While intransitive noninterference is a natural property for any secure OS kernel to enforce, proving that the implementation of any particular general-purpose kernel enforces this property is yet to be achieved. In this paper we take a significant step towards this vision by presenting a machine-checked formulation of intransitive noninterference for OS kernels, and its associated sound and complete unwinding conditions, as well as a scalable proof calculus over nondeterministic state monads for discharging these unwinding conditions across a kernel’s implementation. Our ongoing experience applying this noninterference framework and proof calculus to the seL4 microkernel validates their utility and real-world applicability.