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diff --git a/qemu_mode/README.deferred_initialization_example.md b/qemu_mode/README.deferred_initialization_example.md new file mode 100644 index 00000000..0ba04b79 --- /dev/null +++ b/qemu_mode/README.deferred_initialization_example.md @@ -0,0 +1,201 @@ +# Fuzz ARM32 Python Native Extensions in Binary-only Mode (LLVM fork-based) + +This is an example on how to fuzz Python native extensions in LLVM mode with deferred initialization on ARM32. + +We use Ubuntu x86_64 to run AFL++ and an Alpine ARMv7 Chroot to build the fuzzing target. + +Check [Resources](#resources) for the code used in this example. + +## Setup Alpine ARM Chroot on your x86_64 Linux Host + +### Use systemd-nspawn + +1. Install `qemu-user-binfmt`, `qemu-user-static` and `systemd-container` dependencies. +2. Restart the systemd-binfmt service: `systemctl restart systemd-binfmt.service` +3. Download an Alpine ARM RootFS from https://alpinelinux.org/downloads/ +4. Create a new `alpine_sysroot` folder and extract: `tar xfz alpine-minirootfs-3.17.1-armv7.tar.gz -C alpine_sysroot/` +5. Copy `qemu-arm-static` to Alpine's RootFS: `cp $(which qemu-arm-static) ./alpine/usr/bin/` +6. Chroot into the container: `sudo systemd-nspawn -D alpine/ --bind-ro=/etc/resolv.conf` +7. Install dependencies: `apk update && apk add build-base musl-dev clang15 python3 python3-dev py3-pip` +8. Exit the container with `exit` + +### Alternatively use Docker + +1. Install `qemu-user-binfmt` and `qemu-user-static` +2. Run Qemu container: ```$ docker run --rm --privileged multiarch/qemu-user-static --reset -p yes``` +3. Run Alpine container: ```$ docker run -it --rm arm32v7/alpine sh``` + +## Build AFL++ Qemu Mode with ARM Support + +First, build AFL++ as described [here](https://github.com/AFLplusplus/AFLplusplus/blob/dev/docs/INSTALL.md). Then, run the Qemu build script: + +```bash +cd qemu_mode && CPU_TARGET=arm ./build_qemu_support.sh +``` + +## Compile and Build the Fuzzing Project +Build the native extension and the fuzzing harness for ARM using the Alpine container (check [Resources](#resources) for the code): +```bash +ALPINE_ROOT=<your-alpine-sysroot-directory> +FUZZ=<your-path-to-the-code> +sudo systemd-nspawn -D $ALPINE_ROOT --bind=$FUZZ:/fuzz +CC=$(which clang) CFLAGS="-g" LDSHARED="clang -shared" python3 -m pip install /fuzz +clang $(python3-config --embed --cflags) $(python3-config --embed --ldflags) -o /fuzz/fuzz_harness.a /fuzz/fuzz_harness.c +exit +``` + +Manually trigger bug: +```bash +echo -n "FUZZ" | qemu-arm-static -L $ALPINE_ROOT $FUZZ/fuzz_harness.a +``` + +## Run AFL++ +Make sure to start the forkserver *after* loading all the shared objects by setting the `AFL_ENTRYPOINT` environment variable (see [here](https://aflplus.plus/docs/env_variables/#5-settings-for-afl-qemu-trace) for details): + +Choose an address just before the `while()` loop, for example: +```bash +qemu-arm-static -L $ALPINE_ROOT $ALPINE_ROOT/usr/bin/objdump -d $FUZZ/fuzz_harness.a | grep -A 1 "PyObject_GetAttrString" + +00000584 <PyObject_GetAttrString@plt>: + 584: e28fc600 add ip, pc, #0, 12 +-- + 7c8: ebffff6d bl 584 <PyObject_GetAttrString@plt> + 7cc: e58d0008 str r0, [sp, #8] +... +``` + +Check Qemu memory maps using the instructions from [here](https://aflplus.plus/docs/tutorials/libxml2_tutorial/): +>The binary is position independent and QEMU persistent needs the real addresses, not the offsets. Fortunately, QEMU loads PIE executables at a fixed address, 0x4000000000 for x86_64. +> +> We can check it using `AFL_QEMU_DEBUG_MAPS`. You don’t need this step if your binary is not PIE. + +Setup Python environment variables and run `afl-qemu-trace`: +```bash +PYTHONPATH=$ALPINE_ROOT/usr/lib/python3.10/ PYTHONHOME=$ALPINE_ROOT/usr/bin/ QEMU_LD_PREFIX=$ALPINE_ROOT AFL_QEMU_DEBUG_MAPS=1 afl-qemu-trace $FUZZ/fuzz_harness.a + +... +40000000-40001000 r-xp 00000000 103:03 8002276 fuzz_harness.a +40001000-4001f000 ---p 00000000 00:00 0 +4001f000-40020000 r--p 0000f000 103:03 8002276 fuzz_harness.a +40020000-40021000 rw-p 00010000 103:03 8002276 fuzz_harness.a +40021000-40022000 ---p 00000000 00:00 0 +40022000-40023000 rw-p 00000000 00:00 0 +``` + +Finally, setup Qemu environment variables... +```bash +export QEMU_SET_ENV=PYTHONPATH=$ALPINE_ROOT/usr/lib/python310.zip:$ALPINE_ROOT/usr/lib/python3.10:$ALPINE_ROOT/usr/lib/python3.10/lib-dynload:$ALPINE_ROOT/usr/lib/python3.10/site-packages,PYTHONHOME=$ALPINE_ROOT/usr/bin/ +export QEMU_LD_PREFIX=$ALPINE_ROOT +``` + +... and run AFL++: +```bash +mkdir -p $FUZZ/in && echo -n "FU" > $FUZZ/in/seed +AFL_ENTRYPOINT=0x400007cc afl-fuzz -i $FUZZ/in -o $FUZZ/out -Q -- $FUZZ/fuzz_harness.a +``` + +## Resources + +### setup.py + +```python +from distutils.core import setup, Extension + +module = Extension("memory", sources=["fuzz_target.c"]) + +setup( + name="memory", + version="1.0", + description='A simple "BOOM!" extension', + ext_modules=[module], +) +``` + +### fuzz_target.c + +```c +#define PY_SSIZE_T_CLEAN +#include <Python.h> + +#pragma clang optimize off + +static PyObject *corruption(PyObject* self, PyObject* args) { + char arr[3]; + Py_buffer name; + + if (!PyArg_ParseTuple(args, "y*", &name)) + return NULL; + + if (name.buf != NULL) { + if (strcmp(name.buf, "FUZZ") == 0) { + arr[0] = 'B'; + arr[1] = 'O'; + arr[2] = 'O'; + arr[3] = 'M'; + } + } + + PyBuffer_Release(&name); + Py_RETURN_NONE; +} + +static PyMethodDef MemoryMethods[] = { + {"corruption", corruption, METH_VARARGS, "BOOM!"}, + {NULL, NULL, 0, NULL} +}; + +static struct PyModuleDef memory_module = { + PyModuleDef_HEAD_INIT, + "memory", + "BOOM!", + -1, + MemoryMethods +}; + +PyMODINIT_FUNC PyInit_memory(void) { + return PyModule_Create(&memory_module); +} +``` + +### fuzz_harness.c + +```c +#include <Python.h> + +#pragma clang optimize off + +int main(int argc, char **argv) { + unsigned char buf[1024000]; + ssize_t size; + + Py_Initialize(); + PyObject* name = PyUnicode_DecodeFSDefault("memory"); + PyObject* module = PyImport_Import(name); + Py_DECREF(name); + + if (module != NULL) { + PyObject* corruption_func = PyObject_GetAttrString(module, "corruption"); + + while ((size = read(0, buf, sizeof(buf))) > 0 ? 1 : 0) { + PyObject* arg = PyBytes_FromStringAndSize((char *)buf, size); + + if (arg != NULL) { + PyObject* res = PyObject_CallFunctionObjArgs(corruption_func, arg, NULL); + + if (res != NULL) { + Py_XDECREF(res); + } + + Py_DECREF(arg); + } + } + + Py_DECREF(corruption_func); + Py_DECREF(module); + } + + // Py_Finalize() leaks memory on certain Python versions (see https://bugs.python.org/issue1635741) + // Py_Finalize(); + return 0; +} +``` diff --git a/qemu_mode/README.md b/qemu_mode/README.md index 4ed2f298..92038737 100644 --- a/qemu_mode/README.md +++ b/qemu_mode/README.md @@ -66,6 +66,8 @@ allows to move the forkserver to a different part, e.g., just before the file is opened (e.g., way after command line parsing and config file loading, etc.) which can be a huge speed improvement. +For an example, see [README.deferred_initialization_example.md](README.deferred_initialization_example.md). + ## 4) Persistent mode AFL++'s QEMU mode now supports also persistent mode for x86, x86_64, arm, and |