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

[Dmitry Vyukov <dvyukov@google.com>]

On Wed, Dec 18, 2019 at 11:30 AM Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> 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.

+Andrey, please take a look if CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE
covers everything we need, or we need to disable sysrq entirely.