James Fang of Keen Security has a blog post about Android kernel mitigations:

Emerging Defense in Android Kernel:
There was a time that every Linux kernel hacker loves Android. It comes with a kernel from stone-age with merely any exploit mitigation. Writing exploit with any N-day available was just a walk in the park. Now a days Google, ARM and many other SoC/device vendors have put many efforts hardening the security of Android, including its kernel, which is (in most cases) the last defense against attack. As a group of Android gurus focusing on rooting, we probably facing these defense more than researchers in other fields. In this post we are going to summarize kernel exploit mitigations appeared in the recent 2 years, and sharing our opinions on their effectiveness. Note that we are going to focus on the implementation of mitigations in this post. We may point out its weakness, but we are not going to detail bypassing techniques for each mitigation. […]

http://keenlab.tencent.com/en/2016/06/01/Emerging-Defense-in-Android-Kernel/