Apple devices (e.g., iPhone, MacBook, iPad, and Apple Watch) are high value targets for attackers. Although these devices use different operating systems (e.g., iOS, macOS, iPadOS, watchOS, and tvOS), they are all based on a hybrid kernel called XNU. Existing attacks demonstrated that vulnerabilities in XNU could be exploited to escalate privileges and jailbreak devices. To mitigate these threats, multiple security mechanisms have been deployed in latest systems.
In this paper, we first perform a systematic assessment of deployed mitigations by Apple, and demonstrate that most of them can be bypassed through corrupting a special type of kernel objects, i.e., Mach port objects. We summarize this type of attack as (Mach) Port Object-Oriented Programming (POP). Accordingly, we define multiple attack primitives to launch the attack and demonstrate realistic scenarios to achieve full memory manipulation on recently released systems (i.e., iOS 13 and macOS 10.15). To defend against POP, we propose the Port Ultra SHield (PUSH) system to reduce the number of unprotected Mach port objects. Specifically, PUSH automatically locates potential POP primitives and instruments related system calls to enforce the integrity of Mach port kernel objects. It does not require system modifications and only introduces 2% runtime overhead. The PUSH framework has been deployed on more than 40,000 macOS devices in a leading company. The evaluation of 18 public exploits and one zero-day exploit detected by our system demonstrated the effectiveness of PUSH. We believe that the proposed framework will facilitate the design and implementation of a more secure XNU kernel.