From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Dmitry Vyukov <dvyukov@google.com>,
syzbot <syzbot+b7c3ba8cdc2f6cf83c21@syzkaller.appspotmail.com>,
Jiri Slaby <jirislaby@kernel.org>,
linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4)
Date: Sun, 23 Apr 2023 23:17:57 +0900 [thread overview]
Message-ID: <8904e414-6b3e-8fa3-67e1-709d51965372@I-love.SAKURA.ne.jp> (raw)
In-Reply-To: <2023042347-vaguely-monsieur-63ed@gregkh>
On 2023/04/23 23:03, Greg Kroah-Hartman wrote:
>> Next step is to convert from
>>
>> if (!f_op->$callbackname) {
>> return error;
>> }
>> return f_op->$callbackname($args);
>>
>> to
>>
>> fn = READ_ONCE(f_op->$callbackname);
>> if (!fn) {
>> return error;
>> }
>> return fn($args);
>>
>> pattern.
>
> Why? What does this solve differently than the first one? What can
> change the fops pointer between the check and call path? If something
> can change it, then do NOT make that type of check in the first place
> (or put a proper lock in place.)
__tty_hangup() is changing filp->f_op like
spin_lock(&tty->files_lock);
/* This breaks for file handles being sent over AF_UNIX sockets ? */
list_for_each_entry(priv, &tty->tty_files, list) {
filp = priv->file;
if (filp->f_op->write_iter == redirected_tty_write)
cons_filp = filp;
if (filp->f_op->write_iter != tty_write)
continue;
closecount++;
__tty_fasync(-1, filp, 0); /* can't block */
filp->f_op = &hung_up_tty_fops;
}
spin_unlock(&tty->files_lock);
but filp->f_op readers are (of course) not protected by tty->files_lock.
Like Dmitry mentioned
hung_up_tty_fops does not have splice_read, while other fops have.
, patterns like
if (unlikely(!in->f_op->splice_read))
return warn_unsupported(in, "read");
return in->f_op->splice_read(in, ppos, pipe, len, flags);
is not safe (if compiler generates a code that re-reads the pointer).
Using READ_ONCE() is for preventing the compiler to re-read, and using data_race()
is for teaching KCSAN that race while happens during READ_ONCE()/WRITE_ONCE()
is acceptable.
>> First step (which Dmitry mentioned) is to avoid potential NULL pointer dereferences
>> caused by
>>
>> if (!f_op->$callbackname) {
>> return error;
>> }
>> return f_op->$callbackname($args);
>>
>> pattern, for the next step will touch too many locations to change all at once whereas
>> the first step could be handled by implementing dummy function for all missing $callbackname.
>
> I do not understand, what "callbackname" is the problem here? Just
> splice_read? Something else?
I haven't checked.
> And where would it need to be modified
> and why can't we do it in one place only (i.e. install a default handler
> instead.)
Yes, adding a dummy splice_read callback handler to hung_up_tty_fops is
what I call the first step.
prev parent reply other threads:[~2023-04-23 14:19 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-21 8:18 [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4) syzbot
2023-04-21 8:21 ` Dmitry Vyukov
2023-04-21 15:12 ` Tetsuo Handa
2023-04-21 16:02 ` Tetsuo Handa
2023-04-23 23:34 ` Al Viro
2023-04-23 23:55 ` Tetsuo Handa
2023-04-24 0:44 ` Al Viro
2023-04-24 1:09 ` Tetsuo Handa
2023-04-25 14:47 ` Tetsuo Handa
2023-04-25 16:03 ` Al Viro
2023-04-25 22:09 ` Tetsuo Handa
2023-04-26 11:05 ` [PATCH] tty: tty_io: remove hung_up_tty_fops Tetsuo Handa
2023-04-28 16:27 ` Nathan Chancellor
2023-04-28 16:41 ` Tetsuo Handa
2023-04-28 17:11 ` Al Viro
2023-04-29 10:43 ` Tetsuo Handa
2023-04-28 17:31 ` Greg Kroah-Hartman
2023-04-29 15:21 ` Guenter Roeck
2023-05-01 18:42 ` Geert Uytterhoeven
2023-05-14 1:02 ` [PATCH v2] " Tetsuo Handa
2023-05-30 10:44 ` Greg Kroah-Hartman
2023-05-30 11:57 ` Tetsuo Handa
2023-05-30 12:51 ` Greg Kroah-Hartman
2024-04-27 6:20 ` [PATCH v3] " Tetsuo Handa
2024-04-27 19:02 ` Linus Torvalds
2024-04-28 10:19 ` Tetsuo Handa
2024-04-28 18:50 ` Linus Torvalds
2024-04-29 13:55 ` Marco Elver
2024-04-29 15:38 ` Linus Torvalds
2024-05-01 18:45 ` Paul E. McKenney
2024-05-01 18:56 ` Linus Torvalds
2024-05-01 19:02 ` Paul E. McKenney
2024-05-01 20:14 ` Marco Elver
2024-05-01 21:06 ` Linus Torvalds
2024-05-01 21:20 ` Linus Torvalds
2024-05-01 21:49 ` Paul E. McKenney
2024-05-01 22:32 ` Paul E. McKenney
2024-05-02 16:37 ` Boqun Feng
2024-05-03 23:59 ` Paul E. McKenney
2024-05-04 0:14 ` Linus Torvalds
2024-05-04 5:08 ` Paul E. McKenney
2024-05-04 17:50 ` Linus Torvalds
2024-05-04 18:18 ` Paul E. McKenney
2024-05-04 19:11 ` Linus Torvalds
2024-05-04 19:25 ` Linus Torvalds
2024-05-04 22:17 ` Paul E. McKenney
2024-05-04 22:04 ` Paul E. McKenney
2024-05-02 14:14 ` Marco Elver
2024-05-02 16:42 ` Tetsuo Handa
2024-05-02 17:20 ` Marco Elver
2024-05-02 17:29 ` Linus Torvalds
2024-05-02 18:14 ` Al Viro
2024-05-02 19:29 ` Marco Elver
2024-05-02 23:54 ` Tetsuo Handa
2024-05-03 1:12 ` Linus Torvalds
2023-04-23 13:28 ` [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4) Tetsuo Handa
2023-04-23 14:00 ` Greg Kroah-Hartman
2023-04-23 14:03 ` Greg Kroah-Hartman
2023-04-23 14:17 ` Tetsuo Handa [this message]
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