What if a plane have to make an emergency landing while flying above KP (north korean) airspace?
NO commercial airline would deliberately fly over North Korea unless it was scheduled to land there. China is to the north and west, and South Korea and Japan are not far away to the south. All airliners can fly on a single engine and could certainly reach a suitable airport, so an engine failure would not be a disaster. Even in the case of total engine failure, the plane could glide for quite a distance. In the case of the "Gimli Glider", the 767 in that situation had a glide ratio of about 12:1, so, from 40,000 feet, they could glide about 90 miles.