Fuzzer Configuration#

The kAFL fuzzer configuration can be changed via config files, environment variables and command line switches.

The Dynaconf framework is used behind the scenes for configuration management, so everything that you learn from Dynaconf’s documentation should also be applicable for kAFL fuzzer.

Configuration sources and precedence#

All configuration files are using the YAML format.

kAFL fuzzer packaged default configuration file: kafl_fuzzer/common/config/default_settings.yaml
System-level config file: /etc/xdg/kAFL/settings.yaml
User-level config file: $HOME/.config/kAFL/settings.yaml
Local config file: $PWD/kafl.yaml
Environment variable KAFL_CONFIG_FILE if specified
Command line arguments. Ex: --work_dir /dev/shm/kafl_test_feature
Configuration keys specified by environment variables prefixed by KAFL_. Ex: KAFL_PROCESSES=8

Note: if KAFL_CONFIG_FILE is specified but points to a non-existing file, a validation error will be raised (to warn the user).

Overriding settings from environment variables#

You can override any setting key by exporting an environment variable prefixed by KAFL_.

Example

export KAFL_QEMU_MEMORY=1024
export KAFL_LOG_HRPINTF=TRUE

Configuration keys#

The following section lists all configuration keys avaialble in kAFL, and their corresponding command line switches, if any.

Note

Configuration keys are case insensitive:

qemu_memory: 256
# and
QEMU_MEMORY: 256

`abort_exec`#

Exit kAFL fuzzing a after <n> total executions.

Note

Default: 0

Command-line: --abort-exec

`abort_time`#

Builtin timeout to stop kafl fuzzing after <n> seconds elapsed.

Note

Default: 0

Command-line: --abort-time

`action`#

kAFL debug action.

Choices available:

Action	Description
`benchmark`	Perform performance benchmark
`gdb`	Run payload with GDB server (must compile with redqueen)
`trace`	Peform trace run
`trace-qemu`	Perform trace run and print QEMU stdout
`noise`	Perform run and measure non determinism
`printk`	redirect printk calls to kAFL
`redqueen`	Run redqueen debugger
`redqueen-qemu`	Run redqueen debugger and print QEMU stdout
`verify`	Run verification steps

Note

Command-line: --action.

`afl_arith_max`#

Maximum number of increment/decrement steps in AFL-style arithmetic mutation (only affects deterministic stage, not havoc).

Note

Default: 34

Command-line: --afl-arith-max

`afl_dumb_mode`#

Skip AFL-style deterministic stages (bitflip, arithmetic, interesting mutations).

Note

Default: false

Command-line: --afl-dumb-mode, -D

`afl_skip_zero`#

Skip mutating zero-bytes in AFL-style deterministic stages.

Note

Default: false

Command-line: --afl-skip-zero

`bitmap_size`#

Size of feedback bitmap (must be power of 2).

Note

Default: 65536

Command-line: --bitmap-size

`cpu_offset`#

Modify automated CPU pinning to start at vCPU n and assign Qemu/Worker instances to the next -p vCPUs.

Note

Default: 0

Command-line: --cpu-offset <n>

`debug`#

Enable additional (expensive) debug messages during execution.

switches logging to DEBUG level
toggles QEMU nyx log item (-d nyx), only effective if --log has been specified
stores funky input data in $WORKDIR/funky. See --funky and workdir reference

Implies --verbose.

Note

Default: false

Command-line: --debug

`dict`#

Use file <path> as source of dictionary inputs in havoc stage.

Note

Default: None

Command-line: --dict <path>

`funky`#

kafl fuzz

Validate new (non-crashing/non-timeout) inputs multiple times and add to corpus if consistent new coverage is found on majority of re-runs. Executes payloads 8 times by default and accepts payloads that reproduce with 75% probability. In combination with --debug, non-reproducible inputs are stored in $KAFL_WORKDIR/funky/. Useful with non-deterministic targets.

kafl cov

Output any additional found coverage and payloads in combination with --trace --action noise.

Note

Default: false

Command-line: --funky

`gdbserver`#

Starts QEMU GDB server (Extends QEMU command line with -s -S).

From QEMU command line help:

-s: shorthand for -gdb tcp::1234
-S: freeze CPU at startup (use ‘c’ to start execution)

Note

Default: false

Command-line: --gdbserver

`grimoire`#

Enable the Grimoire fuzzer stage.

Note

Default: false

Command-line: --grimoire

`input`#

kafl cov

Sets the kAFL workdir to be processed for coverage information. Will automatically process files in corpus/{crash,kasan,regular}/ (but not timeout).

kafl debug

Sets the payload file to be used as input for the debugging session.

Note

Default: workdir value

Command-line: --input <path>

`ip0-1-2-3`#

Set the IP filter ranges (code ranges) to be used with Intel PT. The filter ranges may also be set by the agent using the (RANGE_SUBMIT hypercall)[hypercall_api.md].

Not setting a IP filter region is currently not supported.

Note

Default: None

Command-line: --ip0, --ip1, --ip2, --ip3

`iterations`#

Execute the debugged payload <n> times.

Used by noise, trace and trace-qemu debug actions in case of non-deterministic execution.

Note

Default: 5

Command-line: --iterations

Applicable subcommands: debug

`kickstart`#

When no payloads in queue or more workers than available corpus payloads, let the worker kickstart fuzzing with random strings of length n. Useful to test a new harness that has no well defined seeds.

Enabled by default. Set 0 to disable.

Note

Default: 256

Command-line: --kickstart <n>

`log`#

Add an additional file logging handler to $KAFL_WORKDIR/kafl_fuzzer.log. Also redirects Qemu log output to $KAFL_WORKDIR/qemu_trace_NN.log, if any.

Note

Default: false

Command-line: --log

`log_crashes`#

Like log_hprintf, but copy the hprintf log to $KAFL_WORKDIR/logs/ for any new found {crash,kasan,timeout} type payload. Also truncates the main hprintf log after every execution.

This is the recommended guest logging option to collect live guest logs corresponding to new found non-regular payloads, while avoiding OOM due to huge hprintf logs.

Note

Default: false

Command-line: --log-crashes

`log_hprintf`#

Redirect guest hprintf output to $KAFL_WORKDIR/hprintf_NN.log (see PRINTF hypercall).

Creates a linear log of Guest execution across snapshot/restore. Recommended for debugging guest/harness execution.

Note

Default: false

Command-line: --log-hprintf

`payload_size`#

Maximum payload size in bytes. Must be multiple of page size (4096 bytes).

Note

Default: 131072

Command-line: --payload-size <n>

`processes`#

Number of processes to launch for parallelized fuzzing and coverage.

Note

Default: 1

Command-line: --processes <n>

`ptdump_path`#

Path to ptdump executable.

Note

Default: $LIBXDC_ROOT/build/ptdump_static

Command-line: --ptdump-path

`purge`#

Purge the KAFL workdir directory at startup.

Note

Default: false

Command-line: --purge

`qemu_append`#

Kernel command line if --kernel is used.

Corresponds to QEMU -append <cmdline> argument.

Note

Default: nokaslr oops=panic nopti mitigations=off console=ttyS0

Command-line: --qemu-append

`qemu_base`#

Baseline for QEMU command-line.

Note

Default: -enable-kvm -machine kAFL64-v1 -cpu kAFL64-Hypervisor-v1,+vmx -no-reboot -net none -display none

Command-line: --qemu-base

`qemu_bios`#

Set custom BIOS image to be used by QEMU.

Corresponds to QEMU -bios <file> argument.

Note

Default: None

Command-line: --bios

`qemu_extra`#

Extra string appended to QEMU command line. Useful to override specific Qemu flags.

Note

Default: None

Command-line: --qemu-extra

`qemu_image`#

Path to Qemu disk image.

Corresponds to QEMU -drive file=<disk> argument.

Note

Default: None

Command-line: --image

`qemu_initrd`#

Initial ram disk for QEMU.

Corresponds to QEMU -initrd <file> argument.

Note

Default: None

Command-line: --initrd

`qemu_kernel`#

bzImage to use as kernel image for QEMU.

Corresponds to QEMU -kernel <image> argument.

Note

Default: None

Command-line: --kernel

`qemu_memory`#

Amount of memory in MB for QEMU.

Corresponds to QEMU -m <size> argument.

Note

Default: 256

Command-line: --memory <n>, -m <n>

`qemu_path`#

Path to QEMU executable with Nyx patches.

Note

Default: $QEMU_ROOT/x86_64-softmmu/qemu-system-x86_64

Command-line: --qemu-path <path>

`qemu_serial`#

Extend QEMU command line with the configuration value, and then append -chardev file,id=kafl_serial,mux=on,path=$KAFL_WORKDIR/serial_<qemu_pid>.log.

Note

Default: -device isa-serial,chardev=kafl_serial

Command-line: --qemu-serial

`qemu_snapshot`#

Path to VM pre-snapshot directory. Use with manual pre-snapshot creation (LOCK hypercall)[hypercall_api.md]

Note

Default: None

Command-line: --snapshot <path>

`quiet`#

Set Python logging level to WARNING.

Note

Default: false

Command-line: --quiet

`radamsa_path`#

Path to radamsa executable.

Note

Default: $RADAMSA_ROOT/bin/radamsa

Command-line: --radamsa-path <path>

`radamsa`#

Enable radamsa mutation as part of havoc stage.

Note

Default: false

Command-line: --radamsa

`redqueen_simple`#

Modify redqueen to also process ‘simple’ inputs that would likely be found by subsequent deterministic/havoc mutations.

Note

Default: false

Command-line: --redqueen-simple

`redqueen_hammer`#

Enable Redqueen jump table hammering.

Note

Default: false

Command-line: --redqueen-hammer

`redqueen_hashes`#

Enable Redqueen checksum fixer (broken).

Note

Default: false

Command-line: --redqueen-hashes

`redqueen`#

Enable Redqueen analysis + mutation stages.

Note

Default: false

Command-line: --redqueen

`reload`#

Reload the snapshot only on non-regular executions or every <N> regular executions. Set to 0 for fully persistent mode execution. Set to ‘>1’ for partial persistent mode execution.

For targets that support persistent mode, setting ‘–reload’ somewhere in the range of 10-100 tends to yield good performance/stability trade-off.

Note

Default: 1

Command-line: --reload <n>, -R <n>

`resume`#

Tell kAFL to resume operation based on an existing workdir. This is currently limited to kafl cov and kafl debug, where it will cause the Qemu instance to initialize based on existing Nyx snapshot/ and page_cache.* files.

Resuming a complete fuzzer campaign (kafl fuzz) is not yet supported.

Note

Default: false

Command-line: --resume

`seed_dir`#

Specify a directory <path> to use as seed directory. The directory is traversed recursively and files are copied to the kAFL workdir as $KAFL_WORKDIR/imports/seed_xxx, where there are consumed upon fuzzer startup.

Note

Default: None

Command-line: --seed-dir <path>

`sharedir`#

Path to the Qemu ‘sharedir’ directory. Files in this folder will be made available for the agent to download via (REQ_STREAM_DATA hypercall)[hypercall_api.md].

Appends ,sharedir=<value> to the nyx QEMU device.

Note

Default: None

Command-line: --sharedir <path>

`timeout_check`#

When using both timeout_hard and timeout_soft, check that a payload reported as ‘timeout’ also produces a timeout when executing with maximum timeout ‘timeout_hard’.

Note

Default: false

Command-line: --t-check

`timeout_hard`#

Hard execution timeout (seconds)

Note

Default: 4

Command-line: --t-hard

`timeout_soft`#

Soft execution timeout (seconds). Used as initial lower timeout for Qemu instances and then adapted based on seen performance, up to the maximum of timeout_hard.

Note

Default: 0.001

Command-line: --t-soft

`trace_cb`#

Run Qemu in ‘callback-trace’ mode, where PT traces are directly decoded using a libxdc callback function. This tends to significantly slow down execution and results in different coverage bitmaps. Deprecated.

Appends ,edge_cb_trace to the nyx QEMU device.

Note

Default: false

Command-line: --trace-cb

`trace`#

Run Qemu in dump-trace mode, where binary PT traces are directly stored to the workdir by each Qemu process. For any new discovered payloads, kAFL stores the corresponding binary traces to $KAFL_WORKDIR/traces/ for later decoding with kafl cov. This is the recommeded trace mode as it incurrs minimal slow-down and also works for non-deterministic targets.

Appends ,dump_pt_trace to the nyx QEMU device.

Note

Default: false

Command-line: --trace

`verbose`#

Enable verbose output by setting Python logging level to DEBUG.

This option is also useful to dump and debug kafl configuration at load-time.

Example with Fuzzing the Linux kernel tutorial, if --verbose switch were to be added to the command line:

First kAFL will dump the list of loaded configuration files. Check the load order

Loaded configuration files:
['/home/mtarral/kAFL/kafl/fuzzer/kafl_fuzzer/common/config/default_settings.yaml',
 '/home/mtarral/kAFL/kafl/examples/linux-kernel/./kafl_config.yaml']

Then the command-line configuration as parser by argparse

Command line configuration:
{'abort_exec': None,
 'abort_time': None,
 'afl_arith_max': None,
 'afl_dumb_mode': True,
...

Followed by the configuration values extracted from the loaded config files:

Loaded configuration values:
{'env_global': {'CONFIG_FILE': './kafl_config.yaml',
                'ROOT': '/home/mtarral/kAFL/kafl',
                'WORKDIR': '/dev/shm/kafl_mtarral',
                'WORKSPACE': '/home/mtarral/kAFL'},
 'yaml': {'AFL_ARITH_MAX': 34,
          'BITMAP_SIZE': 65536,
          'CPU_OFFSET': 0,
          'DEBUG': False,
...

As the command line supersedes the configuration files, kAFL will override these values and finally dump the definitive configuration:

Final configuration:
{'ABORT_EXEC': 0,
 'ABORT_TIME': 0.0,
 'AFL_ARITH_MAX': 34,
 'AFL_DUMB_MODE': True,

Note

Default: false

Command-line: --verbose

`work_dir`#

The workdir is used by kAFL to accumulate results and communicate with QEMU and other processes. It is the primary location for inspecting the status of a previous or still running kAFL session.

Also, any post-processing steps such as triage or coverage analysis typically builds up on an existing workdir.

Note

Default: /dev/shm/kafl_$USER

Command-line: --workdir

Fuzzer Configuration#

Configuration sources and precedence#

Overriding settings from environment variables#

Configuration keys#

abort_exec#

abort_time#

action#

afl_arith_max#

afl_dumb_mode#

afl_skip_zero#

bitmap_size#

cpu_offset#

debug#

dict#

funky#

gdbserver#

grimoire#

input#

ip0-1-2-3#

iterations#

kickstart#

log#

log_crashes#

log_hprintf#

payload_size#

processes#

ptdump_path#

purge#

qemu_append#

qemu_base#

qemu_bios#

qemu_extra#

qemu_image#

qemu_initrd#

qemu_kernel#

qemu_memory#

qemu_path#

qemu_serial#

qemu_snapshot#

quiet#

radamsa_path#

radamsa#

redqueen_simple#

redqueen_hammer#

redqueen_hashes#

redqueen#

reload#

resume#

seed_dir#

sharedir#

timeout_check#

timeout_hard#

timeout_soft#

trace_cb#

trace#

verbose#

work_dir#

`abort_exec`#

`abort_time`#

`action`#

`afl_arith_max`#

`afl_dumb_mode`#

`afl_skip_zero`#

`bitmap_size`#

`cpu_offset`#

`debug`#

`dict`#

`funky`#

`gdbserver`#

`grimoire`#

`input`#

`ip0-1-2-3`#

`iterations`#

`kickstart`#

`log`#

`log_crashes`#

`log_hprintf`#

`payload_size`#

`processes`#

`ptdump_path`#

`purge`#

`qemu_append`#

`qemu_base`#

`qemu_bios`#

`qemu_extra`#

`qemu_image`#

`qemu_initrd`#

`qemu_kernel`#

`qemu_memory`#

`qemu_path`#

`qemu_serial`#

`qemu_snapshot`#

`quiet`#

`radamsa_path`#

`radamsa`#

`redqueen_simple`#

`redqueen_hammer`#

`redqueen_hashes`#

`redqueen`#

`reload`#

`resume`#

`seed_dir`#

`sharedir`#

`timeout_check`#

`timeout_hard`#

`timeout_soft`#

`trace_cb`#

`trace`#

`verbose`#

`work_dir`#