Back when Google unveiled multi-user support with Android 4.2, one of the first question on many users' minds was how will root access be handled on secondary user accounts? It was a short answer, actually: it wouldn't. Superuser managers like SuperSU simply wouldn't run on anything aside from the primary user.
However, Android developer Chainfire – the mastermind behind SuperSU – knew that it could be done. It's been clear that he has kept this venture on the forefront of his mind, and he's finally broken the barrier.