qasan

QASan (QEMU-AddressSanitizer)

Written and maintaned by Andrea Fioraldi

For just usermode fuzzing, a more updated version of QASan is now embedded into AFL++, use it instead

QASan is a custom QEMU 3.1.1 that detects memory errors in the guest using AddressSanitizer.

As QEMU is a binary code emulator, QASan is a binary-only sanitizer and does not require compiler-based instrumentation.

Dowload it with:

git clone --recursive https://github.com/andreafioraldi/qasan.git

Build

QASan comes in two possible twists, one based on my own ASan implementation and the other on the clang's implementation of ASan.

build.py is the script used to build all.

The flag --system allows you to build full-system QASan, an experimental feature ATM.

asan-giovese

asan-giovese is my implementation of AddressSanitizer. It is in pure C11 and allows you to get useful informations from the target process like stacktraces on allocations and on errors.

It will be the only supported option in future, but at the moment is not already completely thread safe.

This is the default mode, built when you don't specify the --asan-dso flag.

compiler-rt ASan

You need the lief python3 package.

Build using the build.py script specifying the path to the ASan DSO.

./build.py --asan-dso /path/to/libclang_rt.asan-ARCH.so

On Ubuntu 18.04, the path is /usr/lib/llvm-8/lib/clang/8.0.0/lib/linux/libclang_rt.asan-x86_64.so

Note that QASan will not output meaningful stacktraces or error reports when using this mode.

The reported errors show informaton about the QEMU host and so they are not useful for debugging.

other options

Other available build options are:

• --arch to specify the target architecture (default is x86_64)
• --cc and --cxx to specify C and C++ compilers (default clang-8)
• --cross to specify the cross C compiler for libqasan
• --clean to clean builded files

Tested only on Ubuntu 18.04 with x86[_64] and arm[64] targets.

Usage

To simply run a binary under QASan:

./qasan ./program args...

To set the LD path (useful when running cross arch, in this case armhf):

./qasan --prefix /usr/arm-linux-gnueabihf/ ./program args...

Other options are --preload to preload a shared object, --verbose to get a verbose output with memory mappings and --debug to log hooked actions if libqasan is compiled in debug mode.

By default, only the main executable memory accesses are instrumented. To enable the instrumentation of all the libraries, use AFL_INST_LIBS=1.

Beware that glibc have a lot of assumptions on buffer size and a lot of handwritten magic (see this). If you have an error caused by these optimizations you can disable the instrumentation for single functions adding them to libqasan/uninstrument.c.

Fuzzing

To fuzz a binary with QASan and AFL++ use a command similar to the following:

~/AFLplusplus/afl-fuzz -U -i in -o out -m none -- python3 ~/qasan/qasan ./program

It supports all the AFL++ QEMU configurations, AFL_COMPCOV_LEVEL=2 is higly suggested.

To improve speed, set the env variables QASAN_MAX_CALL_STACK=0 and QASAN_SYMBOLIZE=0.

FAQ

When I should use QASan?

If your target binary is PIC x86_64, you should before give a try to retrowrite for static rewriting.

If it fails, or if your binary is for another architecture, QASan is the tool that you want/have to use.

Note that the overhead of AFL++ libdislocator is much lower but it can catch less bugs. This is a short blanket, take your choice.

Another discriminat for the choice is CompareCoverage. If your target has fuzzing roadblocks, you can use QASan+CompCov to fuzz it with Sanitization and Roadblocks bypassing.

QEMU segfaults with big endian archs

See https://bugs.launchpad.net/qemu/+bug/1701798, use the workaround described here.

Performance

Native (slowdown: 1x):

$ time /usr/bin/objdump -g -x /usr/bin/objdump
...
real	0m0,058s
user	0m0,010s
sys	0m0,029s

QEMU (slowdown: 2.4x):

$ time qemu-x86_64 /usr/bin/objdump -g -x /usr/bin/objdump
...
real	0m0,141s
user	0m0,096s
sys	0m0,020s

QASan (slowdown: 3.6x):

$ time ./qasan /usr/bin/objdump -g -x /usr/bin/objdump
...
real	0m0,209s
user	0m0,120s
sys	0m0,032s

Valgrind (slowdown: 17.4x):

$ time valgrind /usr/bin/objdump -g -x /usr/bin/objdump
...
real	0m1,009s
user	0m0,921s
sys	0m0,076s

Cite

If you use QASan for your academic work, you should cite the following pubblication:

• Andrea Fioraldi, Daniele Cono D’Elia, and Leonardo Querzoni. “Fuzzing binaries for memory safety errors with QASan”. In 2020 IEEE Secure Development Conference (SecDev), 2020.

Preprint: https://andreafioraldi.github.io/assets/qasan-secdev20.pdf

Bibtex:

@INPROCEEDINGS{QASan-SecDev20,
  author={Fioraldi, Andrea and D’Elia, Daniele Cono and Querzoni, Leonardo},
  title={Fuzzing Binaries for Memory Safety Errors with {QASan}},
  booktitle={2020 IEEE Secure Development Conference (SecDev)},
  doi={10.1109/SecDev45635.2020.00019},
  pages={23--30},
  year={2020}
}

Video: https://www.youtube.com/watch?v=UtFXU7Nkd8g

Slides: https://andreafioraldi.github.io/assets/qasan-secdev20-slides.pdf