μEmu

A Universal MCU Firmware Emulator for Dynamic Analysis without Any Hardware Dependence

General Idea

Unlike existing MCU firmware dynamic analysis tools that forwards the interactions with unsupported peripherals to the real hardware, μEmu takes the firmware image as input and symbolically executes it by representing unknown peripheral registers as symbols. During symbolic execution, it infers the rules to respond to unknown peripheral accesses. These rules are stored in a knowledge base (KB), which is referred to firmware during dynamic analysis without any hardware dependence. The detail of μEmu design and implementation, please refer to our paper.

Citing our paper

@inproceedings {uEmu,
author = {Wei Zhou and Le Guan and Peng Liu and Yuqing Zhang},
title = {Automatic Firmware Emulation through Invalidity-guided Knowledge Inference},
booktitle = {30th {USENIX} Security Symposium ({USENIX} Security 21)},
year = {2021},
url = {https://www.usenix.org/conference/usenixsecurity21/presentation/zhou},
publisher = {{USENIX} Association},
}

Directory structure of the repo

.
├── LICENSE
├── README.md
├── docs                     # more documentation about implementation and configuration
├── uEmu-helper.py           # helper scripts to configurate μEmu based on configration file (e.g., μEmu.cfg)
├── launch-uEmu-template.sh  # template scripts to launch μEmu
├── launch-AFL-template.sh   # template scripts to launch AFL fuzzer
├── uEmu-config-template.lua # template config file of μEmu and S2E plugins
├── library.lua              # contains convenience functions for the uEmu-config.lua file.
├── uEmu-unit_tests          # uEmu unit test samples and config files
├── uEmu-fuzzing_tests       # uEmu fuzzing test samples (real-world MCU firmware) and config files
├── ptracearm.h
├── vagrant-bootstrap.sh     # bootstrap file for vagrant box
└── Vagrantfile              # configuration file for vagrant box

Vagrant Installation (Recommended)

For easy setup, a vagrant file for a virtual machine with a ready-to-use build of uEmu is provided. After installing vagrant, the following steps are required:

git clone https://github.com/MCUSec/uEmu.git
cd uEmu
vagrant up  # this will take a while, please be patient
vagrant ssh # connect to the virtual machine

Now, from inside the virtual machine, you have the contents of this repository available at /vagrant/.

Note: The VM is configured to use 4 cores and 4096 MB of RAM. You can change this by modifying the Vagrant file.

Source Code Installation

Note:

uEmu builds and runs on Ubuntu 18.04 or 20.04 LTS (64-bit). Earlier versions may still work, but we do not support them anymore.
Since the qemu in arm kvm mode will use ptrace.h which is from the host arm linux kernel, however the real host μEmu is X86, so you have to add ptracearm.h from linux source code (you can directly download it from this repo) to the your local path: /usr/include/x86_64-linux-gnu/asm.

Required packages

You must install a few packages in order to build μEmu manually. The required packages of μEmu is same as the current S2E 2.0, please check out required packages of S2E.

Note: please do not install S2E itself, but only install build, S2E and C++17 dependencies.

Checking out source code

The μEmu source code can be obtained from the my git repository using the following commands.

 export uEmuDIR=/home/user/uEmu  # uEmuDIR must be in your home folder (e.g., /home/user/uEmu)
 sudo apt-get install git-repo   # or follow instructions at https://gerrit.googlesource.com/git-repo/
 cd $uEmuDIR
 repo init -u https://github.com/MCUSec/manifest.git -b uEmu
 repo sync

This will set up the μEmu repositories in $uEmuDIR.

Building μEmu

The μEmu Makefile can be run as follows:

    $ sudo mkdir $uEmuDIR/build
    $ cd $uEmuDIR/build
    $ make -f $uEmuDIR/Makefile && make -f $uEmuDIR/Makefile install
    # Go make some coffee or do whatever you want, this will take some time (approx. 60 mins on a 4-core machine)

By default, the make command compiles and installs μEmu in release mode to $uEmuDIR/build/opt. To change this location, set the S2E_PREFIX environment variable when running make.

To compile μEmu in debug mode, use make install-debug.

Building and Preparing AFL

cd $uEmuDIR/AFL
make
sudo make install

Updating

You can use the same Makefile to recompile μEmu either when changing it yourself or when pulling new versions through repo sync. Note that the Makefile will not automatically reconfigure the packages; for deep changes you might need to either start from scratch by issuing make clean or to force the reconfiguration of specific modules by deleting the corresponding files from the stamps subdirectory.

Usage

All μEmu plugins are enabled and configured with Lua-based configuration file uEmu-config.lua. To provide user a more user-friendly configuration file, we provide python script named uEmu-helper.py to quickly generate Lua-base configure files based on template file uEmu-config-template.lua based on a sample user-defined CFG file.

It will also generate launch scripts to run μEmu and AFL fuzzer based on launch-uEmu-template.sh and launch-AFL-template.sh files.

Please make sure the launch-uEmu-template.sh, launch-AFL-template.sh, uEmu-config-template.lua and library.lua exist and place in the same path e.g., <proj_dir> with uEmu-helper.py.

Usage: python3 <repo_path>/uEmu-helper.py <firmware_name> <config_file_name>  [-h] [--debug] [-kb KBFILENAME] [-s SEEDFILENAME]

positional arguments:
- firmware Tested Firmware. e.g., drone.elf
- config User Configuration File. e.g., firmware.cfg
optional arguments:
- -h, --help Show this help message and exit
- --debug Run μEmu in debug mode. Note that μEmu will output huge log information in a short time and slow down in debug mode, , please ensure you have enough space (e.g., more than 100M). Thus, we recommend only run μEmu with debug mode under KB extraction phase or the beginning of fuzzing phase.
- -kb KBFILENAME, Configure the Knowledge Base filename used for μEmu fuzzing phase and μEmu will run under fuzzing phase. If KB file is not present, μEmu will run KB extraction phase by default.
- -s SEEDFILENAME, Configure the seed filename to bootstrap fuzzing, if absent, μEmu will use random number for fuzzing.

Preparing the user configuration file

You can use the configuration files provided in our unit-tests and real-world-firwmare repos to our unit test samples or real-world samples in the μEmu paper. If you want to test your own firmware, please refer to this instruction and our paper to edit the user configuration file.

Note that incorrect configurations will lead to unexpected behaviors of μEmu like stall or finishing with inaccurate KB.

Example of μEmu workflow

Here, we take WYCNINWYC.elf firmware as a example to show how to run the μEmu with uEmu-helper.py and some attention points in each phase.

KB Extraction Phase:

python3 <repo_path>/uEmu-helper.py <proj_dir>/WYCINWYC.elf <proj_dir>/WYCNINWYC.cfg

After the above command successfully finishes, you could find the launch-uEmu.sh and uEmu-config.lua in your <proj_dir>. Then, you can launch the first-round KB extraction via carry out the launch-uEmu.sh script and you can check uEmu-config.lua to know whether you configurations are actually as your excepted .

After finishing (typically a few minutes), you can find log files and knowledge base (KB) file named as firmwarename-roundnumber-finalstatenumber-unqiuebbnumber_KB.dat (e.g., WYCINWYC.elf-round1-state53-bbnum1069_KB.dat) in s2e-last (referring to the s2e-out-<max>) folder. Detail logs will be printed to s2e-last/debug.txt and important log information will be printed tos2e-last/warnings.txt . More detail about log files please refer to S2E documents .

NOTE:

If you find μEmu cannot be finished KB extraction with a quiet long time (e.g., more than ten minutes), typically the reason is μEmu has been stuck. You should manually abort the execution, then check the configuration file, firmware and log files to find the reason and re-configure and re-run the firmware with μEmu. Thus, we recommend user to enable debug level log information (add --debug in above command) when he first time runs the firmware.

Dynamic Analysis and Fuzzing Phase:

Next, you can run the firmware with learned KB for dynamic analysis. About more detail about KB entries format please refer to kb.md.

The below command is to configure μEmu for running WYCINWYC.elf firmware in dynamic phase with WYCINWYC.elf-round1-state53-tbnum1069_KB KB file and fuzzing seed file small_document.xml.

python3 uEmu-helper.py WYCINWYC.elf WYCNINWYC.cfg -kb WYCINWYC.elf-round1-state53-tbnum1069_KB.dat -s small_document.xml

Since μEmu relies on AFL for fuzzing input. Thus, you first need to launch AFL fuzzur via ./launch-AFL.sh in one terminal to input test-cases and then launch μEmu via ./launch-uEmu.sh in another terminal to consume the test-cases. The fuzzing results is stored in <proj_dir>/ folder. The log and KB files of addition round KB extraction phase are record in folder.

NOTE:

Please abort fuzzing from AFL terminal, and then μEmu will automatically terminate.
We recommend enable auto_mode_switch in configuration file during fuzzing to automatically switch two phase , since complex firmware often use new peripherals on demand.
If no seed file given, μEmu will use 4 bytes random number as initial seed to bootstrap fuzzing.
We enable fuzzing test during dynamic analysis phase by default, but user can disable it (enable_fuzz = false) in configuration file. Then data registers will only used values from KB.
For more advanced configurations during fuzzing phase, please refer to Fuzzer_Config section in instruction.

Debugging with gdb

First, μEmu should be complied with both symbols and all debug information in order to use it with gdb:

sudo make -f $uEmuDIR/Makefile all-debug

Then, you can start μEmu in GDB with ./launch-uEmu.sh debug. The gdb configuration script gdb.ini will be automatically generated in your current folder. More information about debugging, please refer to DebuggingS2E.

bootcap/uEmu