https://people.cs.ksu.edu/~danielwang/BAS/klein-2014-microke...

This is not the kernel that runs on the host CPU. It is the one that handles keys in the security coprocessor. I don’t know of many hacks of that, in practice. There was one where you could guess the pin, and use a timing attack to power down the chip before it persisted the “bad guess” count, which let people brute force pins (with special hardware).

It’s worth noting that the kernel Apple ships is a fork of L4; no idea if they’ve introduced bugs since the paper was written.