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SubjectRe: [PATCH] kconfig: Add kernel config option for fuzz testing.
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On 2019/12/17 17:36, Dmitry Vyukov wrote:
> FWIW we've just disabled sysrq entirely:
> https://github.com/google/syzkaller/blob/master/dashboard/config/bits-syzbot.config#L182
> because random packets over usb can trigger a panic sysrq (again
> almost impossible to reliably filter these out on fuzzer side).

Excuse me, but CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE=0x0 helps only if show_state() etc. are
called via the __handle_sysrq() handler in drivers/tty/sysrq.c .

static void sysrq_handle_showstate(int key)
{
show_state();
show_workqueue_state();
}
static struct sysrq_key_op sysrq_showstate_op = {
.handler = sysrq_handle_showstate,
.help_msg = "show-task-states(t)",
.action_msg = "Show State",
.enable_mask = SYSRQ_ENABLE_DUMP,
};

The k_spec() handler in drivers/tty/vt/keyboard.c calls show_state() etc. without
evaluating sysrq_enabled value.

#define FN_HANDLERS\
fn_null, fn_enter, fn_show_ptregs, fn_show_mem,\
fn_show_state, fn_send_intr, fn_lastcons, fn_caps_toggle,\
fn_num, fn_hold, fn_scroll_forw, fn_scroll_back,\
fn_boot_it, fn_caps_on, fn_compose, fn_SAK,\
fn_dec_console, fn_inc_console, fn_spawn_con, fn_bare_num

typedef void (fn_handler_fn)(struct vc_data *vc);
static fn_handler_fn FN_HANDLERS;
static fn_handler_fn *fn_handler[] = { FN_HANDLERS };

static void fn_show_state(struct vc_data *vc)
{
show_state();
}

static void k_spec(struct vc_data *vc, unsigned char value, char up_flag)
{
if (up_flag)
return;
if (value >= ARRAY_SIZE(fn_handler))
return;
if ((kbd->kbdmode == VC_RAW ||
kbd->kbdmode == VC_MEDIUMRAW ||
kbd->kbdmode == VC_OFF) &&
value != KVAL(K_SAK))
return; /* SAK is allowed even in raw mode */
fn_handler[value](vc);
}

Therefore, we need to guard at either callee side (e.g. show_state_filter())
or caller side (e.g. k_spec()) using kernel config (or something equivalent)
in order to avoid forever calling show_state() from timer function.

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