weijiang Yang wrote:
>
> On 6/27/2023 5:15 AM, Sean Christopherson wrote:
> > And the above is also wrong for host_initiated writes to SHSTK MSRs. E.g. if KVM
> > is running on a CPU that has IBT but not SHSTK, then userspace can write to MSRs
> > that do not exist.
> >
> > Maybe this confusion is just a symptom of the series not providing proper
> > Supervisor Shadow Stack support, but that's still a poor excuse for posting
> > broken code.
> >
> > I suspect you tried to get too fancy. I don't see any reason to ever care about
> > kvm_caps.supported_xss beyond emulating writes to XSS itself. Just require that
> > both CET_USER and CET_KERNEL are supported in XSS to allow IBT or SHSTK, i.e. let
> > X86_FEATURE_IBT and X86_FEATURE_SHSTK speak for themselves. That way, this can
> > simply be:
>
> You're right, kvm_cet_user_supported() is overused.
>
> Let me recap to see if I understand correctly:
>
> 1. Check both CET_USER and CET_KERNEL are supported in XSS before advertise
> SHSTK is supported
>
> in KVM and expose it to guest, the reason is once SHSTK is exposed to guest,
> KVM should support both modes to honor arch integrity.
>
> 2. Check CET_USER is supported before advertise IBT is supported in KVMï¿½ and
> expose IBT, the reason is, user IBT(MSR_U_CET) depends on CET_USER bit while
> kernel IBT(MSR_S_CET) doesn't.

IBT can also used by the kernel...

Just require that both CET_USER and CET_KERNEL are supported to advertise IBT
or SHSTK. I don't see why this is needs to be any more complex than that.

> > bool kvm_cet_is_msr_accessible(struct kvm_vcpu *vcpu, struct msr_data *msr)
> > {
> > if (is_shadow_stack_msr(...))
> > if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK))
> > return false;
> >
> > return msr->host_initiated ||
> > guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
> > }
> >
> > if (!kvm_cpu_cap_has(X86_FEATURE_IBT) &&
> > !kvm_cpu_cap_has(X86_FEATURE_SHSTK))
> > return false;
>
> Move above checks to the beginning?

Why? The is_shadow_stack_msr() would still have to recheck X86_FEATURE_SHSTK,
so hoisting the checks to the top would be doing unnecessary work.

> > return msr->host_initiated ||
> > guest_cpuid_has(vcpu, X86_FEATURE_IBT) ||
> > guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
> > }