How Macs boot securely, or can’t
Earlier this week, I explained how the Signed System Volume (SSV), Data volume and cryptexes are integrated into the boot volume group, to support a secure boot process. This article outlines how modern Macs tackle the problem of booting securely.
The aim of a secure boot process is to ensure that all steps from the Boot ROM to the operating system are verified against any unauthorised change, and the code loaded and run is as intended. A simple operating system might achieve that by running only code contained in a boot ROM, but that’s woefully inadequate for any modern general-purpose operating system such as macOS, which also needs to be updated and upgraded during a Mac’s lifetime. Thus the great bulk of macOS has to be loaded and run from mutable storage, now SSDs. Those and a great deal else require specialised cores, with their own firmware, and features like the Secure Enclave. This is achieved in a cascade, where each step provides access to more of the Mac’s hardware, until many of Sequoia’s 670 kernel extensions are loaded and ready.
Older models of Macs without a T2 chip follow a classic and insecure process when booting. Their Boot ROM loads UEFI firmware, and that in turn loads boot.efi, the macOS booter, without performing any verification. The macOS booter then loads the prelinked kernel from disk, again without verifying it. When the kernel opens the SSV, any checks on that can only be cursory, as Recovery for these Macs doesn’t offer controls in the form of a Startup Security Utility.
