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syzkaller - linux syscall fuzzer

syzkaller is a distributed, unsupervised, coverage-guided Linux syscall fuzzer. It is meant to be used with KASAN (CONFIG_KASAN=y), KTSAN (CONFIG_KTSAN=y), or [KUBSAN] (http://developerblog.redhat.com/2014/10/16/gcc-undefined-behavior-sanitizer-ubsan/) (patch).

Project mailing list: syzkaller@googlegroups.com.

List of found bugs.

This is work-in-progress, some things may not work yet.

Usage

Various components are needed to build and run syzkaller.

  • C compiler with coverage support
  • Linux kernel with coverage additions
  • QEMU and disk image
  • The syzkaller components

Setting each of these up is discussed in the following sections.

C Compiler

Syzkaller is a coverage-guided fuzzer and so needs the kernel to be built with coverage support. Therefore, a recent upstream version of GCC is needed. Coverage support is submitted to gcc in revision 231296. Sync past it and build fresh gcc.

Linux Kernel

As well as adding coverage support to the C compiler, the Linux kernel itself needs to be modified to:

  • add support in the build system for the coverage options (under CONFIG_KCOV)
  • add extra instrumentation on system call entry/exit (for a CONFIG_KCOV build)
  • add code to track and report per-task coverage information.

This is all implemented in this coverage patch; once the patch is applied, the kernel should be configured with CONFIG_KCOV plus CONFIG_KASAN or CONFIG_KTSAN.

QEMU Setup

Syzkaller runs its fuzzer processes inside QEMU virtual machines, so a working QEMU system is needed – see QEMU docs for details.

In particular:

  • The fuzzing processes communicate with the outside world, so the VM image needs to include networking support.
  • The program files for the fuzzer processes are transmitted into the VM using SSH, so the VM image needs a running SSH server.
  • The VM's SSH configuration should be set up to allow root access for the identity that is included in the syz-manager's configuration. In other words, you should be able to do ssh -i $SSHID -p $PORT root@localhost without being prompted for a password (where SSHID is the SSH identification file and PORT is the port that are specified in the syz-manager configuration file).
  • The kernel exports coverage information via a debugfs entry, so the VM image needs to mount the debugfs filesystem at /sys/kernel/debug.

create-image.sh script can be used to create a suitable Linux image.

TODO: Describe how to support other types of VM other than QEMU.

Syzkaller

The syzkaller tools are written in Go, so a Go compiler (>= 1.4) is needed to build them. Build with make, which generates compiled binaries in the bin/ folder.

Configuration

The operation of the syzkaller syz-manager process is governed by a configuration file, passed at invocation time with the -config option. This configuration can be based on the syz-manager/example.cfg; the file is in JSON format with the following keys in its top-level object:

  • http: URL that will display information about the running syz-manager process.
  • workdir: Location of a working directory for the syz-manager process. Outputs here include:
    • <workdir>/instance-x: per VM instance temporary files
    • <workdir>/crashes/crashN-T: crash output files
    • <workdir>/corpus/*: corpus with interesting programs
  • syzkaller: Location of the syzkaller checkout.
  • vmlinux: Location of the vmlinux file that corresponds to the kernel being tested.
  • type: Type of virtual machine to use, e.g. qemu or kvm.
  • count: Number of VMs to run in parallel.
  • procs: Number of parallel test processes in each VM (4 or 8 would be a reasonable number).
  • leak: Detect memory leaks with kmemleak (very slow).
  • kernel: Location of the bzImage file for the kernel to be tested; this is passed as the -kernel option to qemu-system-x86_64.
  • cmdline: Additional command line options for the booting kernel, for example root=/dev/sda1.
  • image: Location of the disk image file for the QEMU instance; a copy of this file is passed as the -hda option to qemu-system-x86_64.
  • sshkey: Location (on the host machine) of an SSH identity to use for communicating with the virtual machine.
  • cpu: Number of CPUs to simulate in the VM (not currently used).
  • mem: Amount of memory (in MiB) for the VM; this is passed as the -m option to qemu-system-x86_64.
  • enable_syscalls: List of syscalls to test (optional).
  • disable_syscalls: List of system calls that should be treated as disabled (optional).
  • suppressions: List of regexps for known bugs.

Running syzkaller

Start the syz-manager process as:

./bin/syz-manager -config my.cfg

The -config command line option gives the location of the configuration file described above.

The syz-manager process will wind up qemu virtual machines and start fuzzing in them. It also reports some statistics on the HTTP address.

Process Structure

The process structure for the syzkaller system is shown in the following diagram; red labels indicate corresponding configuration options.

Process structure for syzkaller

The syz-manager process starts, monitors and restarts several VM instances (support for physical machines is not implemented yet), and starts a syz-fuzzer process inside of the VMs. It is responsible for persistent corpus and crash storage. As opposed to syz-fuzzer processes, it runs on a host with stable kernel which does not experience white-noise fuzzer load.

The syz-fuzzer process runs inside of presumably unstable VMs (or physical machines under test). The syz-fuzzer guides fuzzing process itself (input generation, mutation, minimization, etc) and sends inputs that trigger new coverage back to the syz-manager process via RPC. It also starts transient syz-executor processes.

Each syz-executor process executes a single input (a sequence of syscalls). It accepts the program to execute from the syz-fuzzer process and sends results back. It is designed to be as simple as possible (to not interfere with fuzzing process), written in C++, compiled as static binary and uses shared memory for communication.

Syscall description

syzkaller uses declarative description of syscalls to generate, mutate, minimize, serialize and deserialize programs (sequences of syscalls). Below you can see (hopefully self-explanatory) excerpt from the description:

open(file filename, flags flags[open_flags], mode flags[open_mode]) fd
read(fd fd, buf buffer[out], count len[buf]) len[buf]
close(fd fd)
open_mode = S_IRUSR, S_IWUSR, S_IXUSR, S_IRGRP, S_IWGRP, S_IXGRP, S_IROTH, S_IWOTH, S_IXOTH

The description is contained in sys/sys.txt file.

This is not an official Google product.