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.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);