I imagine, with knowledge of the ISA, can help one can define a verification algorithm, in which, the CPU is probed with some known inputs to gain confidence that there is no backdoor.
There are 2^64 possible values for a single register. It's not possible to probe all possible combinations of values for the over 60 user-accessible registers, to find the single combination which, when calling a specific one of the more than 2^30 possible instructions, silently flips a secret "disable all permissions checking" bit.
Of course if someone can sneak a backdoor into the spec, all bets are off.
For example, even if everything was provably manufactured to spec, it could be laid out in a way that exposes side channels, enables rowhammer-like attacks, and so on.
> Using this defense method, any Trojan that can analyze the entire configurable structure must use complicated logic functions and take up a large silicon area, which greatly increases the possibility of being detected by security tools.
There are live methods of detecting trojans as well, where you have an additional chip checking what the CPU is doing at all times [2].
One of the main thing I have learned in my life is to not underestimate the ingenuity of cryptographers.
[1] https://arxiv.org/pdf/2107.04175
[2] https://re.public.polimi.it/bitstream/11311/1204477/1/DFTS_2...