Exits should block reentry into the tor network
With proposal 110, we blocked the ability of Tor clients to use the Tor protocol for an unbounded amplification attack to destroy the Tor network. However, we still have not completely prevented this attack. It is still possible to tunnel tor over tor by using exits to connect back to other tor nodes. This property can still be used to execute the unbounded amplification attack on the Tor network, or just on the tor directory authorities.
One fix for this would be to add code to exit nodes to implicitly add all of the IP + ORport combinations of all other relays to their exit policy reject lines, or otherwise block this connection at some other level.