We analyze the Ruderman-Kettel-Kasuya-Yosida(RKKY) interaction between magnetic impurities embedded in the helical metal on the surface of three-dimensional topological insulators. Apart from the conventional RKKY terms, the spin-momentum locking of conduction electrons also leads to a significant Dzyaloshinskii-Moriya (DM) interaction between impurity spins. For a chain of magnetic impurities, the DM term can result in single-handed spin helix on the surface. The handedness of spin helix is locked with the sign of Fermi velocity of the emergent Dirac fermions on the surface. We also show the polarization of impurity spins can be controlled via electric voltage for dilute magnetic impurity concentration. ; Comment: 4 pages, 3 figures