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    SubjectRe: [PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()
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    On Wed, 2018-08-29 at 16:40 +0200, Jiri Slaby wrote:
    > On 08/29/2018, 04:23 AM, 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
    > >
    > > I think, tty_ldisc_reinit() should be called with ldisc_sem hold
    > > for
    > > writing, which will protect any reader against line discipline
    > > changes.
    > >
    > > Note: I failed to reproduce the described crash, so obiviously
    > > can't
    > > guarantee that this is the place where line discipline was
    > > switched.
    > >
    > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    > > Cc: Jiri Slaby <jslaby@suse.com>
    > > Cc: stable@vger.kernel.org
    > > Signed-off-by: Dmitry Safonov <dima@arista.com>
    > > ---
    > > drivers/tty/tty_io.c | 9 +++++++--
    > > 1 file changed, 7 insertions(+), 2 deletions(-)
    > >
    > > diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
    > > index 5e5da9acaf0a..3ef8b977b167 100644
    > > --- a/drivers/tty/tty_io.c
    > > +++ b/drivers/tty/tty_io.c
    > > @@ -1267,15 +1267,20 @@ static int tty_reopen(struct tty_struct
    > > *tty)
    > > if (test_bit(TTY_EXCLUSIVE, &tty->flags) &&
    > > !capable(CAP_SYS_ADMIN))
    > > return -EBUSY;
    > >
    > > - tty->count++;
    > > + retval = tty_ldisc_lock(tty, 5 * HZ);
    >
    > Why 5 secs? This would cause random errors on machines under heavy
    > load.

    Yeah, I think MAX_SCHEDULE_TIMEOUT will make more sense here..
    Not sure, why I decided to go with 5*HZ instead.
    Will resend with new timeout, if everything else looks good to you.
    (having in mind my argument for count++ in 1/4)

    >
    > > + if (retval)
    > > + return retval;
    > >
    > > + tty->count++;
    > > if (tty->ldisc)
    > > - return 0;
    > > + goto out_unlock;
    > >
    > > retval = tty_ldisc_reinit(tty, tty->termios.c_line);
    > > if (retval)
    > > tty->count--;
    > >
    > > +out_unlock:
    > > + tty_ldisc_unlock(tty);
    > > return retval;
    >
    > So what about:
    > tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
    > if (!tty->ldisc)
    > ret = tty_ldisc_reinit(tty, tty->termios.c_line);
    > tty_ldisc_unlock(tty);
    >
    > if (!ret)
    > tty->count++;
    >
    > return ret;
    >

    --
    Thanks,
    Dmitry

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