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

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Message-ID: <20190316141410.GA18654@tigerII.localdomain>
Date:   Sat, 16 Mar 2019 23:14:10 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>, 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 (03/16/19 19:18), Tetsuo Handa wrote:
> 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.

Hmm... sysctl, may be?

> > 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?

Right, that's something I would expect 0-day and syzkaller to do.

	-ss