Re: Whoa, hang on
Even if (and I highly doubt this being the case on any modern system) you had a full state machine layout of your entire system and thus could make some assertions that no unexpected states existed, this would still overlook issues outside the logical system state that stray into physics territory (such as the ramhammer technique).
I'm assuming you meant rowhammer, rather than ramhammer.
Rowhammer doesn't work over RS-232...
As a result, I would take askance at any assertions of 100% security, for any kind of interface where information is passed between two systems.
I wouldn't. High-reliability systems have been built for decades - I've built some of them. Whether the current crop of avionics can be considered high-reliability is not something I could answer, not having been in that industry for a few years - but that doesn't mean they couldn't be.