Beacon Fuzz - Update #01

Beacon Fuzz - Progress Update #1

Beacon Fuzz - Update #01

Integrating new Eth2 clients, Fuzzing in the cloud

Sigma Prime is leading the development and maintenance of beacon-fuzz, a differential fuzzing solution for Eth2 clients. This post is part of our series of monthly status updates where we discuss current progress, interesting challenges encountered, and direction for future work. See #00 and the repository's README for more context.


Over the last few weeks we've been making quiet progress, focusing primarily on incorporating new clients, improving consistency and maintainability, and updating the targeted spec. Achievements and points of interest include:

  • Updated to spec v0.9.1
  • Full integration of ZRNT, PySpec, and Lighthouse
  • Nimbus and Trinity integration POCs
  • First bug identified by Beacon Fuzz on Nimbus (attestation processing function)
  • Challenges incorporating Prysm as a second Golang client
  • Continuous fuzzing on Fuzzit
  • Leveraging Python sub-interpreters for concurrently fuzzing multiple Python implementations (in our case, Trinity and PySpec)
  • An additional fuzzing target: attester_slashing/process_attester_slashing()

To Eth2 Spec v0.9.1

All of our fuzzers are now operating on the v0.9.1 Eth2 spec, with matching starter corpora available here.

This was relatively straightforward to implement (at the fuzzer level). The majority of the work was associated with updating the Golang helper library/preprocessor to remove crosslinks and transfers,and freezing client implementations to a suitable commit/tag. It wasn't possible to find a suitable commit in all cases, as some implementations were working on v0.9.1 and v0.9.2 in parallel and had some sections associated with each.

Nimbus and Trinity Integrations

A big thanks to the Nimbus team, who developed a static library nfuzz that implements harnesses for several targets. There is a working POC on our add-nimbus branch (which will shortly be merged to master after a few logging and error-handling changes are finalized). This is currently being exercised on Fuzzit (see Fuzzit section of this blog post for more details). Fuzzit Similarly, Trinity has been successfully integrated. Its harnesses currently live in the add-trinity branch until some spec version differences are resolved (when clients implement spec v0.9.2 or v0.9.3). Some interesting details pertaining to this integration are discussed in more detail in the section below.

Incorporating Additional Implementations With Pragmatic Isolation

For differential fuzzing, it's desirable that each client implementation is exercised in relative isolation with minimal modification. Although sometimes necessary, each modification to the implementation code increases the risk of introducing "false-positive" bugs (which are detected by the fuzzer but are not present in the actual implementation), or masking client bugs such that the fuzzer cannot detect them.

Similarly, we want to avoid crashes caused by interference between clients (e.g. when implementations rely on different versions of the same dependency or rely on the same global state). Although current clients don't clash much, we should not rely on this, and should consider the issue now before it becomes a major headache. Hypothetically, each implementation could achieve effective memory isolation by running in a separate process. However, this introduces substantial overhead and complications with instrumentation. As our current fuzzing engine, libfuzzer, runs the fuzzing harness in a single process, any multi-process solution would also require a significant re-architecture.

So how do we achieve a reasonable level of in-process isolation, where clients do not affect the correctness of others? For clients in different languages (or VM runtimes), we deem it reasonable to leave as-is, realizing that a clashing symbol name is highly unlikely, and we primarily focus on cases where there are multiple clients in the same language.

So far, this is the case for Python (Trinity and Pyspec), and Golang (Prysm and ZRNT). Isolation solutions are specific to each language.

Python and Sub-Interpreters

The CPython interpreter and runtime relies on a global thread state. Naively trying to embed or link multiple interpreters is doomed to fail (as we'd quickly discovered). Fortunately, we stumbled upon a lesser-known feature of the CPython C-API: sub-interpreters. They are quite niche and, currently, Apache/mod_wsgi is one of the few major projects that leverage them.

With a couple C function-calls, we can start a new sub-interpreter that is reasonably isolated - each execution environment maintains its own sys.path, builtins, and other modules. Although currently of limited functionality (they cannot provide true parallelism due to the Global Interpreter Lock, and have no Python-level ability to communicate between interpreters), they are perfect for our use case (we don't care about inter-interpreter communication, and the fuzzer process is single-threaded/serialized anyway).

If you are interested in learning more about Python sub-interpreters, the following links are useful:

Golang Library Linking Errors

When working to integrate Prysm, we found that the existing approach (designed by Guido Vranken) did not extend to more than one Golang client in the same fuzzer. As of now, we can test with ZRNT or Prysm (having a successful POC that integrates their bazel build system), but not both. We are also limited to performing Prysm tests without input preprocessing (which relies on ZRNT).

The rest of this section will examine the issue in more detail, including an exploration of possible approaches along with their identified challenges and associated costs.

Existing approach

The existing approach uses a forked go-fuzz (modified to return output for differential comparison). go-fuzz produces a c-archive static library with provided coverage instrumentation and a harness endpoint. As this archive contains the Go runtime, redefinitions and symbol clashes occur when trying to link more than one of them. Possible approaches generally involve leaving go-fuzz unchanged and resolving linking problems, or modifying or removing go-fuzz from the build process.

A) Renaming symbols

We can use objcopy to rename symbols in the archive such that symbol clashes are avoided. It is difficult to programmatically rename only the symbols that clash, therefore it is more simple and maintainable to use objcopy --prefix-symbols= to prefix all symbols. Unfortunately, Prysm and ZRNT rely on external systems or shared libraries, so renaming everything breaks these references. (ZRNT doesn't use shared libraries, but the cgo runtime references libraries such as stdlib.h and libpthread.)

B) Building as shared libraries

We could modify go-fuzz to produce shared/dynamic libraries with the buildmode.

A combination of this and A), where we rename only externally visible symbols, could also be possible.

C) Combining into a single library built by go-fuzz

This approach involves creating a single Go package such that the harness produced by go-fuzz exercises each Golang client.

While this would likely reduce runtime memory overhead (as only a single Go runtime is used), relative isolation is lost. For Golang clients, this isolation may be less important; especially if they make use of recent semantic import versioning mechanisms that can allow the use of different major versions of the same module. Our remaining concern would be if multiple clients relied on some (unintentionally) shared, global state.

This approach also introduces other drawbacks. go-fuzz expects to produce a single harness from a Fuzz() function. To remain as such, we would need this Fuzz() function to perform the differential comparison between Go clients, returning the result to the c++ differential module only if no differences are found. This breaks some separation of concerns and is effectively a violation of the DRY principle, with its associated costs.

Alternatively, we could further modify the go-fuzz fork to allow the export of multiple harness functions. This increases the differences between our fork and go-fuzz proper, as well as the associated maintenance burden.

One should also consider the increased build complexity involved in combining Prysm's Bazel-based build with the ZRNT's go module.

D) Building without go-fuzz

This approach is similar to C), where only a single library is produced exercising all Golang clients, but now we avoid go-fuzz entirely. This would allow us to have full control over the build, exporting a separate harness interface for each client, and avoiding the need to rely on, and maintain, a fork of go-fuzz. All differential logic also remains contained in the fuzzing::Differential c++ class.

The main cost involved would be to implement our own coverage instrumentation; the difficulty or feasibility of which would greatly vary, depending on the coverage measurements strategy adopted (simply compiling with the clang -fsanitize=fuzzer-no-link flag, or implementing go-specific hooks).

Our First Bug and How to Obtain Useful Debug Info

We have identified our first potential bug thanks to the process_attestation fuzzer. When passed an invalid attestation, Nimbus crashes with an AssertionError instead of returning a handle-able error value (i.e. false) or raising a catchable exception.

See the related GitHub issue on the Nimbus repository for more information.

Currently, the crash corpus directly returned by beacon-fuzz is not very useful to debug with. This is the raw data, before it has passed through preprocessing, so it is different from what is passed to each client harness:

  1. The input is most likely invalid SSZ data that the preprocessing converts to a valid SSZ-encoded object.
  2. The input contains a uint16_t reference to a BeaconState file, which the preprocessing "dereferences", passing the BeaconState and the remaining input to the client harness. Only when combined with the relevant BeaconState does the corpus contain all details required to replicate the bug.

For debugging purposes, it would be much more useful to have the data as passed to the client harness (after preprocessing). To this end, we have produced a corpora-extract CLI tool (working title) that converts a corpus to such data. Better documentation and features to follow.


We have successfully published a few fuzzers to Fuzzit, a fuzzing-as-a-service, cloud platform. The current deployment has the block and shuffle fuzzers exercising ZRNT, Nimbus, and Lighthouse. As Fuzzit requires the upload of pre-built executables, some changes were needed to get our current "run-in-place" fuzzer executables to comply. These changes were performed on the fuzzit branch, and also involve additional manual configuration.

Once built, the current fuzzer executable depends on the following external files:

  • Several shared libraries - both cpython "dynlibs", and system-installed pcre, rocksdb, ssl, leveldb etc.
  • A directory of BeaconState SSZ files specified by the ETH2_FUZZER_STATE_CORPUS_PATH environment variable.
  • Python harness scripts located in absolute paths set at compile-time.

Fortunately, Fuzzit accepts a .tar.gz archive. As long as it contains a ./*fuzzer* executable, everything else is ok and we don't have to try to embed all components and modules into a single binary.

Operation was successful once we used relative paths for runtime references, included relevant shared libraries, and modified the runtime load path to point to the libraries. We modified the Python script paths to be relative to the executable and bundled relevant shared libraries (collected via ldd -v ./fuzzer and referenced at runtime by setting LD_LIBRARY_PATH). However, the current cpython install configuration contains absolute paths and couldn't be moved, so Python clients were disabled for the POC.

The current POC has been sufficient to confirm feasibility of fuzzing on Fuzzit, so automated build of a suitable bundle and CI integration will follow.

Next Steps

Welcome Przmek (aka @cryptomental) to the Beacon Fuzz team!

Przmek has a wealth of fuzzing experience (differential and otherwise) thanks to his efforts with Eth1. He will initially be focussing on resolving the integration issues with multiple Golang clients.

Some of the areas we'll be working on over the upcoming weeks:

  • Upgrade to spec v0.9.3 or v0.9.4 once 3 or more clients have implemented it. (Likely mid-late Jan)
  • Create fuzzing targets for the rest of the state transition functions (epoch state transitions).
  • Improved mutation via libprotobuf-mutator. This involves converting SSZ into protobuf and back, and utilizing libprotobuf-mutator for structure-aware fuzzing. Some work has started here but a working POC is still in progress. Once operational, this is expected to vastly improve fuzzing coverage.
  • Improved use of continuous fuzzing services (fuzzit for now), and to submit a request to Google OSS-Fuzz in the near future.