KVM: nVMX: check for required but missing VMCS12 in KVM_SET_NESTED_STATE

A missing VMCS12 was not causing -EINVAL (it was just read with
copy_from_user, so it is not a security issue, but it is still
wrong).  Test for VMCS12 validity and reject the nested state
if a VMCS12 is required but not present.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -6108,9 +6108,16 @@ 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;
+	/* Empty 'VMXON' state is permitted if no VMCS loaded */
+	if (kvm_state->size < sizeof(*kvm_state) + sizeof(*vmcs12)) {
+		/* See vmx_has_valid_vmcs12.  */
+		if ((kvm_state->flags & KVM_STATE_NESTED_GUEST_MODE) ||
+		    (kvm_state->flags & KVM_STATE_NESTED_EVMCS) ||
+		    (kvm_state->hdr.vmx.vmcs12_pa != -1ull))
+			return -EINVAL;
+		else
+			return 0;
+	}
 
 	if (kvm_state->hdr.vmx.vmcs12_pa != -1ull) {
 		if (kvm_state->hdr.vmx.vmcs12_pa == kvm_state->hdr.vmx.vmxon_pa ||

arch/x86/kvm/vmx/nested.h

@@ -47,6 +47,11 @@ static inline struct vmcs12 *get_shadow_vmcs12(struct kvm_vcpu *vcpu)
 	return to_vmx(vcpu)->nested.cached_shadow_vmcs12;
 }
 
+/*
+ * Note: the same condition is checked against the state provided by userspace
+ * in vmx_set_nested_state; if it is satisfied, the nested state must include
+ * the VMCS12.
+ */
 static inline int vmx_has_valid_vmcs12(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);

tools/testing/selftests/kvm/x86_64/vmx_set_nested_state_test.c

@@ -183,9 +183,19 @@ void test_vmx_nested_state(struct kvm_vm *vm)
 	state->hdr.vmx.smm.flags = KVM_STATE_NESTED_SMM_GUEST_MODE;
 	test_nested_state_expect_einval(vm, state);
 
-	/* Size must be large enough to fit kvm_nested_state and vmcs12. */
+	/*
+	 * Size must be large enough to fit kvm_nested_state and vmcs12
+	 * if VMCS12 physical address is set
+	 */
 	set_default_vmx_state(state, state_sz);
 	state->size = sizeof(*state);
+	state->flags = 0;
+	test_nested_state_expect_einval(vm, state);
+
+	set_default_vmx_state(state, state_sz);
+	state->size = sizeof(*state);
+	state->flags = 0;
+	state->hdr.vmx.vmcs12_pa = -1;
 	test_nested_state(vm, state);
 
 	/*