Syzkaller uses the _RET_IP_ (also known as pc) to decode covered file/function/line, and it employs pc ^ hash(prev_pc) (referred to as signal) to indicate covered edge. If the pc for the same file/line keeps changing across reboots, syzkaller will report incorrect coverage data. Additionally, even if kaslr can be disabled, we cannot get the same covered edge for module because both pc and prev_pc have changed, thus altering pc ^ hash(prev_pc). To facilitate syzkaller coverage, it is crucial for both the core kernel and modules to maintain at the same addresses across reboots. So, the following steps are necessary: - In userspace: 1) To maintain an uninterrupted loading sequence, it is recommended to execute modprobe commands by loading one module at a time, to avoid any interference from the scheduler. 2) Avoid unloading any module during fuzzing. - In kernel: 1) Disable CONFIG_RANDOMIZE_BASE to load the core kernel at the same address consistently. 2) To ensure deterministic module loading at the same address, enabling CONFIG_MODULE_DISABLE_INIT_FREE prevents the asynchronous freeing of init sections. Without this option, there is a possibility that the next module could be loaded into previous freed init pages of a previous loaded module. It is important to note that this option is intended for fuzzing tests only and should not be set as the default configuration in production builds. Signed-off-by: Joey Jiao <quic_jiangenj@xxxxxxxxxxx> --- kernel/module/Kconfig | 13 +++++++++++++ kernel/module/main.c | 3 ++- 2 files changed, 15 insertions(+), 1 deletion(-) diff --git a/kernel/module/Kconfig b/kernel/module/Kconfig index 33a2e991f608..d0df0b5997b0 100644 --- a/kernel/module/Kconfig +++ b/kernel/module/Kconfig @@ -389,4 +389,17 @@ config MODULES_TREE_LOOKUP def_bool y depends on PERF_EVENTS || TRACING || CFI_CLANG +config MODULE_DISABLE_INIT_FREE + bool "Disable freeing of init sections" + default n + depends on !RANDOMIZE_BASE + help + By default, the kernel frees init sections after module is fully + loaded. + + Enabling MODULE_DISABLE_INIT_FREE allows users to prevent the freeing + of init sections. It is particularly helpful for syzkaller fuzzing, + ensuring that the module consistently loads at the same address + across reboots. + endif # MODULES diff --git a/kernel/module/main.c b/kernel/module/main.c index 98fedfdb8db5..d226df3a6cf6 100644 --- a/kernel/module/main.c +++ b/kernel/module/main.c @@ -2593,7 +2593,8 @@ static noinline int do_init_module(struct module *mod) * be cleaned up needs to sync with the queued work - ie * rcu_barrier() */ - if (llist_add(&freeinit->node, &init_free_list)) + if (!IS_ENABLED(CONFIG_MODULE_DISABLE_INIT_FREE) && + llist_add(&freeinit->node, &init_free_list)) schedule_work(&init_free_wq); mutex_unlock(&module_mutex); -- 2.42.0
