Boot process for a Mac with Apple silicon
When a Mac with Apple silicon is turned on, it performs a boot process much like that of iPhone and iPad.
The chip executes code from the Boot ROM in the first step in the chain of trust. macOS secure boot on a Mac with Apple silicon verifies not only the operating system code itself but also the security policies and even kexts (supported, though not recommended) configured by authorised users.
When LLB (which stands for Low Level Bootstrap) is launched, it then verifies the signatures and loads system-paired firmware for intra-SoC cores such as the storage, display, system management and Thunderbolt controllers. LLB is also responsible for loading the LocalPolicy, which is a file signed by the Secure Enclave Processor. The LocalPolicy file describes the configuration that the user has chosen for the system boot and running time security policies. The LocalPolicy has the same data structure format as all other boot objects but it’s signed locally by a private key that’s available only within a particular computer’s Secure Enclave instead of being signed by a central Apple server (like software updates).
To help prevent replay of any previous LocalPolicy, LLB must look up a nonce from the Secure Enclave–attached Secure Storage Component. To do this, it uses the Secure Enclave Boot ROM and makes sure the nonce in the LocalPolicy matches the nonce in the Secure Storage Component. This helps prevent an old LocalPolicy — which could have been configured for lower security — from being reapplied to the system after security has been upgraded. The result is that secure boot on a Mac with Apple silicon helps protect not only against rollback of operating system versions but also against security policy downgrades.
The LocalPolicy file captures whether the operating system is configured for Full, Reduced or Permissive security.
Full Security: The system behaves like iOS and iPadOS and allows only booting software which was known to be the latest that was available at install time.
Reduced Security: LLB is directed to trust “global” signatures, which are bundled with the operating system. This allows the system to run older versions of macOS. Because older versions of macOS inevitably have unpatched vulnerabilities, this security mode is described as Reduced. This is also the policy level required to support booting kernel extensions (kexts).
Permissive Security: The system behaves like Reduced Security in that it uses global signature verification for iBoot and beyond but it also tells iBoot that it should accept some boot objects being signed by the Secure Enclave with the same key used to sign the LocalPolicy. This policy level supports users that are building, signing and booting their own custom XNU kernels.
If the LocalPolicy indicates to LLB that the selected operating system is running in Full Security, LLB evaluates the personalised signature for iBoot. If it’s running in Reduced Security or Permissive Security, it evaluates the global signature. Any signature verification errors cause the system to boot to recoveryOS to provide repair options.
After LLB hands off to iBoot, it loads macOS-paired firmware such as that for the Secure Neural Engine, the Always On Processor and other firmware. iBoot also looks at information about the LocalPolicy handed to it from LLB. If the LocalPolicy indicates that there should be an Auxiliary Kernel Collection (AuxKC), iBoot looks for it on the file system, verifies that it was signed by the Secure Enclave with the same key as the LocalPolicy and verifies that its hash matches a hash stored in the LocalPolicy. If the AuxKC is verified, iBoot places it into memory with the Boot Kernel Collection before locking the full memory region covering the Boot Kernel Collection and AuxKC with the System Coprocessor Integrity Protection (SCIP). If the policy indicates that an AuxKC should be present but it isn’t found, the system continues to boot into macOS without it. iBoot is also responsible for verifying the root hash for the signed system volume (SSV), to check that the file system the kernel will mount is fully integrity verified.