Re: [PATCH] kconfig: Add kernel config option for fuzz testing.

[Tetsuo Handa]

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.