Final 12 months we wrote about how shifting native code in Android from C++ to Rust has resulted in fewer safety vulnerabilities. Many of the elements we talked about then had been system companies in userspace (working below Linux), however these usually are not the one elements sometimes written in memory-unsafe languages. Many security-critical elements of an Android system run in a “bare-metal” setting, outdoors 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’ve got 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 the same operate to a bootloader, and was initially constructed on high of U-Boot, a extensively 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 resulting from out of bounds reminiscence entry, integer underflow and reminiscence corruption. Its VirtIO drivers specifically had a lot of lacking or problematic bounds checks. We mounted the particular points we present in U-Boot, however by leveraging Rust we will keep away from these types 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 neighborhood through the use of and contributing to current crates the place attainable, and publishing a lot of new crates as effectively. For instance, for VirtIO in pVM firmware we’ve hung out fixing bugs and soundness points within the current virtio-drivers crate, in addition to including new performance, and at the moment are 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 assist bare-metal programming on a spread of platforms. These crates are additionally getting used outdoors of Android, reminiscent of in Challenge 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 rapidly come up to the mark. 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 to stack-allocated values out of scope.
One in all our bare-metal Rust course attendees had this to say:
"varieties could 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
Machine drivers are sometimes written in an object-oriented vogue for flexibility, even in C. Rust traits, which could 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 solely at compile time, with no runtime overhead of dynamic dispatch through vtables or structs of operate pointers.
There have been some challenges. Secure 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 packages usually should take care of MMIO and shared reminiscence, which break this assumption. This tends to require a whole lot of unsafe code and uncooked pointers, with restricted instruments for encapsulation. There may be some disagreement within the Rust neighborhood concerning the soundness of references to MMIO house, and the amenities for working with uncooked pointers in steady Rust are presently 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 have to 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 in a position to encapsulate unsafe operations (operations which can trigger undefined behaviour in some circumstances, as a result of they’ve contracts which the compiler can’t verify) in protected wrappers the place we guarantee the required preconditions in order that it’s not attainable for any caller to trigger undefined behaviour. Nonetheless, mapping or unmapping pages in a single a part of this system could make different elements of this system invalid, so we haven’t discovered a manner to supply a totally normal protected interface to this. It ought to be famous that the identical issues apply to a program written in C, the place the programmer at all times has to motive concerning the security of the entire program.
Some individuals adopting Rust for bare-metal use instances have raised issues about binary dimension. We now have 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 significantly optimizing for binary dimension on this case; velocity and correctness had been extra essential. 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 assist is one other concern. Rust is usually effectively supported on the Arm and RISC-V cores that we see most frequently, however assist for extra esoteric architectures (for instance, the Qualcomm Hexagon DSP included in lots of Qualcomm SoCs utilized in Android telephones) could 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 big enchancment over C (or C++), each when it comes to 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 staff engaged on Trusty (the open-source Trusted Execution Setting used on Pixel telephones, amongst others) have been onerous at work including assist for Trusted Purposes written in Rust. For instance, the reference KeyMint Trusted Software implementation is now in Rust. And there’s extra to come back in future Android gadgets, as we proceed to make use of Rust to enhance safety of the gadgets you belief.
