Examining Pointer Authentication on the iPhone XS

In this post I examine Apple's implementation of Pointer Authentication on the A12 SoC used in the iPhone XS, with a focus on how Apple has improved over the ARM standard. I then demonstrate a way to use an arbitrary kernel read/write primitive to forge kernel PAC signatures for the A keys, which is sufficient to execute arbitrary code in the kernel using JOP. The technique I discovered was (mostly) fixed in iOS 12.1.3. In fact, this fix first appeared in the 16D5032a beta while my research was still ongoing.