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TriforceLinuxSyscallFuzzer

• 20160613
• https://github.com/nccgroup/TriforceLinuxSyscallFuzzer
• Jesse Hertz jesse.hertz@nccgroup.trust
• Tim Newsham tim.newsham@nccgroup.trust

New: For those looking to play with TriforceAFL and TLSF, Richard Johnson created a Dockerfile which installs both (and even builds a Linux kernel for you). It's available here https://hub.docker.com/r/moflow/afl-triforce/tags/.

This is a collection of files used to perform system call fuzzing of Linux x86_64 kernels using AFL and QEMU. To use it you will need TriforceAFL from https://github.com/nccgroup/TriforceAFL and a kernel image to fuzz. Scripts assume that TriforceAFL is found in $TAFL or ../TriforceAFL/ (N.B. building testAfl requires that ../TriforceAFL/config.h exist).

Building

To build:

  make

Fuzzing

To run, first install a kernel into ./kern/bzImage and extract /proc/kallsyms into ./kern/kallsyms. Set K=kern environment variable to point to your kernel. Now run:

  make inputs
  ./runFuzz -M M0

Note that the runFuzz script expects a master or slave name, as it always runs in master/slave mode. See the runFuzz script for more usage information.

Also Note that this only creates a small set of example inputs. To test a large number of important system calls, you will probably want to generate one example of each system call, or at least one example for every "shape" of system call. These should be placed in inputs/. See gen2.py for an example.

Reproducing

To reproduce test cases (such as crashes) run:

  ./runTest inputs/ex1
  ./runTest outputs/crashes/id*

You can also run the driver out of the emulated environment with the -t option, with verbose logging with -vv and without actually performing the system calls with -x:

  ./driver -tvvx < inputs/ex1
  strace ./driver -t < inputs/ex1

It is sometimes useful to be able to boot the kernel and interactively run tests. To do so, edit the rootTemplate files as you see fit (for example, to add more test tools to the root filesystem), then run:

  ./runCmd

Other commands other than the shell can be invoked by specifying them as command line arguments to runCmd. Note: when done with the shell, use ^A-c to get the QEMU prompt and type quit.

Debugging

Debugging is easiest with a kernel built with debugging symbols enabled. Use runTest to start the kernel and run a test through the driver, or use runCmd to manually run a test case from the shell. Edit your run script to include the -s option when starting afl-qemu-system-trace. This will enable gdb support on TCP port 1234. Use getvmlinux to extract the vmlinux kernel image from your bzImage kernel and run gdb after the system has booted:

   cp kern/bzImage .
   ./getvmlinux
   gdb ./vmlinux
   target remote :1234
   break somefunction
   continue

You can attach the debugger after runTest has caused a crash or before you manually trigger then bug in runCmd.

Note that Linux sources are compiled with optimization turned on by default. This can make debugging confusing and difficult. You can disable optimization on a file-by-file basis by editing the Linux make file for the subdirectory a file is in and adding CFLAGS_name.o = -O0 to the Makefile. For example editing kernel/Makefile and adding CFLAGS_sys_ni.o = -O0 will disable optimization when building kernel/sys_ni.o.

Utility

The getSyms shell script uses runCmd to execute cat /proc/kallsyms and extract it to a local file named kallsyms. This is typically used to prep your kernel for fuzzing:

• run K=yourKernDir ./getSyms to get kallsyms
• run mv kallsyms yourKernDir to install it

Bugs

Note: When fuzzing a Linux 2.* kernel you will need to enable the CPU timer. When the timer is not enabled panic and logging detection do not seem to operate properly and panics result in hangs. To enable the timer, call startForkserver(1) in driver.c instead of startForkserver(0). This issue does not seem to occur in Linux3.* and Linux4.* kernels.