linux-kernel - Re: [PATCH v3 1/3] KVM: SEV-ES: Prevent MSR access post VMSA encryption

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Date: Tue, 28 May 2024 18:31:01 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Ravi Bangoria <ravi.bangoria@....com>, seanjc@...gle.com,
 nikunj.dadhania@....com
Cc: thomas.lendacky@....com, tglx@...utronix.de, mingo@...hat.com,
 bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
 michael.roth@....com, pankaj.gupta@....com, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, santosh.shukla@....com
Subject: Re: [PATCH v3 1/3] KVM: SEV-ES: Prevent MSR access post VMSA
 encryption

On 5/23/24 14:18, Ravi Bangoria wrote:
> From: Nikunj A Dadhania <nikunj@....com>
> 
> KVM currently allows userspace to read/write MSRs even after the VMSA is
> encrypted. This can cause unintentional issues if MSR access has side-
> effects. For ex, while migrating a guest, userspace could attempt to
> migrate MSR_IA32_DEBUGCTLMSR and end up unintentionally disabling LBRV on
> the target. Fix this by preventing access to those MSRs which are context
> switched via the VMSA, once the VMSA is encrypted.
> 
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@....com>
> ---
>   arch/x86/kvm/svm/svm.c | 18 ++++++++++++++++++
>   1 file changed, 18 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 3d0549ca246f..489b0183f37d 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2834,10 +2834,24 @@ static int svm_get_msr_feature(struct kvm_msr_entry *msr)
>   	return 0;
>   }
>   
> +static bool
> +sev_es_prevent_msr_access(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> +{
> +	return sev_es_guest(vcpu->kvm) &&
> +	       vcpu->arch.guest_state_protected &&
> +	       svm_msrpm_offset(msr_info->index) != MSR_INVALID &&
> +	       !msr_write_intercepted(vcpu, msr_info->index);
> +}
> +
>   static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   {
>   	struct vcpu_svm *svm = to_svm(vcpu);
>   
> +	if (sev_es_prevent_msr_access(vcpu, msr_info)) {
> +		msr_info->data = 0;
> +		return 0;

This should return -EINVAL, not 0.  Likewise below in svm_set_msr().

Paolo

> +	}
> +
>   	switch (msr_info->index) {
>   	case MSR_AMD64_TSC_RATIO:
>   		if (!msr_info->host_initiated &&
> @@ -2988,6 +3002,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>   
>   	u32 ecx = msr->index;
>   	u64 data = msr->data;
> +
> +	if (sev_es_prevent_msr_access(vcpu, msr))
> +		return 0;
> +
>   	switch (ecx) {
>   	case MSR_AMD64_TSC_RATIO:
>