Examining Personal Protection Devices: Hardware and Firmware Research Methodology in Action
Todd Manning, Duo Labs
[…]This paper describes a methodology for retrieving device firmware, and for loading firmware into IDA Pro, a common disassembler. This paper focuses on the disassembly of this firmware, and the discussion of a novel approach to defeating readback protection discovered in one ARM Cortex M implementation. During the course of this research, I developed code for IDA Pro to assist in loading and grooming Cortex M firmware images. The IDAPython code is comprised of two modules. The first module annotates Cortex M vector tables, which gives IDA Pro hints about where code exists in the firmware image. The Cortex M module can automatically annotate firmware with a vector table located at the start of a firmware image, or can be tailored by the end user to work with relocated tables. The second module, called Amnesia, uses byte-level heuristics to detect ARM instructions in the firmware. Amnesia also contains heuristics that operate at the ARM instruction level to determine function boundaries based on common ARM function prologues and epilogues.[…]
Cortex M Firmware (cortex_m_firmware.py): This Cortex M Firmware module grooms an IDA Pro database containing firmware from an ARM Cortex M microcontroller. This module will annotate the firmware vector table, which contains a number of function pointers. This vector table annotation will cause IDA Pro to perform auto analysis against the functions these pointers point to. The Cortex M Firmware module also calls into the Amnesia module to automate discovery of additional code in the firmware image using the Amnesia heuristics.
Amnesia (amnesia.py): Amnesia is an IDAPython module designed to use byte level heuristics to find ARM thumb instructions in undefined bytes in an IDA Pro database. Currently, the heuristics in this module find code in a few different ways. Some instructions identify and define new code by looking for comon byte sequences that correspond to particular ARM opcodes. Other functions in this module define new functions based on sequences of defined instructions.