linux-kernel - [PATCH 2/2] KVM: SVM: Raise #UD if VMMCALL instruction is not intercepted

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Message-ID: <20260106041250.2125920-3-chengkev@google.com>
Date: Tue,  6 Jan 2026 04:12:50 +0000
From: Kevin Cheng <chengkev@...gle.com>
To: seanjc@...gle.com, pbonzini@...hat.com
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, yosry.ahmed@...ux.dev, 
	Kevin Cheng <chengkev@...gle.com>
Subject: [PATCH 2/2] KVM: SVM: Raise #UD if VMMCALL instruction is not intercepted

The AMD APM states that if VMMCALL instruction is not intercepted, the
instruction raises a #UD exception.

Create a vmmcall exit handler that generates a #UD if a VMMCALL exit
from L2 is being handled by L0, which means that L1 did not intercept
the VMMCALL instruction.

Co-developed-by: Sean Christopherson <seanjc@...gle.com>
Co-developed-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
Signed-off-by: Kevin Cheng <chengkev@...gle.com>
---
 arch/x86/kvm/svm/svm.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index fc1b8707bb00c..482495ad72d22 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3179,6 +3179,20 @@ static int bus_lock_exit(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
+static int vmmcall_interception(struct kvm_vcpu *vcpu)
+{
+	/*
+	 * If VMMCALL from L2 is not intercepted by L1, the instruction raises a
+	 * #UD exception
+	 */
+	if (is_guest_mode(vcpu)) {
+		kvm_queue_exception(vcpu, UD_VECTOR);
+		return 1;
+	}
+
+	return kvm_emulate_hypercall(vcpu);
+}
+
 static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
 	[SVM_EXIT_READ_CR0]			= cr_interception,
 	[SVM_EXIT_READ_CR3]			= cr_interception,
@@ -3229,7 +3243,7 @@ static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
 	[SVM_EXIT_TASK_SWITCH]			= task_switch_interception,
 	[SVM_EXIT_SHUTDOWN]			= shutdown_interception,
 	[SVM_EXIT_VMRUN]			= vmrun_interception,
-	[SVM_EXIT_VMMCALL]			= kvm_emulate_hypercall,
+	[SVM_EXIT_VMMCALL]			= vmmcall_interception,
 	[SVM_EXIT_VMLOAD]			= vmload_interception,
 	[SVM_EXIT_VMSAVE]			= vmsave_interception,
 	[SVM_EXIT_STGI]				= stgi_interception,
-- 
2.52.0.351.gbe84eed79e-goog