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AppleMobileFileIntegrity(.kext), which can go by its full name, is an iOS kernel extension which serves as the corner stone of iOS's code entitlements model. It is one of the Sandbox's ( dependencies, along with (which, like on OS X, is responsible for parsing the Sandbox language rules).

Affectionately known as AMFI, this kext can be found in the iOS 5.0 iPod 4,1 kernel around 0x805E499C (start) and 0x805E3EE8 (Initialization function). The latter function registers a MAC policy (using the kernel exported mac_policy_register), which is used to hook various system operations and enforce Apple's tight security policy.

This kext recognizes the task_for_pid-allow entitlement (among others) and is responsible for hooking this Mach call, which retrieves the Mach task port associated with a BSD process identifier. Given this port, one can usurp control of the task/PID, reading and writing its memory, debugging, etc. It is therefore enabled only if the binary is digitally signed with a proper entitlement file, specifying task_for_pid-allow.

The amfi kext recognizes quite a few boot-args, including:

  • PE_i_can_has_debugger (see also patch)
  • amfi_unrestrict_task_for_pid - Allowing the above to proceed even without entitlement
  • amfi_allow_any_signature - Allowing any digital signature on code, not just Apple's
  • amfi_get_out_of_my_way - disable amfi
  • cs_enforcement_disable - Disable code signing enforcement
  • cs_debug - Debug code signing

See also