[3.16.y-ckt stable] Patch "n_tty: Fix unsafe reference to "other" ldisc" has been added to the 3.16.y-ckt tree

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    n_tty: Fix unsafe reference to "other" ldisc

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt25.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

---8<------------------------------------------------------------

>From 95ff4bf5f7517ec99a7b8e8a94e4a7e3a0703682 Mon Sep 17 00:00:00 2001
From: Peter Hurley <peter at hurleysoftware.com>
Date: Sun, 10 Jan 2016 22:40:56 -0800
Subject: n_tty: Fix unsafe reference to "other" ldisc

commit 6d27a63caad3f13e96cf065d2d96828c2006be6b upstream.

Although n_tty_check_unthrottle() has a valid ldisc reference (since
the tty core gets the ldisc ref in tty_read() before calling the line
discipline read() method), it does not have a valid ldisc reference to
the "other" pty of a pty pair. Since getting an ldisc reference for
tty->link essentially open-codes tty_wakeup(), just replace with the
equivalent tty_wakeup().

Signed-off-by: Peter Hurley <peter at hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/tty/n_tty.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index d239fdba29d6..cdf72bcb3d78 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -277,16 +277,13 @@ static void n_tty_check_throttle(struct tty_struct *tty)

 static void n_tty_check_unthrottle(struct tty_struct *tty)
 {
-	if (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
-	    tty->link->ldisc->ops->write_wakeup == n_tty_write_wakeup) {
+	if (tty->driver->type == TTY_DRIVER_TYPE_PTY) {
 		if (chars_in_buffer(tty) > TTY_THRESHOLD_UNTHROTTLE)
 			return;
 		if (!tty->count)
 			return;
 		n_tty_set_room(tty);
-		n_tty_write_wakeup(tty->link);
-		if (waitqueue_active(&tty->link->write_wait))
-			wake_up_interruptible_poll(&tty->link->write_wait, POLLOUT);
+		tty_wakeup(tty->link);
 		return;
 	}