linux-kernel - Re: [syzbot] [kernel?] KCSAN: data-race in __fput / __tty

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Date:   Wed, 26 Apr 2023 07:09:29 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Jiri Slaby <jirislaby@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+b7c3ba8cdc2f6cf83c21@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4)

On 2023/04/26 1:03, Al Viro wrote:
> On Tue, Apr 25, 2023 at 11:47:20PM +0900, Tetsuo Handa wrote:
>> @@ -743,7 +733,8 @@ void tty_vhangup_session(struct tty_struct *tty)
>>   */
>>  int tty_hung_up_p(struct file *filp)
>>  {
>> -	return (filp && filp->f_op == &hung_up_tty_fops);
>> +	/* Accept race with __tty_hangup(). */
>> +	return filp && data_race(((struct tty_file_private *) filp->private_data)->hung);
>>  }
>>  EXPORT_SYMBOL(tty_hung_up_p);
> 
> Umm...  Are you sure we never call that for non-TTY files?  Seeing that
> it's exported and all such...  For internal uses (tty_read(), etc.) the
> check for file being NULL is pointless; for general-purpose primitive
> we probably want to check ->f_op as well...

Indeed. Since drivers/tty/hvc/hvsi.c drivers/tty/n_hdlc.c drivers/tty/n_tty.c
are calling tty_hung_up_p(), this check should be

  filp && filp->f_op == &tty_fops && data_race(((struct tty_file_private *) filp->private_data)->hung);

.

>> I don't know what changes are required for tty_open() and tty_show_fdinfo().
>> I assume that making no change for tty_show_fdinfo() is harmless.
>> But how does "hung_up_tty_fops does not call tty_open()" affects here?
> 
> It doesn't - file->f_op->open() is only called once, and only on fresh
> struct file.

OK. Then, we can continue tty_hung_up_p() approach.