linux-kernel - Re: [syzbot? printk?] no WARN_ON() messages printed before "Kernel panic

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Date:   Sat, 16 Mar 2019 19:18:50 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [syzbot? printk?] no WARN_ON() messages printed before "Kernel
 panic - not syncing: panic_on_warn set ..."

On 2019/03/16 18:11, Dmitry Vyukov wrote:
> Such reports showed up always with low frequency. For all that I
> looked at we also always had a normal  non-truncated version, so I was
> never too worried.
> 
> If something would write from userspace, that would show up in the
> output, or not?
> 
> Perhaps syzkaller somehow manages to lower console output level?

panic() calls console_verbose() before calling

  pr_emerg("Kernel panic - not syncing: %s\n", buf);

line. Then, someone might have changed console_loglevel enough to
suppress printk() output.

> I figured out that we should restrict it from doing
> syslog(SYSLOG_ACTION_CONSOLE_OFF). And I also restricted its access o
> /dev/console. But maybe there is something else? It _should_ not be
> able to write to random sysctl's.

Maybe try running with "ignore_loglevel" kernel command line option added?