SW:Boot

Stage 0

This stage is located in the BootROM. Among others, it loads and executes stage 1 from NOR.

Stage 1 (iBoot1)

This stage is the primary early loader, located in the on-board NOR. This boot stage very roughly goes as follows, given a target partition to boot from:

• Get the volume group UUID (diskutil apfs listVolumeGroups diskX)

• Get the local policy hash:

  • First try the local proposed hash (SEP command 11);
  • If that is not available, get the local blessed hash (SEP command 14)

• Read the local boot policy, located on the iSCPreboot partition at /<volume-uuid>/LocalPolicy/<policy-hash>.img4. This boot policy has the following specific metadata keys:

  • vuid: UUID: Volume group UUID - same as above
  • kuid: UUID: KEK group UUID
  • lpnh: SHA384: Local policy nonce hash
  • rpnh: SHA384: Remote policy nonce hash
  • nsih: SHA384: Next-stage IMG4 hash
  • coih: SHA384: kcOS (custom kernelcache) or fuOS (custom OS) IMG4 hash
  • auxp: SHA384: Auxiliary user-authorized kernel extensions hash
  • auxi: SHA384: Auxiliary kernel cache IMG4 hash
  • auxr: SHA384: Auxiliary kernel extension recept hash
  • prot: SHA384
  • lobo: bool
  • smb0: bool: Permissive security enabled, bit 0
  • smb1: bool: Permissive security enabled, bit 1
  • smb2: bool: Third-party kernel extensions enabled
  • smb3: bool: Manual mobile device management (MDM) enrollment
  • smb4: bool?: MDM device enrollment program disabled
  • sip0: u16: SIP customized
  • sip1: bool: Signed system volume (csrutil authenticated-boot) disabled
  • sip2: bool: CTRR (configurable text region read-only) disabled
  • sip3: bool: boot-args filtering disabled

  And optionally the following linked manifests, each located at /<volume-uuid>/LocalPolicy/<policy-hash>.<id>.im4m

  • auxk: kcOS manifest
  • fuos: fuOS manifest

• If loading the next stage:

  • The boot directory is located at the target partition Preboot subvolume, at path /<volume-uuid>/boot/<local-policy.metadata.nsih>;
  • Decrypt, verify and execute <boot-dir>/usr/standalone/firmware/iBoot.img4 with the device tree and other firmware files in the same directory. No evidence towards other metadata descriptors yet.

• If loading a custom stage (kcOS/fuOS):

  • ...

Stage 2 (iBoot2)

This stage is the OS-level loader, located inside the OS partition and shipped as part of macOS. It loads the rest of the system.