Since the IP packet can be altered at the hardware level a software firewall can't stop this.
Remember you wrote HARDWARE which is the last level in the IP stack. You have your proof right in your statement above.
Graphic to show why hardware wins every time.
Layers 1 and 2 are not OS dependent. If the backdoor is in those layers, the software layer won't see it happen or be able to block the data.
If it is indeed true that there are hardware (firmware or microcode) back doors in AMD and Intel processors, would a software firewall on a PC be sufficient to block it?
Is there any difference between how different OSes handle this; Microsoft, Mac and Linux?
If not, is there anything that would work?
Please, no conjecture, verification preferred.