cargo fuzz Targets for Wasmtime

This crate defines various libFuzzer fuzzing targets for Wasmtime, which can be run via cargo fuzz.

These fuzz targets just glue together pre-defined test case generators with oracles and pass libFuzzer-provided inputs to them. The test case generators and oracles themselves are independent from the fuzzing engine that is driving the fuzzing process and are defined in wasmtime/crates/fuzzing.

Safety warning

Fuzzers exist to generate random garbage and then try running it. You should not trust these fuzz targets: they could in theory try to read or write files on your disk, send your private data to reporters, or do anything else. If they succeed at doing something malicious, they are doing a great job at identifying dangerous bugs and we're proud of them.

In addition, some of these fuzz targets use other libraries, such as to test that our implementation matches other WebAssembly runtimes. We have not reviewed those runtimes or libraries for safety, security, correctness, supply-chain attacks, or any other properties. Software used only during fuzzing is not subject to our usual cargo vet requirements.

Paragraphs 7 and 8 of the license which this work is distributed to you under are especially important here: We disclaim all warranties and liability if running some fuzz target causes you any harm.

Therefore, if you are at all concerned about the safety of your computer, then you should either not run these fuzz targets, or only run them in a sandbox with sufficient isolation for your threat model.

Example

To start fuzzing run the following command, where $MY_FUZZ_TARGET is one of the available fuzz targets:

cargo fuzz run $MY_FUZZ_TARGET

Available Fuzz Targets

At the time of writing, we have the following fuzz targets:

• api_calls: stress the Wasmtime API by executing sequences of API calls; only the subset of the API is currently supported.
• compile: Attempt to compile libFuzzer's raw input bytes with Wasmtime.
• compile-maybe-invalid: Attempt to compile a wasm-smith-generated Wasm module with code sequences that may be invalid.
• cranelift-fuzzgen: Generate a Cranelift function and check that it returns the same results when compiled to the host and when using the Cranelift interpreter; only a subset of Cranelift IR is currently supported.
• cranelift-icache: Generate a Cranelift function A, applies a small mutation to its source, yielding a function A', and checks that A compiled + incremental compilation generates the same machine code as if A' was compiled from scratch.
• differential: Generate a Wasm module, evaluate each exported function with random inputs, and check that Wasmtime returns the same results as a choice of another engine: the Wasm spec interpreter (see the wasm-spec-interpreter crate), the wasmi interpreter, V8 (through the v8 crate), or Wasmtime itself run with a different configuration.
• instantiate: Generate a Wasm module and Wasmtime configuration and attempt to compile and instantiate with them.
• instantiate-many: Generate many Wasm modules and attempt to compile and instantiate them concurrently.
• spectests: Pick a random spec test and run it with a generated configuration.
• table_ops: Generate a sequence of externref table operations and run them in a GC environment.

The canonical list of fuzz targets is the .rs files in the fuzz_targets directory:

ls wasmtime/fuzz/fuzz_targets/

Corpora

While you can start from scratch, libFuzzer will work better if it is given a corpus of seed inputs to kick start the fuzzing process. We maintain a corpus for each of these fuzz targets in a dedicated repo on github.

You can use our corpora by cloning it and placing it at wasmtime/fuzz/corpus:

git clone \
    https://github.com/bytecodealliance/wasmtime-libfuzzer-corpus.git \
    wasmtime/fuzz/corpus

Reproducing a Fuzz Bug

When investigating a fuzz bug (especially one found by OSS-Fuzz), use the following steps to reproduce it locally:

1. Download the test case (either the "Minimized Testcase" or "Unminimized Testcase" from OSS-Fuzz will do).
2. Run the test case in the correct fuzz target:

   cargo +nightly fuzz run <target> <test case>

   If all goes well, the bug should reproduce and libFuzzer will dump the failure stack trace to stdout
3. For more debugging information, run the command above with RUST_LOG=debug to print the configuration and WebAssembly input used by the test case (see uses of log_wasm in the wasmtime-fuzzing crate).

Target specific options

cranelift-fuzzgen

Fuzzgen supports passing the FUZZGEN_ALLOWED_OPS environment variable, which when available restricts the instructions that it will generate.

Running FUZZGEN_ALLOWED_OPS=ineg,ishl cargo fuzz run cranelift-fuzzgen will run fuzzgen but only generate ineg or ishl opcodes.

cranelift-icache

The icache target also uses the fuzzgen library, thus also supports the FUZZGEN_ALLOWED_OPS environment variable as described in the cranelift-fuzzgen section above.