syzkaller is an unsupervised coverage-guided kernel fuzzer.
Linux kernel fuzzing has the most support,
gvisor are supported to varying degrees.
Initially, syzkaller was developed with Linux kernel fuzzing in mind, but now it's being extended to support other OS kernels as well. Most of the documentation at this moment is related to the Linux kernel. For other OS kernels check: Akaros, FreeBSD, Fuchsia, NetBSD, Windows, gVisor.
This is not an official Google product.