linux-kernel - Re: [PATCH v8 22/26] KVM: VMX: Set up interception for CET MSRs

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Date: Mon, 15 Jan 2024 17:58:54 +0800
From: Yuan Yao <yuan.yao@...ux.intel.com>
To: Yang Weijiang <weijiang.yang@...el.com>
Cc: seanjc@...gle.com, pbonzini@...hat.com, dave.hansen@...el.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	peterz@...radead.org, chao.gao@...el.com,
	rick.p.edgecombe@...el.com, mlevitsk@...hat.com, john.allen@....com
Subject: Re: [PATCH v8 22/26] KVM: VMX: Set up interception for CET MSRs

On Thu, Dec 21, 2023 at 09:02:35AM -0500, Yang Weijiang wrote:
> Enable/disable CET MSRs interception per associated feature configuration.
> Shadow Stack feature requires all CET MSRs passed through to guest to make
> it supported in user and supervisor mode while IBT feature only depends on
> MSR_IA32_{U,S}_CETS_CET to enable user and supervisor IBT.
>
> Note, this MSR design introduced an architectural limitation of SHSTK and
> IBT control for guest, i.e., when SHSTK is exposed, IBT is also available
> to guest from architectual perspective since IBT relies on subset of SHSTK
> relevant MSRs.
>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 42 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 064a5fe87948..08058b182893 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -692,6 +692,10 @@ static bool is_valid_passthrough_msr(u32 msr)
>  	case MSR_LBR_CORE_TO ... MSR_LBR_CORE_TO + 8:
>  		/* LBR MSRs. These are handled in vmx_update_intercept_for_lbr_msrs() */
>  		return true;
> +	case MSR_IA32_U_CET:
> +	case MSR_IA32_S_CET:
> +	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
> +		return true;
>  	}
>
>  	r = possible_passthrough_msr_slot(msr) != -ENOENT;
> @@ -7767,6 +7771,42 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
>  		vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
>  }
>
> +static void vmx_update_intercept_for_cet_msr(struct kvm_vcpu *vcpu)
> +{
> +	bool incpt;
> +
> +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
> +		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
> +
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP,
> +					  MSR_TYPE_RW, incpt);
> +		if (guest_cpuid_has(vcpu, X86_FEATURE_LM))

Looks this leading to MSR_IA32_INT_SSP_TAB not intercepted
after below steps:

Step 1. User space set cpuid w/  X86_FEATURE_LM, w/  SHSTK.
Step 2. User space set cpuid w/o X86_FEATURE_LM, w/o SHSTK.

Then MSR_IA32_INT_SSP_TAB won't be intercepted even w/o SHSTK
on guest cpuid, will this lead to inconsistency when do
rdmsr(MSR_IA32_INT_SSP_TAB) from guest in this scenario ?

> +			vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
> +						  MSR_TYPE_RW, incpt);
> +		if (!incpt)
> +			return;
> +	}
> +
> +	if (kvm_cpu_cap_has(X86_FEATURE_IBT)) {
> +		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_IBT);
> +
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
> +					  MSR_TYPE_RW, incpt);
> +	}
> +}
> +
>  static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -7845,6 +7885,8 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>
>  	/* Refresh #PF interception to account for MAXPHYADDR changes. */
>  	vmx_update_exception_bitmap(vcpu);
> +
> +	vmx_update_intercept_for_cet_msr(vcpu);
>  }
>
>  static u64 vmx_get_perf_capabilities(void)
> --
> 2.39.3
>
>