With the development of multi-messenger astronomy and the detection of
astrophysical neutrinos, we are in a golden age of new cosmic ray (CR)
measurements and studies. Supernova remnants are believed as the leading
candidate for the source of Galactic CRs. Recent observations, however,
challenge the well-developed theory of shock acceleration of CRs in
supernova remnants. I will reexamine the assumptions in the standard
shock acceleration model and discuss the key missing physics for shocks
propagating in the turbulent and inhomogeneous interstellar medium. A
more realistic shock acceleration model holds the promise to understand
the observational puzzles and the origin of Galactic CRs.