seL4 + TrustZone: Spanning both worlds
Arm TrustZone partitions a SoC’s hardware resources into two worlds: the Secure World for the security subsystem, and the Normal World for everything else. By configuring seL4 to span both worlds, we obtain a firmware design with greater assurance and flexibility than traditional secure monitor firmware. Such a configuration will become even more interesting and useful in the coming months as the Armv8.4-SecEL2 extension for secure virtualization becomes available in silicon.
To both motivate and demonstrate such a firmware design, we present IceCap, an experimental platform from Arm Research for virtualization and general-purpose confidential compute, based on seL4. By implementing the design introduced above, IceCap binds hardware isolation boundaries enforced by TrustZone with software isolation boundaries enforced by seL4 to fit a strong threat model without sacrificing simplicity of design and implementation. Our preliminary results suggest that this world-spanning configuration of seL4 on AArch64 is worth the attention of the seL4 community.
In addition to the firmware design described above, IceCap also features a dynamic adaptation of CapDL, a seL4 userland written entirely in the Rust programming language, and ultra-lightweight virtualization with a virtual machine monitor consisting of <1kLOC.