This document discusses porting the Tock operating system to the OpenTitan project. It provides background on OpenTitan and Tock, describes the status of the porting work, and highlights a deep dive into implementing USB and CTAP support on Tock running on OpenTitan hardware. Key points covered include OpenTitan using the Ibex RISC-V core, Tock being designed for small platforms without MMUs and enforcing security through Rust, the interface for Tock applications, and modules already supported through the mainline Tock project.

1. © 2020 Western Digital Corporation or its affiliates. All rights reserved. 4/19/2021
Porting Tock to OpenTitan
Alistair Francis <alistair.francis@wdc.com>
December 2020

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Todays Talk
• What is OpenTitan?
• What is Tock?
• What do Tock apps look like?
• Status of Tock on OpenTitan
• Deep dive into USB/CTAP support in Tock on OpenTitan

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OpenTitan
• OpenTitan is an open source silicon Root
of Trust (RoT) project
• Backed by lowRISC, ETH, Google, Nuvoton,
Western Digital and others
• OpenTitan uses the Ibex RV32 core as its
main processor
– Includes a variety of IP blocks for
security, including AES, HMAC

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Tock
• Tock is an embedded operating system
written in Rust
• It’s designed for small platforms without
an MMU
– Tock's design centers around protection,
both from potentially malicious
applications and from device drivers
• Tock uses the Rust language to enforce
security and safety protections in the
kernel

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Tock Apps
• Apps can be written in C (libtock-c) or Rust
(libtock-rs)
– C apps have better support
– OpenTitan likes Rust as it provides more safety
– Other languages are possible as well
• Tock uses the ARM® MPU or RISC-V PMP to
isolate apps
• Apps only have access to their own
memory and can’t interfere with the kernel
or other apps
– There are kernel calls and IPC to allow
communication between apps
OpenTitan Hardware
OpenTitan Boot ROM
Tock
U2F App RNG App Other App

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App interface to kernel
• Four main system call categories
– allow
– subscribe
– command
– yield

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Tock on OpenTitan
• The majority of work happens on mainline
Tock
• Mainline Tock supports:
– AES
– Flash Controller
– GPIOs
– HMAC
– I2C
– Power Manager
– Timers
– UART
– USB/CTAP
• Upcoming work
– ePMP
– Key/Value filesystem
– System call (syscall) Filtering
– Storage ACLs

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CTAP2 Client To Authenticator Protocol
• CTAP is a spec developed by the FIDO
Alliance that specifies the protocol for a
host to communicate with a cryptographic
authenticator device.
– Is used by browsers and Yubikeys for
WebAuthn
• Ctap Rust crate already exists:
https://crates.io/crates/ctap2-
authenticator This Photo by MesserWoland is licensed under CC BY-SA

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Tock Kernel Changes for USB

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Demo

11. © 2019 Western Digital Corporation or its affiliates. All rights reserved. 4/19/2021