Bringing Rust to the Pixel Baseband

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.

Code size by category:

• Rust implemented Shim that calls Hickory-proto on receiving a DNS response: 4KB
• core, alloc, compiler_builtins (reusable, one-time cost): 17KB
• Hickory-proto library and dependencies: 350KB
• Sum: 371KB

We built prototypes and measured size with size-optimized settings. Expectedly, hickory_proto is not designed with embedded use in mind, and is not optimized for size. As the Pixel modem is not tightly memory constrained, we prioritized community support and code quality, leaving code size optimizations as future work.

However, the additional code size may be a blocker for other embedded systems. This could be addressed in the future by adding additional feature flags to conditionally compile only required functionality. Implementing this modularity would be a valuable future work.

Hook-up Rust to modem firmware

Before building the Rust DNS library, we defined several Rust unit tests to cover basic arithmetic, dynamic allocations, and FFI to verify the integration of Rust with the existing modem firmware code base.

Compile Rust code to staticlib

While using cargo is the default choice for compilation in the Rust ecosystem, it presents challenges when integrating it into existing build systems. We evaluated two options:

1. Using cargo to build a staticlib before the modem builds. Then add the produced staticlib into the linking step.
2. Directly work with rustc and integrate the Rust compilation steps into the existing modem build system.

Option #1 does not scale if we are going to add more Rust components in the future, as linking multiple staticlibs may cause duplicated symbol errors. We chose option #2 as it scales more easily and allows tighter integration into our existing build system. Our existing C/C++ codebase uses Pigweed to drive the primary build system. Pigweed supports Rust targets (example) with direct calls to rustc through rust tools defined in GN.

We compiled all the Rust crates, including hickory-proto, its dependencies, and core, compiler_builtin, alloc, to rlib. Then, we created a staticlib target with a single lib.rs file which references all the rlib crates using extern crate keywords.

Build core, alloc, and compiler_builtins

Android’s Rust Toolchain distributes source code of core, alloc, and compiler_builtins, and we leveraged this for the modem. They can be included to the build graph by adding a GN target with crate_root pointing to the root lib.rs of each crate.

Pixel modem firmware already has a well-tested and specialized global memory allocation system to support some dynamic memory allocations. alloc support was added by implementing the GlobalAlloc with FFI calls to the allocators C APIs:

Pixel modem firmware already implements a backend for the Pigweed crash facade as the global crash handler. Exposing it into Rust panic_handler through FFI unifies the crash handling for both Rust and C/C++ code.

Link Rust staticlib

The Pixel modem firmware linking has a step that calls the linker to link all the objects generated from C/C++ code. By using llvm-ar -x to extract object files from the Rust combined staticlib and supplying them to the linker, the Rust code appears in the final modem image.

There was a performance issue we experienced due to weak symbols during linking. The inclusion of Rust core and compiler-builtin caused unexpected power and performance regressions on various tests. Upon analysis, we realized that modem optimized implementations of memset and memcpy provided by the modem firmware are accidentally replaced by those defined in compiler_builtin. It seems to happen because both compiler_builtin crate and the existing codebase defines symbols as weak, linker has no way to figure out which one is weaker. We fixed the regression by stripping the compiler_builtin crate before linking using a one line shell script.

Integrating hickory-proto

Expose Rust API and calling back to C++

For the DNS parser, we declared the DNS response parsing API in C and then implemented the same API in Rust.

The Rust function returns an integer standing for the error code. The received DNS answers in the DNS response are required to be updated to in-memory data structures that are coupled with the original C implementation, therefore, we use existing C functions to do it. The existing C functions are dispatched from the Rust implementation.

In our case, the DNS responding parsing function API is simple enough for us to hand write, while the callbacks back to C functions for handling the response have complex data type conversions. Therefore, we leveraged bindgen to generate FFI code for the callbacks.

Build third-party crates

Even with all features disabled, hickory-proto introduces more than 30 dependent crates. Manually written build rules are difficult to ensure correctness and scale poorly when upgrading dependencies into new versions.

Fuchsia has developed cargo-gnaw to support building their third party Rust crates. Cargo-gnaw works by invoking cargo metadata to resolve dependencies, then parse and generate GN build rules. This ensures correctness and ease of maintenance.

Conclusion

The Pixel 10 series of phones marks a pivotal moment, being the first Pixel device to integrate a memory-safe language into its modem.

While replacing one piece of risky attack surface is itself valuable, this project lays the foundation for future integration of memory-safe parsers and code into the cellular baseband, ensuring the baseband’s security posture will continue to improve as development continues.

Special thanks to Armando Montanez, Bjorn Mellem, Boky Chen, Cheng-Yu Tsai, Dominik Maier, Erik Gilling, Ever Rosales, Hungyen Weng, Ivan Lozano, James Farrell, Jeffrey Vander Stoep, Jiacheng Lu, Jingjing Bu, Min Xu, Murphy Stein, Ray Weng, Shawn Yang, Sherk Chung, Stephan Chen, Stephen Hines.