linux-kernel - [PATCH v13 12/21] KVM: VMX: Set up interception for CET MSRs

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Message-ID: <20250821133132.72322-13-chao.gao@intel.com>
Date: Thu, 21 Aug 2025 06:30:46 -0700
From: Chao Gao <chao.gao@...el.com>
To: kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: chao.gao@...el.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	hpa@...or.com,
	john.allen@....com,
	mingo@...hat.com,
	minipli@...ecurity.net,
	mlevitsk@...hat.com,
	pbonzini@...hat.com,
	rick.p.edgecombe@...el.com,
	seanjc@...gle.com,
	tglx@...utronix.de,
	weijiang.yang@...el.com,
	x86@...nel.org,
	xin@...or.com
Subject: [PATCH v13 12/21] KVM: VMX: Set up interception for CET MSRs

From: Yang Weijiang <weijiang.yang@...el.com>

Enable/disable CET MSRs interception per associated feature configuration.

Pass through CET MSRs that are managed by XSAVE, as they cannot be
intercepted without also intercepting XSAVE. However, intercepting XSAVE
would likely cause unacceptable performance overhead.
MSR_IA32_INT_SSP_TAB is not managed by XSAVE, so it is intercepted.

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 architectural perspective since IBT relies on subset of SHSTK
relevant MSRs.

Suggested-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
Tested-by: Mathias Krause <minipli@...ecurity.net>
Tested-by: John Allen <john.allen@....com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Signed-off-by: Chao Gao <chao.gao@...el.com>
---
v13
- document the real reason why MSRs are pass-thru'd
- Rename the local variable that indicates whether interception is
  needed.
---
 arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 4fc1dbba2eb0..adf5af30e537 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4101,6 +4101,8 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)
 
 void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
 {
+	bool intercept;
+
 	if (!cpu_has_vmx_msr_bitmap())
 		return;
 
@@ -4146,6 +4148,23 @@ void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
 		vmx_set_intercept_for_msr(vcpu, MSR_IA32_FLUSH_CMD, MSR_TYPE_W,
 					  !guest_cpu_cap_has(vcpu, X86_FEATURE_FLUSH_L1D));
 
+	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
+		intercept = !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
+
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP, MSR_TYPE_RW, intercept);
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP, MSR_TYPE_RW, intercept);
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP, MSR_TYPE_RW, intercept);
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP, MSR_TYPE_RW, intercept);
+	}
+
+	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK) || kvm_cpu_cap_has(X86_FEATURE_IBT)) {
+		intercept = !guest_cpu_cap_has(vcpu, X86_FEATURE_IBT) &&
+			    !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
+
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET, MSR_TYPE_RW, intercept);
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET, MSR_TYPE_RW, intercept);
+	}
+
 	/*
 	 * x2APIC and LBR MSR intercepts are modified on-demand and cannot be
 	 * filtered by userspace.
-- 
2.47.3