  Nodal line semimetals are characterized by nontrivial bulk-band crossings,
giving rise to almost flat drumhead-like surface states (DSS), which provide an
attractive playground where interaction can induce symmetry-broken states and
potential emergent phases. Here, we show that electronic interaction causes
Stoner ferromagnetic instability in the DSS while the bulk remains
non-magnetic, which together with spin-orbit coupling drive the surface states
into a 2D Chern insulator. We show that each DSS carries a half-monopole
charge, which for systems containing two pieces of DSS yield a net Chern number
$\mathcal{C}=-1$. We show that this phenomenology is robust against
chiral-symmetry breaking, that gives a finite dispersion to the DSS. Our work
shows that nodal line semimetals are a promising platform to implement surface
Chern insulators and dissipation-less electron transport by exploiting enhanced
interaction effects of the DSS.
