PHB p. 168: Polearm Master
You gain the following benefits:
- When you take the Attack action and attack with only a glaive, halberd, or quarterstaff, you can use a bonus action to make a melee attack with the opposite end of the weapon. This attack uses the same ability modifier as the primary attack. The weapon's damage die for this attack is a d4, and it deals bludgeoning damage. [...]
Do I use the same reach as the primary weapon as well?