You can fork the JVM, but the patent license will only cover you if you are pass the compatibility kit, which Oracle may withhold whenever they feel like it. S

You can fork the JVM, but the patent license will only cover you if you are pass the compatibility kit, which Oracle may withhold whenever they feel like it. So from a copyright perspective you are fine (that's the point of the GPL) but from a trademark perspective you may not call it Java, and from a patent perspective there's nothing stopping Oracle from attempting to crush you in jurisdictions like the US with bad patent laws.