Re: [PATCH v3 14/21] KVM:VMX: Add a synthetic MSR to allow userspace to access GUEST_SSP

[Binbin Wu]

On 5/11/2023 12:08 PM, Yang Weijiang wrote:
> Introduce a host-only synthetic MSR, MSR_KVM_GUEST_SSP, so that the VMM
> can read/write the guest's SSP, e.g. to migrate CET state.  Use a synthetic
> MSR, e.g. as opposed to a VCPU_REG_, as GUEST_SSP is subject to the same
> consistency checks as the PL*_SSP MSRs, i.e. can share code.
>
> Co-developed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
>   arch/x86/include/uapi/asm/kvm_para.h |  1 +
>   arch/x86/kvm/vmx/vmx.c               | 15 ++++++++++++---
>   arch/x86/kvm/x86.c                   |  4 ++++
>   3 files changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> index 6e64b27b2c1e..7af465e4e0bd 100644
> --- a/arch/x86/include/uapi/asm/kvm_para.h
> +++ b/arch/x86/include/uapi/asm/kvm_para.h
> @@ -58,6 +58,7 @@
>   #define MSR_KVM_ASYNC_PF_INT	0x4b564d06
>   #define MSR_KVM_ASYNC_PF_ACK	0x4b564d07
>   #define MSR_KVM_MIGRATION_CONTROL	0x4b564d08
> +#define MSR_KVM_GUEST_SSP	0x4b564d09
>   
>   struct kvm_steal_time {
>   	__u64 steal;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 0ccaa467d7d3..72149156bbd3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2095,9 +2095,13 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		break;
>   	case MSR_IA32_U_CET:
>   	case MSR_IA32_PL3_SSP:
> +	case MSR_KVM_GUEST_SSP:
>   		if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
>   			return 1;
> -		kvm_get_xsave_msr(msr_info);
> +		if (msr_info->index == MSR_KVM_GUEST_SSP)
> +			msr_info->data = vmcs_readl(GUEST_SSP);
According to the change of the kvm_cet_is_msr_accessible() below,
kvm_cet_is_msr_accessible() will return false for MSR_KVM_GUEST_SSP, 
then this code is unreachable?

> +		else
> +			kvm_get_xsave_msr(msr_info);
>   		break;
>   	case MSR_IA32_DEBUGCTLMSR:
>   		msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
> @@ -2413,15 +2417,20 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		break;
>   	case MSR_IA32_U_CET:
>   	case MSR_IA32_PL3_SSP:
> +	case MSR_KVM_GUEST_SSP:
>   		if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
>   			return 1;
>   		if (is_noncanonical_address(data, vcpu))
>   			return 1;
>   		if (msr_index == MSR_IA32_U_CET && (data & GENMASK(9, 6)))
>   			return 1;
> -		if (msr_index == MSR_IA32_PL3_SSP && (data & GENMASK(2, 0)))
> +		if ((msr_index == MSR_IA32_PL3_SSP ||
> +		     msr_index == MSR_KVM_GUEST_SSP) && (data & GENMASK(2, 0)))
>   			return 1;
> -		kvm_set_xsave_msr(msr_info);
> +		if (msr_index == MSR_KVM_GUEST_SSP)
> +			vmcs_writel(GUEST_SSP, data);
> +		else
> +			kvm_set_xsave_msr(msr_info);
>   		break;
>   	case MSR_IA32_PERF_CAPABILITIES:
>   		if (data && !vcpu_to_pmu(vcpu)->version)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2e3a39c9297c..baac6acebd40 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -13642,6 +13642,10 @@ bool kvm_cet_is_msr_accessible(struct kvm_vcpu *vcpu, struct msr_data *msr)
>   	    !guest_cpuid_has(vcpu, X86_FEATURE_IBT))
>   		return false;
>   
> +	/* The synthetic MSR is for userspace access only. */
> +	if (msr->index == MSR_KVM_GUEST_SSP)
> +		return false;
> +
>   	if (msr->index == MSR_IA32_PL3_SSP &&
>   	    !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK))
>   		return false;