While massive MIMO based underlay cognitive radio (CR) networks are a promising concept in the next generation of wireless networks to increase spectral efficiency, reusing the same pilot sequences in both networks cause pilot contamination leading to residual interference. Thus, this paper investigates the affects of pilot contamination on a random CR network underlaid upon a random primary network where both networks employ path loss inversion based power control. A Matern cluster process is considered for the underlay system, while homogeneous Poisson point processes are considered for the primary transmitters and receivers. We derive the moment generating function of the normalized aggregate interference at an underlay receiver, its first two moments, and the outage probability. Finally, it is shown that the underlay cluster radius, ensured received power levels through power control and the different node densities have a significant effect on the outage of an underlay receiver.