The CG must be ahead of the neutral point, but that doesn't mean it can't produce a positive lift. The important thing is to have a greater angle of attack on the main wings than on the tailplane. That is what provides stability.
This site gives a very good explanation to longitudinal (angle of attack) stability:
Angle of Attack Stability, Trim, and Spiral Dives [Ch. 6 of See How It Flies]
According to the author of the article above, you can even get a C172 to produce a positive lift on the tailplane with aft CG. You can read about the experiment in the article.