Bringing Rust to the Pixel Baseband

Posted by Jiacheng Lu, Software Engineer, Google Pixel Team

Google is continuously advancing the security of Pixel devices. We have been focusing on hardening the cellular baseband modem against exploitation. Recognizing the risks associated within the complex modem firmware, Pixel 9 shipped with mitigations against a range of memory-safety vulnerabilities. For Pixel 10, Google is advancing its proactive security measures further. Following our previous discussion on “Deploying Rust in Existing Firmware Codebases”, this post shares a concrete application: integrating a memory-safe Rust DNS(Domain Name System) parser into the modem firmware. The new Rust-based DNS parser significantly reduces our security risk by mitigating an entire class of vulnerabilities in a risky area, while also laying the foundation for broader adoption of memory-safe code in other areas.

Here we share our experience of working on it, and hope it can inspire the use of more memory safe languages in low-level environments.

Why Modem Memory Safety Can’t Wait

In recent years, we have seen increasing interest in the cellular modem from attackers and security researchers. For example, Google’s Project Zero gained remote code execution on Pixel modems over the Internet. Pixel modem has tens of Megabytes of executable code. Given the complexity and remote attack surface of the modem, other critical memory safety vulnerabilities may remain in the predominantly memory-unsafe firmware code.

Why DNS?

The DNS protocol is most commonly known in the context of browsers finding websites. With the evolution of cellular technology, modern cellular communications have migrated to digital data networks; consequently, even basic operations such as call forwarding rely on DNS services.

DNS is a complex protocol and requires parsing of untrusted data, which can lead to vulnerabilities, particularly when implemented in a memory-unsafe language (example: CVE-2024-27227). Implementing the DNS parser in Rust offers value by decreasing the attack surfaces associated with memory unsafety.

Picking a DNS library

DNS already has a level of support in the open-source Rust community. We evaluated multiple open source crates that implement DNS. Based on criteria shared in earlier posts, we identified hickory-proto as the best candidate. It has excellent maintenance, over 75% test coverage, and widespread adoption in the Rust community. Its pervasiveness shows its potential as the de-facto DNS choice and long term support. Although hickory-proto initially lacked no_std support, which is needed for Bare-metal environments (see our previous post on this topic), we were able to add support to it and its dependencies.

Adding no_std support

The work to enable no_std for hickory-proto is mostly mechanical. We shared the process in a previous post. We undertook modifications to hickory_proto and its dependencies to enable no_std support. The upstream no_std work also results in a no_std URL parser, beneficial to other projects.

• https://github.com/hickory-dns/hickory-dns/pull/2104
• https://github.com/servo/rust-url/pull/831
• https://github.com/krisprice/ipnet/pull/58

The above PRs are great examples of how to extend no_std support to existing std-only crates.

Code size study

Code size is the one of the factors that we evaluated when picking the DNS library to use.