Last job was at Panasonic Avionics' cross-town rival, Thales Avionics. From the many Panasonic veterans working at Thales, they mentioned our architectures were quite similar, so I will use what I know about Thales' system to make what could be a close comparison.
You have a system called ACARS, which is an ancient technology that sends data from the cockpit to the IFE system (GPS, airspeed, etc). ACARS I believe is a 1-way protocol (think of it as an RSS feed where you can subscribe for updates) The only bit of communication that can go the other direction (I think) is BITE (built-in test environment) which is a centralized way for a technician to see status messages and error codes sent from individual components (such as each seat-back screen, power distribution boxes, switches, wireless access points, etc). The IFE systems we developed had separate satellite systems for internet connectivity from that of the cabin, including separate antennas. From my understanding, hacking an IFE system could mean gaining access to CC info, but it would be very very unlikely to take control of the plane from a seatback screen.
