linux-kernel - [PATCH 02/11] KVM: nSVM: Add missing consistency check for EFER, CR0, CR4, and CS

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Message-ID: <20251104195949.3528411-3-yosry.ahmed@linux.dev>
Date: Tue,  4 Nov 2025 19:59:40 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
	Jim Mattson <jmattson@...gle.com>,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: [PATCH 02/11] KVM: nSVM: Add missing consistency check for EFER, CR0, CR4, and CS

According to the APM Volume #2, 15.5, Canonicalization and Consistency
Checks (24593—Rev. 3.42—March 2024), the following condition (among
others) results in a #VMEXIT with VMEXIT_INVALID (aka SVM_EXIT_ERR):

  EFER.LME, CR0.PG, CR4.PAE, CS.L, and CS.D are all non-zero.

Add the missing consistency check. This is functionally a nop because
the nested VMRUN results in SVM_EXIT_ERR in HW, which is forwarded to
L1, but KVM makes all consistency checks before a VMRUN is actually
attempted.

Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
---
 arch/x86/kvm/svm/nested.c | 7 +++++++
 arch/x86/kvm/svm/svm.h    | 1 +
 2 files changed, 8 insertions(+)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 9a534f04bdc83..9866b2fd8f32a 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -380,6 +380,11 @@ static bool __nested_vmcb_check_save(struct kvm_vcpu *vcpu,
 		    CC(!(save->cr0 & X86_CR0_PE)) ||
 		    CC(!kvm_vcpu_is_legal_cr3(vcpu, save->cr3)))
 			return false;
+
+		if (CC((save->cr4 & X86_CR4_PAE) &&
+		       (save->cs.attrib & SVM_SELECTOR_L_MASK) &&
+		       (save->cs.attrib & SVM_SELECTOR_DB_MASK)))
+			return false;
 	}
 
 	/* Note, SVM doesn't have any additional restrictions on CR4. */
@@ -473,6 +478,8 @@ static void __nested_copy_vmcb_save_to_cache(struct vmcb_save_area_cached *to,
 	 * Copy only fields that are validated, as we need them
 	 * to avoid TOC/TOU races.
 	 */
+	to->cs = from->cs;
+
 	to->efer = from->efer;
 	to->cr0 = from->cr0;
 	to->cr3 = from->cr3;
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 0a2908e22d746..efcf923264c55 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -142,6 +142,7 @@ struct kvm_vmcb_info {
 };
 
 struct vmcb_save_area_cached {
+	struct vmcb_seg cs;
 	u64 efer;
 	u64 cr4;
 	u64 cr3;
-- 
2.51.2.1026.g39e6a42477-goog