KVM: x86: nSVM: Cache and validate vmcb12 g_pat

When KVM_X86_QUIRK_NESTED_SVM_SHARED_PAT is disabled and nested paging is
enabled in vmcb12, validate g_pat at emulated VMRUN and cause an immediate
VMEXIT with exit code VMEXIT_INVALID if it is invalid, as specified in the
APM, volume 2: "Nested Paging and VMRUN/VMEXIT."

Fixes: 3d6368ef580a ("KVM: SVM: Add VMRUN handler")
Signed-off-by: Jim Mattson <jmattson@google.com>
Link: https://patch.msgid.link/20260407190343.325299-4-jmattson@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

2 files changed, 20 insertions, 4 deletions

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index cd8f5f3f5e33..98701c23bc99 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -411,7 +411,8 @@ static bool nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
 
 /* Common checks that apply to both L1 and L2 state.  */
 static bool nested_vmcb_check_save(struct kvm_vcpu *vcpu,
-				   struct vmcb_save_area_cached *save)
+				   struct vmcb_save_area_cached *save,
+				   bool check_gpat)
 {
 	if (CC(!(save->efer & EFER_SVME)))
 		return false;
@@ -446,6 +447,15 @@ static bool nested_vmcb_check_save(struct kvm_vcpu *vcpu,
 	if (CC(!kvm_valid_efer(vcpu, save->efer)))
 		return false;
 
+	/*
+	 * If userspace contrives to get an invalid g_pat into vmcb02 by
+	 * disabling KVM_X86_QUIRK_NESTED_SVM_SHARED_PAT in a race with
+	 * this check, it should be prepared for the KVM_EXIT_FAIL_ENTRY
+	 * that will follow.
+	 */
+	if (check_gpat && CC(!kvm_pat_valid(save->g_pat)))
+		return false;
+
 	return true;
 }
 
@@ -453,7 +463,8 @@ int nested_svm_check_cached_vmcb12(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
-	if (!nested_vmcb_check_save(vcpu, &svm->nested.save) ||
+	if (!nested_vmcb_check_save(vcpu, &svm->nested.save,
+				    l2_has_separate_pat(vcpu)) ||
 	    !nested_vmcb_check_controls(vcpu, &svm->nested.ctl))
 		return -EINVAL;
 
@@ -566,6 +577,7 @@ static void __nested_copy_vmcb_save_to_cache(struct vmcb_save_area_cached *to,
 
 	to->rax = from->rax;
 	to->cr2 = from->cr2;
+	to->g_pat = from->g_pat;
 
 	svm_copy_lbrs(to, from);
 }
@@ -1982,13 +1994,16 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
 
 	/*
 	 * Validate host state saved from before VMRUN (see
-	 * nested_svm_check_permissions).
+	 * nested_svm_check_permissions). Note that the g_pat field is not
+	 * validated, because (a) it may have been clobbered by SMM before
+	 * KVM_GET_NESTED_STATE, and (b) it is not loaded at emulated
+	 * #VMEXIT.
 	 */
 	__nested_copy_vmcb_save_to_cache(&save_cached, save);
 	if (!(save->cr0 & X86_CR0_PG) ||
 	    !(save->cr0 & X86_CR0_PE) ||
 	    (save->rflags & X86_EFLAGS_VM) ||
-	    !nested_vmcb_check_save(vcpu, &save_cached))
+	    !nested_vmcb_check_save(vcpu, &save_cached, false))
 		goto out_free;
 
 
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 5261c3f12424..fb7360231d8b 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -167,6 +167,7 @@ struct vmcb_save_area_cached {
 	u64 isst_addr;
 	u64 rax;
 	u64 cr2;
+	u64 g_pat;
 	u64 dbgctl;
 	u64 br_from;
 	u64 br_to;