Suggested reading:
* https://www.maven-silicon.com/blog/privilege-levels-in-arm-a...
* https://projectzero.google/2017/02/lifting-hyper-visor-bypas...
* https://en.wikipedia.org/wiki/ARM_architecture_family#Securi...
* https://docs.samsungknox.com/admin/fundamentals/whitepaper/s... (in which Samsung uses firmware running under the kernel to protect against kernel exploits)