PolyTracker

PolyTracker is a tool for the Automated Lexical Annotation and Navigation of Parsers, a backronym devised solely for the purpose of referring to it as The ALAN Parsers Project. It is a an LLVM pass that instruments the programs it compiles to track which bytes of an input file are operated on by which functions. It outputs a JSON file containing the function-to-input-bytes mapping. Unlike dynamic instrumentation alternatives like Taintgrind, PolyTracker imposes negligible performance overhead for almost all inputs, and is capable of tracking every byte of input at once. PolyTracker started as a fork of the LLVM DataFlowSanitizer and takes much inspiration from the Angora Fuzzer.

PolyTracker can be used in conjunction with PolyFile to automatically determine the semantic purpose of the functions in a parser.

Quickstart

PolyTracker is controlled via a Python script called polytracker. You can install it by running

pip3 install polytracker

PolyTracker requires a very particular system environment to run, so almost all users are likely to run it in a virtualized environment. Luckily, polytracker makes this easy. All you need to do is have docker installed, then run:

polytracker docker pull

and

polytracker docker run

The latter command will mount the current working directory into the PolyTracker Docker container, and allow you to build and run instrumented programs.

The polytracker control script—which you can run from either your host system or from inside the Docker container—has a variety of commands, both for instrumenting programs as well as analyzing the resulting artifacts. For example, you can explore the dataflows in the execution, reconstruct the instrumented program's control flow graph, and even extract a context free grammar matching the inputs accepted by the program. You can explore these commands by running

polytracker --help

Instrumenting a simple C/C++ program

The following assumes you are working inside of the Docker container.

If you have a C target, you can instrument it by invoking the C compiler and passing the --instrument-target before your cflags

${CC} --instrument-target -g -o my_target my_target.c 

Repeat the same steps above for a cxx file by invoking ${CXX} instead of ${CC}

Dependencies and Prerequisites

PolyTracker has only been tested on x86_64 Linux. (Notably, the DataFlow Sanitizer that PolyTracker builds upon does not work on macOS.)

PolyTracker depends on gllvm to create whole program bitcode archives and to extract bitcode from targets.

PolyTracker depends on python3.7+

The following tools and libraries are required to run PolyTracker:

• LLVM version 7 or 7.1; other later versions may work but have not been tested. The builds in the official Ubuntu Bionic repository appear to be broken; we suggest building LLVM from source or installing it from the official LLVM repositories

Manually building the examples

Check out this Git repository. From the root, build the base PolyTracker Docker image:

docker build -t trailofbits/polytracker . 

This will create a Docker container with PolyTracker built, and the CC environment variable set to polybuild. Simply add the code to be instrumented to this container, and as long as its build process honors the CC environment variable, the resulting binary will be instrumented.

For a demo of PolyTracker running on the MuPDF parser run this command:

docker build -t trailofbits/polytracker-demo -f examples/pdf/Dockerfile-mupdf.demo .

Mutool_track will be build in /polytracker/the_klondike/mupdf/build/debug. Running mutool_track will output polytracker.json which contains the information provided by the taint analysis. Its reccomended to use this json with PolyFile.

For a demo of PolyTracker running on Poppler utils version 0.84.0 run this command:

docker build -t trailofbits/polytracker-demo -f examples/pdf/Dockerfile-poppler.demo .

All the poppler utils will be located in /polytracker/the_klondike/poppler-0.84.0/build/utils.

cd /polytracker/the_klondike/poppler-0.84.0/build/utils
POLYPATH=some_pdf.pdf ./pdfinfo_track some_pdf.pdf

Building PolyTracker from Source

The following tools are required to build PolyTracker:

• CMake
• Ninja (ninja-build on Ubuntu)
• Python 3.7 and pip, for testing purposes (apt-get -y install python3.7 python3-pip)

First, make sure that the LLVM 7 binaries have priority in your PATH, e.g.,

export PATH="/usr/lib/llvm-7/bin:${PATH}"

Next, from the root directory of this repository, run

mkdir build && cd build
cmake -G Ninja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ .. && ninja install

This builds and installs two compilers: polybuild and polybuild++. These are wrappers around gclang and gclang++, respectively, and will add the PolyTracker instrumentation.

Instrumenting a Program with PolyTracker

All that is required is to modify the program's build system to use polybuild/polybuild++ instead of its default compiler. The easiest way to do this is to set the compiler environment variables to them:

export CC=`which polybuild`
export CXX=`which polybuild++`

For example, let's work through how to build MuPDF with PolyTracker instrumentation:

git clone --recursive git://git.ghostscript.com/mupdf.git
cd mupdf
git submodule update --init
make -j10 HAVE_X11=no HAVE_GLUT=no prefix=./bin install

Or if you would like to build the debug version, as we do in our Dockerfile:

make -j10 HAVE_X11=no HAVE_GLUT=no prefix=./bin debug

Then, find the build util you want to instrument, run gllvm's get-bc to extract the bitcode from the target, then instrument it with polybuild

get-bc -b target
${CC}/{CXX} --instrument-bitcode target.bc -o target_track --libs <any libs go here> 

If you aren't sure about what libraries you might need to link for a complex target, the enviornment variable WLLVM_ARTIFACT_STORE sets a directory that contains a mainfest that logs all build commands and artifacts used. You should be able to rebuild the target completely using information in the mantifest and the artifacts.

Environment Variables

PolyTracker accepts configuration paramters in the form of environment variables to avoid recompiling target programs. The current environment variables PolyTracker supports is:

POLYPATH: The path to the file to mark as tainted 

POLYTTL: This value is an initial "strength" value for taint nodes, when new nodes are formed, the average is taken. When the TTL value is 0, the node is considered clean. 

POLYSTART: Start offset to track 

POLYEND: End offset to track

POLYOUTPUT: Provides a path/prefix to output polytracker information too

POLYCONFIG: Provides a path to a JSON file specifying setings

WLLVM_ARTIFACT_STORE: Provides a path to an exisiting directory to store artifact/manifest for all build targets

Configuration Files

Rather than setting environment variables on every run, you can make a configuration file.

Example:

{
    "POLYSTART": 1,
    "POLYEND": 3,
    "POLYTTL": 16
}

Polytracker will search for config files in the following way:

1. If POLYCONFIG is specified, it will grab it from there
2. Checks current directory there is a polytracker_config.json
3. Checks the .config directory under the users home ("~/.config/polytracker/polytracker_config.json")

If a configuration isn't specified or if certain settings aren't tweaked, defaults are set if possible. Some settings like POLYPATH do not have defaults, so if POLYPATH isn't specified via environment variable or config, an error will be thrown.

Running an Instrumented Program

The PolyTracker instrumentation looks for the POLYPATH environment variable to specify which input file's bytes are meant to be tracked. (Note: PolyTracker can in fact track multiple input files—and really any file-like stream such as network sockets—however, we have thus far only exposed the capability to specify a single file. This will be improved in a future release.)

The instrumented software will write its output to polytracker_process_sets.json and polytracker_forest.bin in the current directory.

For example, with our instrumented version of MuPDF, run

POLYPATH=input.pdf POLYTTL=32 ./mutool_track info input.pdf

On program exit, those artifacts will be created in the current directory. These artifacts can be inspected and manipulated by the polytracker tool on the host system. The artifacts are documented here.

The optional POLYTRACE enviornment variable can be set to POLYTRACE=1 to produce a basic-block level trace of the program. This trace data will be included in the output JSON file under the "trace" key.

Creating custom ignore lists from pre-built libraries

Attempting to build large software projects can be time consuming, especially older/unsupported ones. It's even more time consuming to try and modify the build system such that it supports changes, like dfsan's/our instrumentation.

There is a script located in polytracker/scripts that you can run on any ELF library and it will output a list of functions to ignore. We use this when we do not want to track information going through a specific library like libpng, or other sub components of a program. The Dockerfile-listgen.demo exists to build common open source libraries so we can create these lists.

This script is a slightly tweaked version of what DataFlowSanitizer has, which focuses on ignoring system libraries. The original script can be found in dfsan_rt.

Current Status and Known Issues

Taints will not propagate through dynamically loaded libraries unless those libraries were compiled from source using PolyTracker, or there is specific support for the library calls implemented in PolyTracker. There is currently support for propagating taint throught the majority of uninstrumented C standard library calls. To be clear, programs that use uninstrumented functions will still run normally, however, operations performed by unsupported library calls will not propagate taint. We are currently working on adding robust support for C++ programs, but currently the best results will be from C programs.

Snapshotting is currently deprecated and not supported in the latest version.

If there are issues with Docker please do a system prune and build with --no-cache for both PolyTracker and whatever demo you are trying to run.

The worst case performance of PolyTracker is exercised when a single byte in memory is simultaneously tainted by a large number of input bytes from the source file. This is most common when instrumenting compression and cryptographic algorithms that have large block sizes. There are a number of mitigations for this behavior currently being researched and developed.

License and Acknowledgements

This research was developed by Trail of Bits with funding from the Defense Advanced Research Projects Agency (DARPA) under the SafeDocs program as a subcontractor to Galois. It is licensed under the Apache 2.0 lisense. © 2019, Trail of Bits.

Maintainers

Carson Harmon
Evan Sultanik
Brad Larsen

[email protected]