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

[Yang, Weijiang]

On 6/27/2023 10:55 PM, Sean Christopherson wrote:
> 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 KVM� 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.

The arch control for user/kernel mode CET is the big source of 
complexity of the helpers.

Currently, CET_USER bit manages IA32_U_CET and IA32_PL3_SSP.

And CET_KERNEL bit manages PL{0,1,2}_SSP,

but architectural control/enable of IBT(user or kernel) is through 
IA32_{U,S}_CET, the former is

XSAVE-managed, but the latter is not.

 Checking both before enable the features  would make things much 
easier, but looks like

CET_KERNEL check for kernel IBT is excessive, just want to get your 
opinion on this. Thanks!

> > > 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.

Yeah, just considered from readability perspective for the change, but 
it does introduce

unnecessary check. Will follow it.

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