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

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Date:   Fri, 4 Aug 2023 15:21:01 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Chao Gao <chao.gao@...el.com>,
        Yang Weijiang <weijiang.yang@...el.com>, peterz@...radead.org,
        john.allen@....com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, rick.p.edgecombe@...el.com,
        binbin.wu@...ux.intel.com
Subject: Re: [PATCH v5 11/19] KVM:VMX: Emulate read and write to CET MSRs

On Fri, Aug 04, 2023, Paolo Bonzini wrote:
> On 8/4/23 23:27, Sean Christopherson wrote:
> > > > +
> > > > +	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);
> > 
> > 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.
> 
> No, please don't.  Allowing host-initiated accesses is what makes it
> possible to take the list of MSR indices and pass it blindly to KVM_GET_MSR
> and KVM_SET_MSR.

I don't see how that can work today.  Oooh, the MSRs that don't exempt host_initiated
are added to the list of MSRs to save/restore, i.e. KVM "silently" supports
MSR_AMD64_OSVW_ID_LENGTH and MSR_AMD64_OSVW_STATUS.

And guest_pv_has() returns true unless userspace has opted in to enforcement.

Sad panda.

That means we need to figure out a solution for KVM stuffing GUEST_SSP on RSM,
which is a "host" write but a guest controlled value.