Apple PAC, Four Years Later: Reverse Engineering the Customized Pointer Authentication Hardware Implementation on Apple M1
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Presented at BlackHat USA 2023 by
Since 2018, Apple has deployed PA on all Apple-silicon-based products to improve the security of the system. There have been many incredible security researchers who have conducted in-depth analyses of the PA protection deployed by Apple. Notably, in the 2019 analysis blog "Examining Pointer Authentication on the iPhone XS" by Brandon Azad from Project Zero, he discovered that Apple uses some unknown "dark magic" on the PA hardware. So the customized PA on Apple Silicon can defend against cross-EL/Key attacks without software support. However, four years have passed and there is still no research to reveal the causes of these cross attack mitigations.We found that this customized PA (Apple PA) was also deployed on Apple M1, and in addition, we found that Apple deployed hardware read/write protection for PA key registers. This hardware protection makes Apple PA difficult to reverse engineer since we can not control the value of PA key registers after Apple PA is enabled. To bypass this protection, we propose new techniques based on another hardware feature on Apple M1: Virtualization Host Extension (VHE). Based on these new techniques, we successfully revealed the new PA hardware (registers and instruction behaviors) introduced by Apple. More specifically, we found that Apple introduces three new diversifiers and a per-key-type key transformation to implement four types of cross attack (Cross-VM/Key/Boot/EL) mitigations on the hardware level.