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

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Message-ID: <126deeee-0a34-bfad-27d6-ff39c93c1e25@amd.com>
Date:   Wed, 6 Sep 2023 15:34:15 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <seanjc@...gle.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 9/6/23 15:26, Sean Christopherson wrote:
> 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;

That looks good to me!

Thanks,
Tom