Fuzz Modules

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Rust fuzzing is supported through the libfuzzer-sys crate, which provides bindings to LLVM's libFuzzer fuzzing engine. For more information, see the libfuzzer-sys repository as well as the LLVM libFuzzer project page.

The rust_fuzz module produces a fuzzer binary which begins fuzzing when it's run (similar to cc_fuzz modules). As the fuzzer leverages the libFuzzer fuzzing engine, it can take a number of arguments to control fuzzing. These are enumerated in the libFuzzer documentation.

rust_fuzz modules are an extension of rust_binary modules, and as such share the same properties and considerations. Additionally, they implement many of the same properties and functionality as do the cc_fuzz modules.

Writing a basic Rust fuzzer

You can define a fuzz module in an Android.bp build file with this code:

rust_fuzz {
    name: "example_rust_fuzzer",
    srcs: ["fuzzer.rs"],

    // Config for running the target on fuzzing infrastructure can be set under
    // fuzz_config. This shares the same properties as cc_fuzz's fuzz_config.
    fuzz_config: {
        fuzz_on_haiku_device: true,
        fuzz_on_haiku_host: false,
    },

    // Path to a corpus of sample inputs, optional. See https://llvm.org/docs/LibFuzzer.html#corpus
    corpus: ["testdata/*"],

    // Path to a dictionary of sample byte sequences, optional. See https://llvm.org/docs/LibFuzzer.html#dictionaries
    dictionary: "example_rust_fuzzer.dict",
}

The fuzzer.rs file contains a simple fuzzer:

fn heap_oob() {
    let xs = vec![0, 1, 2, 3];
    let val = unsafe { *xs.as_ptr().offset(4) };
    println!("Out-of-bounds heap value: {}", val);
}

fuzz_target!(|data: &[u8]| {
    let magic_number = 327;
    if data.len() == magic_number {
        heap_oob();
    }
});

Here fuzz_target!(|data: &[u8]| { /* fuzz using data here */ }); defines the fuzz-target entry-point called by the libFuzzer engine. The data argument is a sequence of bytes provided by the libFuzzer engine to be manipulated as input to fuzz the targeted function.

In this example fuzzer, only the length of the data gets checked to determine whether to call the heap_oob function, the calling of which results in an out-of-bounds read. libFuzzer is a coverage-guided fuzzer, so it quickly converges on the problematic length as it determines that the first 326 B of data don't result in new execution paths.

Locate this example, in-tree, at tools/security/fuzzing/example_rust_fuzzer/. To view a slightly more complex example of another fuzzer (which fuzzes a rustlib dependency) in-tree, see the legacy_blob_fuzzer.

For guidance on how to write structure-aware Rust fuzzers, see the Rust Fuzz book, the official documentation for the Rust Fuzz project.