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

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Date: Wed, 29 May 2024 16:14:10 +0530
From: Ravi Bangoria <ravi.bangoria@....com>
To: Paolo Bonzini <pbonzini@...hat.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, ravi.bangoria@....com
Subject: Re: [PATCH v3 1/3] KVM: SEV-ES: Prevent MSR access post VMSA
 encryption

On 5/28/2024 10:01 PM, Paolo Bonzini wrote:
> 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().

Sure.

Thanks,
Ravi