We investigate the electronic properties of HfO2 grain boundaries employing a simple Σ5 (310)/[001] grain boundary model based on the cubic phase. Our calculations show the emergence of unoccupied defect states 0.4 eV below the conduction band due to the under-coordination of certain Hf ions in the grain boundary. They also show that migration of metal interstitials such as Hf and Ti to the grain boundary is energetically favorable, turning the grain boundary region metallic. This scenario may create leakage paths in poly-crystalline HfO2 or serve as the conduction mechanism in resistive random access memories.