Final yr we wrote about how shifting native code in Android from C++ to Rust has resulted in fewer safety vulnerabilities. A lot of the elements we talked about then have been system providers in userspace (working underneath Linux), however these are usually not the one elements usually written in memory-unsafe languages. Many security-critical elements of an Android system run in a “bare-metal” setting, exterior of the Linux kernel, and these are traditionally written in C. As a part of our efforts to harden firmware on Android gadgets, we’re more and more utilizing Rust in these bare-metal environments too.

To that finish, we now have rewritten the Android Virtualization Framework’s protected VM (pVM) firmware in Rust to supply a reminiscence protected basis for the pVM root of belief. This firmware performs an analogous operate to a bootloader, and was initially constructed on prime of U-Boot, a broadly used open supply bootloader. Nonetheless, U-Boot was not designed with safety in a hostile setting in thoughts, and there have been quite a few safety vulnerabilities present in it because of out of bounds reminiscence entry, integer underflow and reminiscence corruption. Its VirtIO drivers particularly had various lacking or problematic bounds checks. We mounted the precise points we present in U-Boot, however by leveraging Rust we are able to keep away from these kinds of memory-safety vulnerabilities in future. The brand new Rust pVM firmware was launched in Android 14.

As a part of this effort, we contributed again to the Rust group through the use of and contributing to present crates the place doable, and publishing various new crates as properly. For instance, for VirtIO in pVM firmware we’ve frolicked fixing bugs and soundness points within the present virtio-drivers crate, in addition to including new performance, and are actually serving to preserve this crate. We’ve revealed crates for making PSCI and different Arm SMCCC calls, and for managing web page tables. These are only a begin; we plan to launch extra Rust crates to help bare-metal programming on a spread of platforms. These crates are additionally getting used exterior of Android, reminiscent of in Venture Oak and the bare-metal part of our Complete Rust course.

Coaching engineers

Many engineers have been positively stunned by how productive and nice Rust is to work with, offering good high-level options even in low-level environments. The engineers engaged on these initiatives come from a spread of backgrounds. Our complete Rust course has helped skilled and novice programmers shortly come on top of things. Anecdotally the Rust sort system (together with the borrow checker and lifetimes) helps keep away from making errors which can be simply made in C or C++, reminiscent of leaking tips that could stack-allocated values out of scope.

One in every of our bare-metal Rust course attendees had this to say:

"varieties will be constructed that usher in all of Rust's niceties and safeties and 
but nonetheless compile right down to extraordinarily environment friendly code like writes
of constants to memory-mapped IO."

97% of attendees that accomplished a survey agreed the course was value their time.

Benefits and challenges

Gadget drivers are sometimes written in an object-oriented trend for flexibility, even in C. Rust traits, which will be seen as a type of compile-time polymorphism, present a helpful high-level abstraction for this. In lots of instances this may be resolved totally at compile time, with no runtime overhead of dynamic dispatch through vtables or structs of operate pointers.

There have been some challenges. Protected Rust’s sort system is designed with an implicit assumption that the one reminiscence this system must care about is allotted by this system (be it on the stack, the heap, or statically), and solely utilized by this system. Naked-metal applications usually need to take care of MMIO and shared reminiscence, which break this assumption. This tends to require quite a lot of unsafe code and uncooked pointers, with restricted instruments for encapsulation. There’s some disagreement within the Rust group concerning the soundness of references to MMIO area, and the services for working with uncooked pointers in secure Rust are at present considerably restricted. The stabilisation of offset_of, slice_ptr_get, slice_ptr_len, and different nightly options will enhance this, however it’s nonetheless difficult to encapsulate cleanly. Higher syntax for accessing struct fields and array indices through uncooked pointers with out creating references would even be useful.

The concurrency launched by interrupt and exception handlers may also be awkward, as they usually must entry shared mutable state however can’t depend on having the ability to take locks. Higher abstractions for crucial sections will assist considerably, however there are some exceptions that may’t virtually be disabled, reminiscent of web page faults used to implement copy-on-write or different on-demand web page mapping methods.

One other situation we’ve had is that some unsafe operations, reminiscent of manipulating the web page desk, can’t be encapsulated cleanly as they’ve security implications for the entire program. Normally in Rust we’re capable of encapsulate unsafe operations (operations which can trigger undefined behaviour in some circumstances, as a result of they’ve contracts which the compiler can’t examine) in protected wrappers the place we guarantee the mandatory preconditions in order that it isn’t doable for any caller to trigger undefined behaviour. Nonetheless, mapping or unmapping pages in a single a part of this system could make different components of this system invalid, so we haven’t discovered a method to supply a totally common protected interface to this. It must be famous that the identical issues apply to a program written in C, the place the programmer at all times has to purpose concerning the security of the entire program.

Some individuals adopting Rust for bare-metal use instances have raised issues about binary dimension. We have now seen this in some instances; for instance our Rust pVM firmware binary is round 460 kB in comparison with 220 kB for the sooner C model. Nonetheless, this isn’t a good comparability as we additionally added extra performance which allowed us to take away different elements from the boot chain, so the general dimension of all VM boot chain elements was comparable. We additionally weren’t notably optimizing for binary dimension on this case; velocity and correctness have been extra vital. In instances the place binary dimension is crucial, compiling with dimension optimization, being cautious about dependencies, and avoiding Rust’s string formatting equipment in launch builds normally permits comparable outcomes to C.

Architectural help is one other concern. Rust is usually properly supported on the Arm and RISC-V cores that we see most frequently, however help for extra esoteric architectures (for instance, the Qualcomm Hexagon DSP included in lots of Qualcomm SoCs utilized in Android telephones) will be missing in comparison with C.

The way forward for bare-metal Rust

Total, regardless of these challenges and limitations, we’ve nonetheless discovered Rust to be a major enchancment over C (or C++), each by way of security and productiveness, in all of the bare-metal use instances the place we’ve tried it thus far. We plan to make use of it wherever sensible.

In addition to the work within the Android Virtualization Framework, the group engaged on Trusty (the open-source Trusted Execution Setting used on Pixel telephones, amongst others) have been arduous at work including help for Trusted Functions written in Rust. For instance, the reference KeyMint Trusted Utility implementation is now in Rust. And there’s extra to return in future Android gadgets, as we proceed to make use of Rust to enhance safety of the gadgets you belief.