linux-kernel - Re: [PATCHv5 3/7] tty: Hold tty_ldisc_lock() during tty

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Date:   Tue, 18 Sep 2018 15:19:17 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Daniel Axtens <dja@...ens.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Nathan March <nathan@...net>,
        Pasi Kärkkäinen <pasik@....fi>,
        Peter Hurley <peter@...leysoftware.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rong, Chen" <rong.a.chen@...el.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Jiri Slaby <jslaby@...e.cz>,
        syzbot+3aa9784721dfb90e984d@...kaller.appspotmail.com,
        stable@...r.kernel.org, Jiri Slaby <jslaby@...e.com>
Subject: Re: [PATCHv5 3/7] tty: Hold tty_ldisc_lock() during tty_reopen()

On Tue, 2018-09-18 at 15:47 +0200, Greg Kroah-Hartman wrote:
> On Tue, Sep 18, 2018 at 12:52:54AM +0100, Dmitry Safonov wrote:
> > tty_ldisc_reinit() doesn't race with neither tty_ldisc_hangup()
> > nor set_ldisc() nor tty_ldisc_release() as they use tty lock.
> > But it races with anyone who expects line discipline to be the same
> > after hoding read semaphore in tty_ldisc_ref().
> > 
> > We've seen the following crash on v4.9.108 stable:
> > 
> > BUG: unable to handle kernel paging request at 0000000000002260
> > IP: [..] n_tty_receive_buf_common+0x5f/0x86d
> > Workqueue: events_unbound flush_to_ldisc
> > Call Trace:
> >  [..] n_tty_receive_buf2
> >  [..] tty_ldisc_receive_buf
> >  [..] flush_to_ldisc
> >  [..] process_one_work
> >  [..] worker_thread
> >  [..] kthread
> >  [..] ret_from_fork
> > 
> > tty_ldisc_reinit() should be called with ldisc_sem hold for
> > writing,
> > which will protect any reader against line discipline changes.
> > 
> > Backport-first: b027e2298bd5 ("tty: fix data race between
> > tty_init_dev
> > and flush of buf")
> 
> What does this mean?
> 
> Does this require that patch for a stable patch?  If so, just do:
>  Cc: stable@...r.kernel.org  # b027e2298bd5 ("tty: fix data race
> between tty_init_dev and flush of buf")
> in the signed-off-by area.  The stable documentation should describe
> this pretty well.  If not, we can modify it to make it more obvious.
> 
> can you fix this up for the next resend of this series?

Sure, sorry about that non-obvious tag.

-- 
Thanks,
             Dmitry