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Date:   Mon, 12 Apr 2021 14:04:55 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Samo Pogacnik <samo_pogacnik@t-2.net>,
        Petr Mladek <pmladek@suse.com>,
        Jiri Slaby <jirislaby@kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        John Ogness <john.ogness@linutronix.de>,
        linux-kernel@vger.kernel.org,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: How to handle concurrent access to /dev/ttyprintk ?
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On Mon, Apr 12, 2021 at 08:25:27PM +0900, Tetsuo Handa wrote:
> On 2021/04/12 19:44, Greg Kroah-Hartman wrote:
> > And trying to "open exclusive only" just does not work, the kernel can
> > not enforce that at all, sorry.  Any driver that you see trying to do
> > that is trivial to work around in userspace, making the kernel code
> > pointless.
> 
> You mean something like below cannot be used?
> 
> diff --git a/drivers/char/ttyprintk.c b/drivers/char/ttyprintk.c
> index 6a0059e508e3..57200569918a 100644
> --- a/drivers/char/ttyprintk.c
> +++ b/drivers/char/ttyprintk.c
> @@ -84,14 +84,26 @@ static int tpk_printk(const unsigned char *buf, int count)
>  	return count;
>  }
>  
> +static DEFINE_MUTEX(open_close_lock);

Hah, nope, does not work at all!

Think about sending an open file descriptor all around the system, or
through a pipe, your "open only once" check does not prevent that at
all.

> > Like any tty port, if you have multiple accesses, all bets are off and
> > hilarity ensues.  Just don't do that and expect things to be working
> > well.
> 
> Since syzkaller is a fuzzer, syzkaller happily opens /dev/ttyprintk from
> multiple threads. Should we update syzkaller to use CONFIG_TTY_PRINTK=n ?

Why?  Can you not hit the same tty code paths from any other tty driver
being open multiple times?  Why is ttyprintk somehow "special" here?

thanks,

greg k-h