Awesome

Simple Shannon Baseband Loader for IDA Pro

This is a simple firmware loader plugin to load Samsung Exynos "Shannon" modem images in IDA Pro or IDA Home ARM. This loader is designed to perform the most important taks to load a Shannon image, furthermore it should be easy to understand and customize.

The loader should work with most Samsung Exynos modem images containing a TOC header including crash dumps. Compatible images can be found e.g. in updates for Exynos based phones. The typical file name is modem.bin. Sometimes the images are compressed using lz4. Uncompress them before loading, using lz4 utility present on most Linux distros.

The loader was tested with a larger set of images from ancient (e.g. G8700, S7) to new (e.g. S22, S24). Loading 2024 build images works fine including task identification.

How To Use This Loader

To use the loader just install shannon_load.py inside your IDA Pro or IDA Home ARM loader folder and the other python files into the IDA python folder. install.sh will assist you with this task, if you want to do it manually please take a look inside. For IDA Pro 9.0, 8.3 and 8.4 and IDA Home ARM run the installer with one of the following commandline options:

-d <dir> specify IDA installation directory
-r run IDA after installation
-a auto mode, find IDA installation dir automatically
-t test mode, combines -a -r and enabled logging in IDA
-h print help

Once installed open a modem.bin file extracted from an OTA update in IDA. The loader should detect the TOC format and load the image accordingly. The postprocessor script will add additional segment information after the initial analysis has finished. Adding the segments right from the start will confuse and slow down the analysis process.

The postprocessing module does most of the magic and it will take a bit to run, please be patient. All steps the post processor performs are in the individual files copied to <IDAHOME>/python/. You can use them as individual python modules if needed. See the post processor for details. A complete analysis of a modem image takes about 10 minutes to complete on average hardware.

How The Loader Works

The TOC header is a simple structure the loader will read to create a database of the file with proper aligned segments and entry points. For more insights just look at the code contained in this reponsitory.

The loader will recognize a TOC based Shannon modem binary and load it. After basic processing and initial auto analysis, exessive post-processing is performed. The post processing workflow will perform the following tasks:

• restore debug trace entries and structures
• rename functions based on various string references
• rename supplementary services functions (ss_*) based on debug log functions
• identify the hardware and mpu init function
• restore the mpu table and map memory accordingly
• identify MMU related mnemonics and label them
• identify the scatter loader
• perform scatter loading and decompression (LZ77-like compression scheme)
• find important platform abstraction layer functions
• identify and label all task init functions

After that your idb or i64 should be ready to go so you can focus on reverse engineering the modem.

About Samsung Shannon

History

Shannon is an IC series by Samsung LSI. Most notable product in the series is the Shannon baseband processor. Shannon is the name of a range of ICs and not the name of the modem itself. You might find other Shannon ICs made by Samsung. A typical set of Shannon ICs consists of a baseband, RF transceiver, powermanagement and a envelope tracking IC. All of them have different part numbers. E.g. Shannon baseband is 5300, the RF transceiver is 5510, PMIC is 5200 and so on. So don't get confused, these are all different parts. You won't find much about Shannon on Samsungs website, they refer to the baseband as Exynos Modem instead.

Nowadays the baseband IC is most of the time integrated into the Exynos SOC directly. For special configs, standalone baseband ICs are still sold. User of the stand alone solution are i.e. automotive or IoT vendors.

Historically the Shannon baseband was at least developed since 2005. During the early days it went under the name of CMC. The CMC220 was the first LTE version of the IC which launched in 2011, shortly after the first Exynos processor. Samsung is still celebrating it in some of their company presentations.

The name "Shannon" is a homage to Claude Shannon, father of the information theory and the information age. There are other references to important scientists in the field which can be found in Samsung hard and firmware. I.e. some radio related parts are called Marconi, as reference to Guglielmo Marconi a pioneer in the field of radio transmission and inventor of the first working antenna.

RTOS

The RTOS used by Samsung for Shannon ICs is called ShannonOS. I believe that ShannonOS is a re-branded Nucleus core. Old versions of CMC even identify it by the original name. The system is build using the ARM RVCT compiler with slowly increasing version numbers.

On top of the RTOS, Samsung created a Platform Abstraction Layer (PAL) as interface to the lower level functionality operating the hardware. Platform abstraction layer are a common design pattern found in a lot of embedded development projects. Typically the hardware related layer would be called Hardware Abstraction Layer (HAL). In old modems the HAL was never explictly mentioned or labeled as such, new modems (2023 onwards) even have HAL management tasks and a clear seperation. Possibly because the low level / core functionality was taken from Nucleus and never part of the initial inhouse design.

Inside the PAL, functionality like task management and other higher level management operations are run. The most interesting functionality of the baseband such as packet parsers for GSM/LTE/5G or logging (DM) etc. are run in individual tasks scheduled by the PAL. The loader will identify the individual tasks for you and label them, see shannon_pal_reconstructor.py for details.

If you look at a crash dump or find the respective functionality inside modem.bin, you will encounter the following log banner. It gives away some information about the system and processor:

===================================================
            DEVELOPMENT PLATFORM
 - ARM Emulation Baseboard | Cortex-R7
 - Software Build Date : 
 - Software Builder    : 
 - Compiler Version    : ARM RVCT 50.6 [Build 422]
    Platform Abstraction Layer (PAL) Powered by
               CP Platform Part
===================================================

The modem image is build with ARM RVCT as shown above. A mapping of the RVCT version to toolchain used can be found here. Build 422 as shown above maps to Keil MDK 5.22, RVCT/Arm Compiler version 5.06u4. This version of RCVT can be found on the ARM website here. This loader will detect installed ARM legacy compilers on Linux and ask to set their includes as compiler options for further analysis. Furthermore, multiple premade FLIRT signatures to identify RVCT functions inside the binary are included.

Cortex-R and Cortex-A

Around the S20 release a few things changed, Shannon basebands were upgraded from Cortex-R to Cortex-A. During this time additional stack checks were introduced as well. But stack cookies seem to be static, given the random values used it's unclear if the developers really understood the reason for this mitigation.

Another new addition from the core change was the MMU (Memory Management Unit). The MMU would technically provide domain security and advanced management capabilities, while the MPU only offers basic mapping and protection. The advanced MMU features are disabled by setting the translation table to zero and domain security to minus one for a lot of the earlier images. Tensor and recent S22+ images seem to use the MMU much more. ShannonOS uses a continuous address space until today.

The loader will identify MRC/MCR instructions in the image and comment them for you, see shannon_mpu.py for details.

IDA Compatibility And Installation

Tested with IDA Pro 9.x (9.0), 8.x (8.3 to 8.4 SP2) as well as IDA Home ARM. Might work on versions starting from 7.x using the revamped idapython API.

Since I work on Linux the install.sh is a bash script. If you are using OSX or Windows you can perform the installation manually by copying the files in their respective directories to install the loader manually:

Bugs

This code is WIP and should be used as such. If you encounter a modem image not proper processed, please fill a bug report so I can fix the loader for it. Make sure to note the exact version of the modem image you are trying to process so I can locate the file.

Motivation

I took a Shannon related trainings and had to work with Ghidra. After working with IDA for 20 years I feel much more comfortable using it compared to the dragon engine. So I started the work on this loader before another Shannon centric training this year. Doing things from scratch allowed me to get much more into the details of each and every aspect. The loader as it is resembles my idea of how I want it to work and which features I think are needed to do proper research on the bianary.

Highly Recommended Trainings

For those of you who want to get more into baseband topics I can highly recommend the following trainings. Since these were conference trainings they are not regularly repeated classes. Keep an eye open they are happening again.

• https://hardwear.io/netherlands-2022/training/reverse-engineering-emulation-dynamic-testing-cellular-baseband-firmware.php
• https://www.offensivecon.org/trainings/2024/exploiting-basebands-and-application-processors.html

Noteable Works

Some Links to notable work related to the Shannon baseband:

License

MIT License, have fun.