How much load is on the buck input side?
You can connect FETs in anti-series (i.e., gates together, sources together, wire drain-to-drain for load) for a bidirectional switch, of course it incurs twice the Rds(on), which makes P-ch MOS that much less desirable. There are protection controllers (reverse, UVLO, OVLO, surge) that handle this, if you want a one-chip solution (and don't mind paying for it).
Tim