Atmosphere is a full-featured microkernel developed in Rust and verified with Verus. Conceptually Atmosphere is similar to the line of L4 microkernels. Atmosphere pushes most kernel functionality to user-space, e.g., device drivers, network stack, file systems, etc. The microkernel supports a minimal set of mechanisms to implement address spaces, page-tables, coarse-grained memory management, and threads of execution that together with address spaces implement an abstraction of a process. Each process has a page table and a collection of schedulable threads. Atmosphere allows threads to control layout of their virtual address space through a collection of system calls that support mapping and unmapping pages as well as receiving pages from other threads via communication endpoints. To simplify verification, at the moment Atmosphere relies on a big-lock synchronization.
We develop all code in Rust and prove its functional correctness, i.e., refinement of a high-level specification with Verus (a Dafny-like automated verification engine for Rust). Similar to prior work, we carefully design the kernel to keep verification complexity under control. Still, Verus allows us to implement typical kernel data structures like linked lists, support verified memory allocation, develop proofs about page tables, etc.
A combination of Verus and Rust significantly reduces verification effort. On average our code has proof-to-code ratio of 7.5:1 which is significantly lower than in prior approaches. Moreover, Rust and Verus allow us to reason about a microkernel with a feature-rich interface that is conceptually similar to the line of classical L4 microkernels.