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

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Date:   Tue, 27 Jun 2023 07:55:21 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Weijiang Yang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        rppt@...nel.org, binbin.wu@...ux.intel.com,
        rick.p.edgecombe@...el.com, john.allen@....com,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v3 13/21] KVM:VMX: Emulate reads and writes to CET MSRs

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);
> > }