linux-kernel - Re: [PATCH 3/9] KVM: SVM: Don't intercept #GP for SEV guests

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Message-ID: <61dcbb64-2f2a-175a-e207-79398e80184c@oracle.com>
Date:   Thu, 20 Jan 2022 14:30:13 +0000
From:   Liam Merwick <liam.merwick@...cle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Liam Merwick <liam.merwick@...cle.com>
Subject: Re: [PATCH 3/9] KVM: SVM: Don't intercept #GP for SEV guests

On 20/01/2022 01:07, Sean Christopherson wrote:
> Never intercept #GP for SEV guests as reading SEV guest private memory
> will return cyphertext, i.e. emulating on #GP can't work as intended.
> 

"ciphertext" seems to be the convention.


> Cc: stable@...r.kernel.org
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Brijesh Singh <brijesh.singh@....com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Liam Merwick <liam.merwick@...cle.com>

> ---
>   arch/x86/kvm/svm/svm.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 85703145eb0a..edea52be6c01 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -312,7 +312,11 @@ int svm_set_efer(struct kvm_vcpu *vcpu, u64 efer)
>   				return ret;
>   			}
>   
> -			if (svm_gp_erratum_intercept)
> +			/*
> +			 * Never intercept #GP for SEV guests, KVM can't
> +			 * decrypt guest memory to workaround the erratum.
> +			 */
> +			if (svm_gp_erratum_intercept && !sev_guest(vcpu->kvm))
>   				set_exception_intercept(svm, GP_VECTOR);
>   		}
>   	}
> @@ -1010,9 +1014,10 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
>   	 * Guest access to VMware backdoor ports could legitimately
>   	 * trigger #GP because of TSS I/O permission bitmap.
>   	 * We intercept those #GP and allow access to them anyway
> -	 * as VMware does.
> +	 * as VMware does.  Don't intercept #GP for SEV guests as KVM can't
> +	 * decrypt guest memory to decode the faulting instruction.
>   	 */
> -	if (enable_vmware_backdoor)
> +	if (enable_vmware_backdoor && !sev_guest(vcpu->kvm))
>   		set_exception_intercept(svm, GP_VECTOR);
>   
>   	svm_set_intercept(svm, INTERCEPT_INTR);