fair game

Short definition from Wiktionary

Noun

1. 1
   Actions permissible by the rules.
2. 2
   A goal or an object that may legitimately be sought.
3. 3
   An acceptable subject or target for criticism, scrutiny, mockery, or attack.
4. 4
   Used other than figuratively or idiomatically: see fair, game.: a game that is fair, that does not involve cheating, etc.