usb: cdc-acm: Fix acm_tty_hangup() vs. acm_tty_close() race

[Not upstream as it was fixed differently for 3.3 with a much more
"intrusive" rework of the driver - gregkh]

There is a race condition involving acm_tty_hangup() and acm_tty_close()
where hangup() would attempt to access tty->driver_data without proper
locking and NULL checking after close() has potentially already set it
to NULL.  One possibility to (sporadically) trigger this behavior is to
perform a suspend/resume cycle with a running WWAN data connection.

This patch addresses the issue by introducing a NULL check for
tty->driver_data in acm_tty_hangup() protected by open_mutex and exiting
gracefully when hangup() is invoked on a device that has already been
closed.
Signed-off-by: Thilo-Alexander Ginkel <thilo@ginkel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/class/cdc-acm.c

@@ -554,10 +554,18 @@ static void acm_port_down(struct acm *acm)
 
 static void acm_tty_hangup(struct tty_struct *tty)
 {
-	struct acm *acm = tty->driver_data;
-	tty_port_hangup(&acm->port);
+	struct acm *acm;
+
 	mutex_lock(&open_mutex);
+	acm = tty->driver_data;
+
+	if (!acm)
+		goto out;
+
+	tty_port_hangup(&acm->port);
 	acm_port_down(acm);
+
+out:
 	mutex_unlock(&open_mutex);
 }