[PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()

[Dmitry Safonov <dima@arista.com>]

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);
+	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;
 }
 
-- 
2.13.6