linux-kernel - Re: [PATCH 1/9] KVM: x86: SVM: Emulate reads and writes to shadow stack MSRs

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Message-ID: <65781f864b94db127a7a17e52835326a8a6a9ea0.camel@redhat.com>
Date:   Thu, 02 Nov 2023 20:00:36 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     John Allen <john.allen@....com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        weijiang.yang@...el.com, rick.p.edgecombe@...el.com,
        seanjc@...gle.com, x86@...nel.org, thomas.lendacky@....com,
        bp@...en8.de
Subject: Re: [PATCH 1/9] KVM: x86: SVM: Emulate reads and writes to shadow
 stack MSRs

On Tue, 2023-10-10 at 20:02 +0000, John Allen wrote:
> Set up interception of shadow stack MSRs. In the event that shadow stack
> is unsupported on the host or the MSRs are otherwise inaccessible, the
> interception code will return an error. In certain circumstances such as
> host initiated MSR reads or writes, the interception code will get or
> set the requested MSR value.
> 
> Signed-off-by: John Allen <john.allen@....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 f283eb47f6ac..6a0d225311bc 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2859,6 +2859,15 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		if (guest_cpuid_is_intel(vcpu))
>  			msr_info->data |= (u64)svm->sysenter_esp_hi << 32;
>  		break;
> +	case MSR_IA32_S_CET:
> +		msr_info->data = svm->vmcb->save.s_cet;
> +		break;
> +	case MSR_IA32_INT_SSP_TAB:
> +		msr_info->data = svm->vmcb->save.isst_addr;
> +		break;
> +	case MSR_KVM_SSP:
> +		msr_info->data = svm->vmcb->save.ssp;
> +		break;
>  	case MSR_TSC_AUX:
>  		msr_info->data = svm->tsc_aux;
>  		break;
> @@ -3085,6 +3094,15 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  		svm->vmcb01.ptr->save.sysenter_esp = (u32)data;
>  		svm->sysenter_esp_hi = guest_cpuid_is_intel(vcpu) ? (data >> 32) : 0;
>  		break;
> +	case MSR_IA32_S_CET:
> +		svm->vmcb->save.s_cet = data;
> +		break;
> +	case MSR_IA32_INT_SSP_TAB:
> +		svm->vmcb->save.isst_addr = data;
> +		break;
> +	case MSR_KVM_SSP:
> +		svm->vmcb->save.ssp = data;
> +		break;
>  	case MSR_TSC_AUX:
>  		/*
>  		 * TSC_AUX is usually changed only during boot and never read

Looks good, except that if my complaint about turning the fake 'MSR_KVM_SSP' into
a first class register with an ioctl to load/save it is accepted, then
there will be a new vendor callback to read/write it instead of doing it in svm_get_msr/svm_set_msr.


Besides this,

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky