We develop backstepping state feedback control to stabilize a moving shockwave in a freeway segment under bilateral boundary actuations of traffic flow. A moving shockwave, consisting of light traffic upstream of the shockwave and heavy traffic downstream, is usually caused by changes of local road situations. The density discontinuity travels upstream and drivers caught in the shockwave experience transitions from free to congested traffic. Boundary control design in this article brings the shockwave front to a static setpoint position, hindering the upstream propagation of traffic congestion. The traffic dynamics are described with Lighthil-Whitham-Richard model, leading to a system of two first-order hyperbolic partial differential equations (PDEs). Each represents the traffic density of a spatial domain segregated by the moving interface. By Rankine-Hugoniot condition, the interface position is driven by flux discontinuity and thus governed by an ordinary differential equation (ODE) dependent on the PDE states. The control objective is to stabilize both the PDE states of traffic density and the ODE state of moving shock position to setpoint values. Using delay representation and backstepping method, we design predictor feedback controllers to cooperatively compensate state-dependent input delays to the ODE. From Lyapunov stability analysis, we show local stability of the closed-loop system in <inline-formula><tex-math notation="LaTeX">H^1</tex-math></inline-formula> norm with an arbitrarily fast convergence rate. The stabilization result is demonstrated by a numerical simulation and the total travel time of the open-loop system is reduced by <inline-formula><tex-math notation="LaTeX">12 \%</tex-math></inline-formula> in the closed loop.