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kernel_configs.md

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Linux kernel configs

List of recommended kernel configs for syzkaller. See syzbot config for a reference config.

Syzkaller features

To enable coverage collection, which is extremely important for effective fuzzing:

CONFIG_KCOV=y
CONFIG_KCOV_INSTRUMENT_ALL=y
CONFIG_KCOV_ENABLE_COMPARISONS=y
CONFIG_DEBUG_FS=y

Note that CONFIG_KCOV_ENABLE_COMPARISONS feature also requires gcc8+ and the following commits if you are testing an old kernel:

    kcov: support comparison operands collection
    kcov: fix comparison callback signature

To detect memory leaks using the Kernel Memory Leak Detector (kmemleak):

CONFIG_DEBUG_KMEMLEAK=y

To show code coverage in web interface:

CONFIG_DEBUG_INFO=y

For detection of enabled syscalls and kernel bitness:

CONFIG_KALLSYMS=y
CONFIG_KALLSYMS_ALL=y

For better sandboxing:

CONFIG_NAMESPACES=y
CONFIG_UTS_NS=y
CONFIG_IPC_NS=y
CONFIG_PID_NS=y
CONFIG_NET_NS=y
CONFIG_CGROUP_PIDS=y
CONFIG_MEMCG=y

For namespace sandbox:

CONFIG_USER_NS=y

For running in VMs make kvmconfig is generally required.

Debian images produced by tools/create-image.sh also require:

CONFIG_CONFIGFS_FS=y
CONFIG_SECURITYFS=y

It is recommended to disable the following config (and required if your kernel doesn't have commits arm64: setup: introduce kaslr_offset() and kcov: make kcov work properly with KASLR enabled):

# CONFIG_RANDOMIZE_BASE is not set

Bug detection configs

Syzkaller is meant to be used with KASAN (available upstream with CONFIG_KASAN=y), KTSAN (prototype available), KMSAN (prototype available), or KUBSAN (available upstream with CONFIG_UBSAN=y).

Enable KASAN for use-after-free and out-of-bounds detection:

CONFIG_KASAN=y
CONFIG_KASAN_INLINE=y

For testing with fault injection enable the following configs (syzkaller will pick it up automatically):

CONFIG_FAULT_INJECTION=y
CONFIG_FAULT_INJECTION_DEBUG_FS=y
CONFIG_FAULT_INJECTION_USERCOPY=y
CONFIG_FAILSLAB=y
CONFIG_FAIL_PAGE_ALLOC=y
CONFIG_FAIL_MAKE_REQUEST=y
CONFIG_FAIL_IO_TIMEOUT=y
CONFIG_FAIL_FUTEX=y

Note: you also need the following commits if you are testing an old kernel:

    fault-inject: support systematic fault injection
    fault-inject: simplify access check for fail-nth
    fault-inject: fix wrong should_fail() decision in task context
    fault-inject: add /proc/<pid>/fail-nth

Any other debugging configs, the more the better, here are some that proved to be especially useful:

CONFIG_LOCKDEP=y
CONFIG_PROVE_LOCKING=y
CONFIG_DEBUG_ATOMIC_SLEEP=y
CONFIG_PROVE_RCU=y
CONFIG_DEBUG_VM=y
CONFIG_REFCOUNT_FULL=y
CONFIG_FORTIFY_SOURCE=y
CONFIG_HARDENED_USERCOPY=y
CONFIG_LOCKUP_DETECTOR=y
CONFIG_SOFTLOCKUP_DETECTOR=y
CONFIG_HARDLOCKUP_DETECTOR=y
CONFIG_BOOTPARAM_HARDLOCKUP_PANIC=y
CONFIG_DETECT_HUNG_TASK=y
CONFIG_WQ_WATCHDOG=y

Increase hung/stall timeout to reduce false positive rate:

CONFIG_DEFAULT_HUNG_TASK_TIMEOUT=140
CONFIG_RCU_CPU_STALL_TIMEOUT=100