  In this project, we will compute the form factors relevant for $B \to K^*(\to
K \pi)\ell^+\ell^-$ decays. To map the finite-volume matrix elements computed
on the lattice to the infinite-volume $B \to K \pi$ matrix elements, the $K
\pi$ scattering amplitude needs to be determined using L\"uscher's method. Here
we present preliminary results from our calculations with $2+1$ flavors of
dynamical clover fermions. We extract the $P$-wave scattering phase shifts and
determine the $K^*$ resonance mass and the $K^* K \pi$ coupling for two
different ensembles with pion masses of $317(2)$ and $178(2)$ MeV.
