There is every possibility, however, that there is open-source code in the Linux kernel that, at runtime, interacts with specific microcode instructions that can backdoor a system. Runtime remote backdoor triggers are more useful anyway, because the one thing the NSA can't do is hide from network sniffers. (Of course the best way to hide would be piggybacking on something like automatic software update requests - which I happen to disable, as a nod to my tinfoil wearing brethren.)