linux-kernel - Re: [PATCH V2] KVM: SEV: Update SEV-ES shutdown intercepts with more metadata

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Date:   Wed, 6 Sep 2023 13:26:16 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Peter Gonda <pgonda@...gle.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] KVM: SEV: Update SEV-ES shutdown intercepts with more metadata

On Wed, Sep 06, 2023, Tom Lendacky wrote:
> On 9/6/23 15:11, Sean Christopherson wrote:
> > On Wed, Sep 06, 2023, Tom Lendacky wrote:
> > > On 9/6/23 10:14, Peter Gonda wrote:
> > > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > > > index 956726d867aa..cecf6a528c9b 100644
> > > > --- a/arch/x86/kvm/svm/svm.c
> > > > +++ b/arch/x86/kvm/svm/svm.c
> > > > @@ -2131,12 +2131,14 @@ static int shutdown_interception(struct kvm_vcpu *vcpu)
> > > >    	 * The VM save area has already been encrypted so it
> > > >    	 * cannot be reinitialized - just terminate.
> > > >    	 */
> > > > -	if (sev_es_guest(vcpu->kvm))
> > > > -		return -EINVAL;
> > > > +	if (sev_es_guest(vcpu->kvm)) {
> > > > +		kvm_run->exit_reason = KVM_EXIT_SHUTDOWN;
> > > > +		return 0;
> > > > +	}
> > > 
> > > Just a nit... feel free to ignore, but, since KVM_EXIT_SHUTDOWN is also set
> > > at the end of the function and I don't think kvm_vcpu_reset() clears the
> > > value from kvm_run, you could just set kvm_run->exit_reason on entry and
> > > just return 0 early for an SEV-ES guest.
> > 
> > kvm_run is writable by userspace though, so KVM can't rely on kvm_run->exit_reason
> > for correctness.
> > 
> > And IIUC, the VMSA is also toast, i.e. doing anything other than marking the VM
> > dead is futile, no?
> 
> I was just saying that "kvm_run->exit_reason = KVM_EXIT_SHUTDOWN;" is in the
> shutdown_interception() function twice now (at both exit points of the
> function) and can probably just be moved to the top of the function and be
> common for both exit points, now, right?
> 
> I'm not saying to get rid of it, just set it sooner.

Ah, I thought you were saying bail early from kvm_vcpu_reset().  I agree that not
having completely split logic would be ideal.  What about this?

	/*
	 * VMCB is undefined after a SHUTDOWN intercept.  INIT the vCPU to put
	 * the VMCB in a known good state.  Unfortuately, KVM doesn't have
	 * KVM_MP_STATE_SHUTDOWN and can't add it without potentially breaking
	 * userspace.  At a platform view, INIT is acceptable behavior as
	 * there exist bare metal platforms that automatically INIT the CPU
	 * in response to shutdown.
	 *
	 * The VM save area for SEV-ES guests has already been encrypted so it
	 * cannot be reinitialized, i.e. synthesizing INIT is futile.
	 */
	if (!sev_es_guest(vcpu->kvm)) {
		clear_page(svm->vmcb);
		kvm_vcpu_reset(vcpu, true);
	}

	kvm_run->exit_reason = KVM_EXIT_SHUTDOWN;
	return 0;