Received-SPF: pass (google.com: domain of linux-kernel+bounces-161132-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Precedence: bulk
MIME-Version: 1.0
References: <e1fe6a44-3021-62ad-690a-69146e39e1ac@I-love.SAKURA.ne.jp>
 <20230424004431.GG3390869@ZenIV> <8e21256a-736e-4c2d-1ff4-723775bcac46@I-love.SAKURA.ne.jp>
 <2fca7932-5030-32c3-dd61-48dd78e58e11@I-love.SAKURA.ne.jp>
 <20230425160344.GS3390869@ZenIV> <1b405689-ea0a-6696-6709-d372ce72d68c@I-love.SAKURA.ne.jp>
 <5cebade5-0aa9-506c-c817-7bcf098eba89@I-love.SAKURA.ne.jp>
 <c95c62ba-4f47-b499-623b-05627a81c601@I-love.SAKURA.ne.jp>
 <2023053005-alongside-unvisited-d9af@gregkh> <8edbd558-a05f-c775-4d0c-09367e688682@I-love.SAKURA.ne.jp>
 <2023053048-saved-undated-9adf@gregkh> <18a58415-4aa9-4cba-97d2-b70384407313@I-love.SAKURA.ne.jp>
In-Reply-To: <18a58415-4aa9-4cba-97d2-b70384407313@I-love.SAKURA.ne.jp>
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sat, 27 Apr 2024 12:02:32 -0700
Message-ID: <CAHk-=wgSOa_g+bxjNi+HQpC=6sHK2yKeoW-xOhb0-FVGMTDWjg@mail.gmail.com>
Subject: Re: [PATCH v3] tty: tty_io: remove hung_up_tty_fops
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Dmitry Vyukov <dvyukov@google.com>, 
	syzbot <syzbot+b7c3ba8cdc2f6cf83c21@syzkaller.appspotmail.com>, 
	linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, 
	Nathan Chancellor <nathan@kernel.org>, Arnd Bergmann <arnd@kernel.org>, Al Viro <viro@zeniv.linux.org.uk>, 
	Jiri Slaby <jirislaby@kernel.org>
Content-Type: text/plain; charset="UTF-8"

On Fri, 26 Apr 2024 at 23:21, Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> syzbot is reporting data race between __tty_hangup() and __fput(), for
> filp->f_op readers are not holding tty->files_lock.

Hmm. I looked round, and we actually have another case of this:
snd_card_disconnect() also does

                        mfile->file->f_op = &snd_shutdown_f_ops;

and I don't think tty->files_lock (or, in the sound case,
&card->files_lock) is at all relevant, since the users of f_ops don't
use it or care.

That said, I really think we'd be better off just keeping the current
model, and have the "you get one or the other". For the two cases that
do this, do that f_op replacement with a WRITE_ONCE(), and just make
the rule be that you have to have all the same ops in both the
original and the shutdown version.

I do *not* think it's at all better to replace (in two different
places) the racy f_op thing with another racy 'hungup' flag.

The sound case is actually a bit more involved, since it tries to deal
with module counts. That looks potentially bogus. It does

                        fops_get(mfile->file->f_op);

after it has installed the snd_shutdown_f_ops, but in snd_open() it
has done the proper

        replace_fops(file, new_fops);

which actually drops the module count for the old one. So the sound
case seems to possibly leak a module ref on disconnect. That's a
separate issue, though.

                      Linus

                    Linus