[imtringued]

None of what you said makes any sense. You're mixing "UB" and bugs.

UB is about declaring programs invalid with a snide "don't do that", not about incorrect execution due to an incorrect specification. E.g. speculative execution running in privileged mode due to a prior syscall is just a plain hardware bug. It's not undefined behavior. In fact, the bug in question is extremely well defined.

The closest thing to reading undefined behavior is reading a "don't care" or VHDL's 'U' value from a std_ulogic and even those are well defined in simulation, just not in physical hardware, but even there they can only ever be as bad as reading a garbage value. Since a lot of the hardware design is non-programmable, there is also usually no way to exploit it.