Operating system integrity
Apple’s operating system software is designed with security at its core. This design includes a hardware root of trust — leveraged to enable secure boot — and a secure software update process that’s quick and safe. Apple’s operating systems also use their purpose-built silicon-based hardware capabilities to help prevent exploitation as the system runs. These runtime features protect the integrity of trusted code while it is being executed. In short, Apple’s operating system software helps mitigate attack and exploit techniques — whether those originate from a malicious app, from the web or through any other channel. Protections listed here are available on devices with supported Apple-designed SoCs, including iOS, iPadOS, tvOS, watchOS and now macOS on a Mac with Apple silicon.
Feature | A10 | A11, S3 | A12, A13, A14 S4–S9 | A15, A16, A17 | M1, M2, M3 |
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See Note 1 below. | |
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See Note 2 below. |
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Note 1: Page Protection Layer (PPL) requires that the platform execute only signed and trusted code; this is a security model that isn’t applicable in macOS.
Note 2: Secure Page Table Monitor (SPTM) is supported on A15, A16 and A17 and replaces Page Protection Layer on supported platforms.
Kernel Integrity Protection
After the operating system kernel completes initialisation, Kernel Integrity Protection (KIP) is enabled to help prevent modifications of kernel and driver code. The memory controller provides a protected physical memory region that iBoot uses to load the kernel and kernel extensions. After startup is complete, the memory controller denies writes to the protected physical memory region. The Application Processor’s Memory Management Unit (MMU) is configured to help prevent mapping privileged code from physical memory outside the protected memory region and to help prevent writeable mappings of physical memory within the kernel memory region.
To prevent reconfiguration, the hardware used to enable KIP is locked after the boot process is complete.
Fast Permission Restrictions
Starting with the Apple A11 Bionic and S3 SoCs, a new hardware primitive was introduced. This primitive, Fast Permission Restrictions, includes a CPU register that quickly restricts permissions per thread. With Fast Permission Restrictions (also known as APRR registers), supported operating systems can remove execute permissions from memory without the overhead of a system call and a page table walk or flush. These registers provide one more level of mitigation for attacks from the web, particularly for code compiled at runtime (just-in-time compiled) — because memory can’t be effectively executed at the same time it’s being read from and written to.
System Coprocessor Integrity Protection
Coprocessor firmware handles many critical system tasks — for example, the Secure Enclave, the image sensor processor and the motion coprocessor. Therefore, its security is a key part of the security of the overall system. To prevent modification of coprocessor firmware, Apple uses a mechanism called System Coprocessor Integrity Protection (SCIP).
SCIP works much like Kernel Integrity Protection (KIP): At boot time, iBoot loads each coprocessor’s firmware into a protected memory region, one that’s reserved and separate from the KIP region. iBoot configures each co-processor’s memory unit to help prevent:
Executable mappings outside its part of the protected memory region
Writeable mappings inside its part of the protected memory region
Also, to configure SCIP for the Secure Enclave at boot time, the Secure Enclave operating system is used. After the boot process is complete, the hardware used to enable SCIP is locked. This is designed to prevent reconfiguration.
Pointer Authentication Codes
Pointer Authentication Codes (PACs) are used to protect against exploitation of memory corruption bugs. System software and built-in apps use PAC to help prevent modification of function pointers and return addresses (code pointers). PAC uses five secret 128-bit values to sign kernel instructions and data and each user space process has its own B keys. Items are salted and signed as indicated below.
Item | Key | Salt |
|---|---|---|
Function Return Address | IB | Storage address |
Function Pointers | IA | 0 |
Block Invocation Function | IA | Storage address |
Objective-C Method Cache | IB | Storage address + Class + Selector |
C++ V-Table Entries | IA | Storage address + Hash (mangled method name) |
Computed Goto Label | IA | Hash (function name) |
Kernel Thread State | GA | • |
User Thread State Registers | IA | Storage address |
C++ V-Table Pointers | DA | 0 |
The signature value is stored in the unused padding bits at the top of the 64-bit pointer. The signature is verified before use and the padding is restored to help ensure a functioning pointer address. Failure to verify results in an abort. This verification increases the difficulty of many attacks such as a return-oriented programming (ROP) attack, which attempts to trick the device into executing existing code maliciously by manipulating function return addresses stored on the stack.
Page Protection Layer
Page Protection Layer (PPL) in iOS, iPadOS and watchOS is designed to prevent user space code from being modified after code signature verification is complete. Building on Kernel Integrity Protection and Fast Permission Restrictions, PPL manages the page table permission overrides to make sure only the PPL can alter protected pages containing user code and page tables. The system provides a massive reduction in attack surface by supporting systemwide code integrity enforcement, even in the face of a compromised kernel. This protection isn’t offered in macOS because PPL is only applicable on systems where all executed code must be signed.
Secure Page Table Monitor and Trusted Execution Monitor
Secure Page Table Monitor (SPTM) and Trusted Execution Monitor (TXM) are designed to work together to help protect page tables for both user and kernel processes against modification, even when attackers have kernel write capabilities and can bypass control flow protections. SPTM does this by utilising a higher privilege level than the kernel, and utilising the lower privileged TXM to actually enforce the policies that govern code execution. This system is designed such that a TXM compromise doesn’t automatically translate to an SPTM bypass due to this privilege separation and the governing of trust between them. In A15, A16 and A17 SOCs, SPTM (in combination with TXM) replaces the PPL, providing a smaller attack surface that doesn’t rely on trust of the kernel, even during early boot. SPTM also relies on new silicon primitives that are an evolution of the Fast Permission Restrictions that PPL utilises.