Re: [PATCH] printk: Monitor change of console loglevel.

[Dmitry Vyukov]

On Thu, May 23, 2019 at 11:57 AM Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
>
> Well, the culprit of this problem might be syz_execute_func().
>
>   https://twitter.com/ed_maste/status/1131165065485398016
>
> Then, blacklisting specific syscalls/arguments might not work.
> We will need to guard specific paths on the kernel side using
> some kernel config option...

Yes, that's a nasty issue. We could stop running random code, or
setuid into nobody, but then we will lose lots of test coverage...

> Anyway, Andrew, will you send this patch to linux-next.git ?
> syzbot would identify which syz_execute_func() call is triggering
> this problem.
>
> From 96e0741839f56c461f85d83e20bf5ae6baac9a3a Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Date: Thu, 23 May 2019 05:57:52 +0900
> Subject: [PATCH] printk: Monitor change of console loglevel.
>
> We are seeing syzbot reports [1] where printk() messages prior to panic()
> are missing for unknown reason. To test whether it is due to some testcase
> changing console loglevel, let's panic() as soon as console loglevel has
> changed. This patch is intended for testing on linux-next.git only, and
> will be removed after we found what is wrong.
>
> [1] https://lkml.kernel.org/r/127c9c3b-f878-174f-7065-66dc50fcabcf@xxxxxxxxxxxxxxxxxxx
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
> Cc: Petr Mladek <pmladek@xxxxxxxx>
> ---
>  kernel/printk/printk.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 1888f6a..5326015 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3343,3 +3343,23 @@ void kmsg_dump_rewind(struct kmsg_dumper *dumper)
>  EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
>
>  #endif
> +
> +#ifdef CONFIG_DEBUG_AID_FOR_SYZBOT
> +static int initial_loglevel;
> +static void check_loglevel(struct timer_list *timer)
> +{
> +       if (console_loglevel < initial_loglevel)
> +               panic("Console loglevel changed (%d->%d)!", initial_loglevel,
> +                     console_loglevel);
> +       mod_timer(timer, jiffies + HZ);
> +}
> +static int __init loglevelcheck_init(void)
> +{
> +       static DEFINE_TIMER(timer, check_loglevel);
> +
> +       initial_loglevel = console_loglevel;
> +       mod_timer(&timer, jiffies + HZ);
> +       return 0;
> +}
> +late_initcall(loglevelcheck_init);
> +#endif
> --
> 1.8.3.1