[PATCHv5 3/7] tty: Hold tty_ldisc_lock() during tty_reopen()

From: Dmitry Safonov
Date: Mon Sep 17 2018 - 19:53:35 EST

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")
Cc: stable@xxxxxxxxxxxxxxx

Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Jiri Slaby <jslaby@xxxxxxxx>
Reviewed-by: Jiri Slaby <jslaby@xxxxxxx>
Reported-by: syzbot+3aa9784721dfb90e984d@xxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: Mark Rutland <mark.rutland@xxxxxxx>
Tested-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
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_ldisc_unlock(tty);
return retval;