  Polarized attosecond $\gamma-$rays may offer excitation and hyperfine
tracking of reactions relevant to nuclear physics, astrophysics, high-energy
physics, etc. However, unfortunately, generation of a feasible and
easy-to-deploy source is still a great challenge. Here, we put forward a novel
method for producing ultra-collimated high-brilliance polarized attosecond
$\gamma-$rays via the interaction of an unpolarized electron beam with a
solid-density plasma. As a relativistic electron beam enters a solid-density
plasma, it can be modulated into high-density clusters via the self-modulation
instability of itself and further into attosecond slices due to its own hosing
instability. This is accompanied by the generation of similar pulse-width
$\gamma-$slices via nonlinear Compton scattering. The severe hosing instability
breaks the symmetry of the excited electromagnetic fields, resulting in net
linear polarization of $\gamma-$slices, which challenges the conventional
perception that the interaction of an axially symmetric unpolarized electron
beam with a uniform plasma cannot generate polarized radiation. In addition, we
also obtain high-quality electron microbunches which may serve as an
alternative source for prebunched free-electron lasers.
