Security

Boot process of Mac computers

When a Mac computer with the Apple T2 Security Chip chip is turned on, the chip executes code from read-only memory known as Boot ROM. This immutable code, referred to as the hardware root of trust, is laid down during chip fabrication and is audited for vulnerabilities and implicitly trusted. The Boot ROM code contains the Apple Root CA public key, which is used to verify that the iBoot bootloader is signed by Apple’s private key before allowing it to load. This is the first step in the chain of trust. iBoot verifies the kernel and kernel extension code on the T2 chip, which subsequently verifies the Intel UEFI firmware. The UEFI firmware and the associated signature are initially available only to the T2 chip.

After verification, the UEFI firmware image is mapped into a portion of the T2 chip memory. This memory is made available to the Intel CPU through the enhanced Serial Peripheral Interface (eSPI). When the Intel CPU first boots, it fetches the UEFI firmware through eSPI from the integrity-checked, memory-mapped copy of the firmware located on the T2 chip.

The evaluation of the chain of trust continues on the Intel CPU, with the UEFI firmware evaluating the signature for boot.efi, which is the macOS bootloader. The Intel-resident macOS secure boot signatures are stored in the same Image4 format used for iOS, iPadOS, and T2 chip secure boot, and the code that parses the Image4 files is the same hardened code from the current iOS and iPadOS secure boot implementation. Boot.efi in turn verifies the signature of a new file, called immutablekernel. When secure boot is enabled, the immutablekernel file represents the complete set of Apple kernel extensions required to boot macOS. The secure boot policy terminates at the handoff to the immutablekernel, and after that, macOS security policies (such as System Integrity Protection and signed kernel extensions) take effect.

If there are any errors or failures in this process, the Mac enters macOS Recovery mode, Apple T2 Security Chip Recovery mode, or Apple T2 Security Chip DFU mode.

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Apple Platform Security

Boot process of Mac computers