test: add Mayhem and basic fuzzer

[xansec]

Hi! My name's Alex from ForAllSecure, and I'm opening this PR on behalf of one of the many "heroes'' who has integrated Mayhem fuzz testing into your project. The "Heroes" program involves integrating fuzz testing to open source projects, in order to help secure the world's software. These integrations generally (but not always) include:

• A Dockerfile (for building the fuzz target)
• A Mayhemfile (for specifying fuzzing configuration)
• A Github Action (for fuzzing as part of CI)
• A fuzz target
  In this target, it looks like we've also added a couple of new Cargo files to lock in dependencies and easily build the fuzz targets. You can take a look at the status of the latest fuzzing results here: https://mayhem.forallsecure.com/mayhemheroes/boringtun?coverage_target=new-mac

Let me know if you have any questions about this PR!

[Noah-Kennedy]

@jeff-hiner I'm gonna leave this to you. Is there anything in particular you really want to see fuzzing on? I can imagine that noise might be a great candidate for fuzzing.

[Noah-Kennedy]

What's the reason this is commented out?

[xansec]

I'm pretty sure that fuzz target was removed by the original author after the initial PR (possibly due to failing to build). However, the code still exists under boringtun/fuzz/fuzz_targets/check_tun_name.rs. I can remove it entirely if you'd prefer a cleaner merge.

[jeff-hiner]

@jeff-hiner I'm gonna leave this to you. Is there anything in particular you really want to see fuzzing on? I can imagine that noise might be a great candidate for fuzzing.

The base crypto stuff is now all external now, but yeah it would be good to get fuzzing on packet inputs to encapsulate and decapsulate. The timing is kinda bad though because I'm messing around with the function signatures for vectorization.

My expectation is that trying to decapsulate random data on an established handshake should virtually always return an error (if you can get aead to collide in this scenario please write a paper). Encapsulating is a bit more complicated, but essentially if you pass it a valid IPv4/IPv6 packet within the MTU bounds you should get something reasonable out the other end.