The bootloader is signed?
How would that work? If your bootloader is signed then you can go ahead and boot any OS?
Including a malicious one?
If it were that easy it would not be a problem.
All that would be required is that the Grub folks get their key out to the motherboard manufacturers and then *anybody* or *anything* could boot whatever they damn well please via grub, including a Grub loaded malware infestation.
Good lord, if you don't have a clue what you are on about then please try and refrain from commenting. All it does is spread confusion.