RLBox is a toolkit for sandboxing third party C libraries, that are being used by C++ code (support for other languages is in the works). RLBox was originally developed for Firefox1, which has been shipping with it since 2020.

The RLBox toolkit consists of:

  1. A C++ framework (RLBox) that makes it easy to retrofit existing application code to safely interface with sandboxed libraries.

  2. A Wasm backend (based on wasm2c) for isolating (sandboxing) C libraries.

In this section, we provide an overview of the RLbox framework, its reason for being, and a high level sketch of how it works. In the next section, we will provide a tutorial that provides an end-to-end example of applying RLbox to a simple application.

Why RLBox

Work on RLbox began several years ago while attempting to add fine grain isolation third party libraries in the Firefox renderer. Initially we attempted this process without any support from a framwork like RLBox, instead attempting to manually deal with all the details sandboxing such as sanitizing untrusted inputs, and reconciling ABI differences between the sandbox and host application.

This went poorly, it was tedious, error prone, and did nothing to abstract the details of the underlying sandbox from the developer. We had basically no hope that this would result in code that was maintainable, or that normal Mozilla developers who were unfamiliar with the gory details of our system would be able to sandbox new library, let alone maintain existing ones.

So we scrapped this manual approach and build RLBox1.

RLbox automates many of the low level details of sandboxing and allows you, as a security engineer or application developer, to instead focus just on what you need to do to sandbox your particular application.

To sandbox a library — and thus to move to a world where the library is no longer trusted — we need to modify this application-library boundary. For example, we need to add security checks in Firefox to ensure that any value from the sandboxed library is properly validated before it is used. Otherwise, the library (when compromised) may be able to abuse Firefox code to hijack its control flow 1. The RLBox API is explicitly designed to make retrofitting of existing application code simpler and less error-prone.2

What does RLBox provide?

RLBox ensures that a sandboxed library is memory isolated from the rest of the application — the library cannot directly access memory outside its designated region — and that all boundary crossings are explicit. This ensures that the library cannot, for example, corrupt Firefox's address space. It also ensures that Firefox cannot inadvertently expose sensitive data to the library. The figure below illustrates this idea.

RLBox explicitly isolates the library data and control flow from the application

Memory isolation is enforced by the underlying sandboxing mechanism (e.g., using Wasm3) from the start, when you create the sandbox with create_sandbox(). Explicit boundary crossings are enforced by RLBox (either at compile- or and run-time). For example, with RLBox you can't call library functions directly; instead, you must use the invoke_sandbox_function() method. Similarly, the library cannot call arbitrary Firefox functions; instead, it can only call functions that you expose with the register_callback() method. (To simplify the sandboxing task, though, RLBox does expose a standard library as described in the Standard Library.)

When calling a library function, RLBox copies simple values into the sandbox memory before calling the function. For larger data types, such as structs and arrays, you can't simply pass a pointer to the object. This would leak ASLR and, more importantly, would not work: sandboxed code cannot access application memory. So, you must explicitly allocate memory in the sandbox via malloc_in_sandbox() and copy application data to this region of memory (e.g., via strlcpy).

RLBox similarly copies simple return values and callback arguments. Larger data structures, however, must (again) be passed by sandbox-reference, i.e., via a reference/pointer to sandbox memory.

To ensure that application code doesn't unsafely use values that originate in the sandbox -- and may thus be under the control of an attacker -- RLBox considers all such values as untrusted and taints them. Tainted values are essentially opaque values (though RLBox does provide some basic operators on tainted values). To use a tainted value, you must unwrap it by (typically) copying the value into application memory -- and thus out of the reach of the attacker -- and verifying it. Indeed, RLBox forces application code to perform the copy and verification in sync using verification functions (see this).