Three-phase active rectifier - Confusion between rectifier and inverter mode
In a perfect world, the diodes would never conduct. A mosfet channel can hapilly pass current in either direction and diodes have much worse volt drop than mosfet channels do. So ideally you want the mosfets to be turned on whenever current is flowing through them in either direction.

In the real world we can't turn mosfets on and of instantly and we don't want to create a current flow path between the two DC rails. So there will likely be a dead time when neither mosfet is turned on and current is forced through the body diodes.