The Caliptra Root of Trust project, a collaboration between AMD, Google, Microsoft and NVIDIA within the CHIPS Alliance, is steadily heading towards its 2.0 release – an effort Antmicro is actively contributing to. They’ve recently described their implementation of User mode in the RISC-V VeeR EL2 core along with extended Physical Memory Protection, and related to this work, they’ve introduced support for VeeR EL2 with User Mode and PMP to the Tock embedded OS. The main goal of this implementation was to test the mode switching feature of the VeeR EL2 core with PMP enabled.
In this article we’ll describe implementation details, including Antmicro’s contributions to Tock, libtock-c and tockloader. We’ll also show an example in which a Tock application running on a VeeR EL2 core simulated in Verilator performs forbidden memory accesses, demonstrating that PMP enforces proper User mode constraints.
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Tock is an embedded operating system written in Rust, designed for running multiple concurrent, mutually distrustful applications on Cortex-M and RISC-V based platforms. As it provides isolation between components to ensure safety and security, it’s a popular choice for Root of Trust projects, such as OpenTitan used e.g. in Google’s Open Se Cura project that Antmicro has also been involved with.
When adding support for a new platform in Tock, we need to consider three layers of support: architecture, chip and board. Tock already supports several RISC-V platforms as well as the RISC-V privilege spec with Machine, Supervisor and User modes. When introducing support for VeeR in Tock, we were able to reuse the implementation of RISC-V-specific components, i.e. the general architecture support, machine timer and interrupt controller.
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Caliptra
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