KVM: nSVM: call nested_svm_load_cr3 on nested state load

While KVM's MMU should be fully reset by loading of nested CR0/CR3/CR4
by KVM_SET_SREGS, we are not in nested mode yet when we do it and therefore
only root_mmu is reset.

On regular nested entries we call nested_svm_load_cr3 which both updates
the guest's CR3 in the MMU when it is needed, and it also initializes
the mmu again which makes it initialize the walk_mmu as well when nested
paging is enabled in both host and guest.

Since we don't call nested_svm_load_cr3 on nested state load,
the walk_mmu can be left uninitialized, which can lead to a NULL pointer
dereference while accessing it if we happen to get a nested page fault
right after entering the nested guest first time after the migration and
we decide to emulate it, which leads to the emulator trying to access
walk_mmu->gva_to_gpa which is NULL.

Therefore we should call this function on nested state load as well.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210401141814.1029036-3-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/nested.c

@@ -215,24 +215,6 @@ static bool nested_svm_vmrun_msrpm(struct vcpu_svm *svm)
 	return true;
 }
 
-static bool svm_get_nested_state_pages(struct kvm_vcpu *vcpu)
-{
-	struct vcpu_svm *svm = to_svm(vcpu);
-
-	if (WARN_ON(!is_guest_mode(vcpu)))
-		return true;
-
-	if (!nested_svm_vmrun_msrpm(svm)) {
-		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
-		vcpu->run->internal.suberror =
-			KVM_INTERNAL_ERROR_EMULATION;
-		vcpu->run->internal.ndata = 0;
-		return false;
-	}
-
-	return true;
-}
-
 static bool nested_vmcb_check_controls(struct vmcb_control_area *control)
 {
 	if (CC(!vmcb_is_intercept(control, INTERCEPT_VMRUN)))
@@ -1330,6 +1312,28 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
 	return ret;
 }
 
+static bool svm_get_nested_state_pages(struct kvm_vcpu *vcpu)
+{
+	struct vcpu_svm *svm = to_svm(vcpu);
+
+	if (WARN_ON(!is_guest_mode(vcpu)))
+		return true;
+
+	if (nested_svm_load_cr3(&svm->vcpu, vcpu->arch.cr3,
+				nested_npt_enabled(svm)))
+		return false;
+
+	if (!nested_svm_vmrun_msrpm(svm)) {
+		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+		vcpu->run->internal.suberror =
+			KVM_INTERNAL_ERROR_EMULATION;
+		vcpu->run->internal.ndata = 0;
+		return false;
+	}
+
+	return true;
+}
+
 struct kvm_x86_nested_ops svm_nested_ops = {
 	.check_events = svm_check_nested_events,
 	.triple_fault = nested_svm_triple_fault,