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

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Date:   Mon, 24 Apr 2023 08:55:58 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+b7c3ba8cdc2f6cf83c21@...kaller.appspotmail.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4)

On 2023/04/24 8:34, Al Viro wrote:
> As for the original report - add a (failing) ->splice_read() in hung_ut_tty_fops
> to deal with the original problem.

Yes, adding a dummy splice_read callback is OK for avoiding NULL pointer dereference.
But we need more changes for avoiding KCSAN race reporting.

Are you OK with https://lkml.kernel.org/r/6bec279c-07b3-d6f1-0860-4d6b136a2025@I-love.SAKURA.ne.jp
which will require touching so many locations ?

If you want tty layer handle this race without rewriting all f_op dereferences,
we would need to replace

  filp->f_op = &hung_up_tty_fops;

with

  data_race(filp->some_flags_for_tty = true);

rather than

  data_race(filp->f_op = &hung_up_tty_fops);

and check

  if (data_race(filp->some_flags_for_tty)) {
    return error;
  }

 from each "struct tty_operations" callback function.