As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
INFLIGHT, CREW REPORTED SMOKE IN CABIN AND SMELLED LIKE BURNED RUBBER. RETURNED TO DFW AND LANDED WITHOUT INCIDENT. FOUND APU OIL LEAK ON LEFT SIDE WITH INGESTION INTO AIR CONDITIONING SYSTEM. REPLACED APU ON 13 DEC AT DFW. SYSTEM GROUND CHECKED NORMAL OPERATION.