x86/kvm/nVMX: allow bare VMXON state migration

It is perfectly valid for a guest to do VMXON and not do VMPTRLD. This
state needs to be preserved on migration.

Cc: stable@vger.kernel.org
Fixes: 8fcc4b5923af5de58b80b53a069453b135693304
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -14827,13 +14827,6 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
 	if (!page_address_valid(vcpu, kvm_state->vmx.vmxon_pa))
 		return -EINVAL;
 
-	if (kvm_state->size < sizeof(kvm_state) + sizeof(*vmcs12))
-		return -EINVAL;
-
-	if (kvm_state->vmx.vmcs_pa == kvm_state->vmx.vmxon_pa ||
-	    !page_address_valid(vcpu, kvm_state->vmx.vmcs_pa))
-		return -EINVAL;
-
 	if ((kvm_state->vmx.smm.flags & KVM_STATE_NESTED_SMM_GUEST_MODE) &&
 	    (kvm_state->flags & KVM_STATE_NESTED_GUEST_MODE))
 		return -EINVAL;
@@ -14863,6 +14856,14 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
 	if (ret)
 		return ret;
 
+	/* Empty 'VMXON' state is permitted */
+	if (kvm_state->size < sizeof(kvm_state) + sizeof(*vmcs12))
+		return 0;
+
+	if (kvm_state->vmx.vmcs_pa == kvm_state->vmx.vmxon_pa ||
+	    !page_address_valid(vcpu, kvm_state->vmx.vmcs_pa))
+		return -EINVAL;
+
 	set_current_vmptr(vmx, kvm_state->vmx.vmcs_pa);
 
 	if (kvm_state->vmx.smm.flags & KVM_STATE_NESTED_SMM_VMXON) {