Own a fab, design and verify your own chips, produce them under a strict control and cross-checking, same for all other parts, assembly, transportation, storage, and operation. Background-check everyone you employ. This is basically how high-assurance military hardware is handled.
The question for the rest of us is: can we buy trustable hardware? Even more specifically, can we buy if for the peanuts we're used to pay, instead of buying the whole wad of production chains as the military does?
Here the answer is negative in the strict sense: we can't be certain. But the answer is mostly-somehow-positive in many cases, because nobody cares enough to add subtle behaviors to MCUs that go for cents and control light switches and kids toys. The more advanced is your machine though, and the more there is a reason to bug it, e.g. to have an ability to remotely disable affected devices, or worse.
In theory it should be possible to manufacture IC's at home, similar to 3D printing. And a (very!) few people have even done so.
A whole lot of progress in IC manufacturing is making things smaller. No reason one couldn't include the fabrication machinery itself in that quest.
Another approach is to fabricate IC's in a blank state, and have end users 'burn' its function. FPGAs are a thing, iirc even some fuse-programmed ones exist. Some gate arrays consist(ed? not sure if made anymore) of bulk, prefabricated logic, with a layer of customer-specified routing on top. Something like that could lend itself well to 'diy-at-home'.
All these would sidestep much of the "trust 3rd party" problem.
So yeah it's possible. Technology (or more precisely: equipment on the market) just isn't quite there yet.
Own a fab, design and verify your own chips, produce them under a strict control and cross-checking, same for all other parts, assembly, transportation, storage, and operation. Background-check everyone you employ. This is basically how high-assurance military hardware is handled.
The question for the rest of us is: can we buy trustable hardware? Even more specifically, can we buy if for the peanuts we're used to pay, instead of buying the whole wad of production chains as the military does?
Here the answer is negative in the strict sense: we can't be certain. But the answer is mostly-somehow-positive in many cases, because nobody cares enough to add subtle behaviors to MCUs that go for cents and control light switches and kids toys. The more advanced is your machine though, and the more there is a reason to bug it, e.g. to have an ability to remotely disable affected devices, or worse.