KVM: nVMX: really fix the size checks on KVM_SET_NESTED_STATE

The offset for reading the shadow VMCS is sizeof(*kvm_state)+VMCS12_SIZE,
so the correct size must be that plus sizeof(*vmcs12).  This could lead
to KVM reading garbage data from userspace and not reporting an error,
but is otherwise not sensitive.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -5427,7 +5427,7 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
 	    vmcs12->vmcs_link_pointer != -1ull) {
 		struct vmcs12 *shadow_vmcs12 = get_shadow_vmcs12(vcpu);
 
-		if (kvm_state->size < sizeof(*kvm_state) + 2 * sizeof(*vmcs12))
+		if (kvm_state->size < sizeof(*kvm_state) + VMCS12_SIZE + sizeof(*vmcs12))
 			return -EINVAL;
 
 		if (copy_from_user(shadow_vmcs12,