tty: pty: Fix race condition between release_one_tty and pty_write

Especially when a linked tty is used such as pty, the linked tty
port's buf works have not been cancelled while master tty port's
buf work has been cancelled. Since release_one_tty and flush_to_ldisc
run in workqueue threads separately, when pty_cleanup happens and
link tty port is freed, flush_to_ldisc tries to access freed port
and port->itty, eventually it causes a panic.
This patch utilizes the magic value with holding the tty_mutex to
check if the tty->link is valid.

Fixes: 2b022ab7542d ("pty: cancel pty slave port buf's work in tty_release")
Signed-off-by: Sahara <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/pty.c

···
 	if (tty->stopped)
 		return 0;
 
+	mutex_lock(&tty_mutex);
+	if (to->magic != TTY_MAGIC) {
+		mutex_unlock(&tty_mutex);
+		return -EIO;
+	}
+
 	if (c > 0) {
 		spin_lock_irqsave(&to->port->lock, flags);
 		/* Stuff the data into the input queue of the other end */
···
 			tty_flip_buffer_push(to->port);
 		spin_unlock_irqrestore(&to->port->lock, flags);
 	}
+	mutex_unlock(&tty_mutex);
 	return c;
 }
 

drivers/tty/tty_io.c

···
 	struct tty_driver *driver = tty->driver;
 	struct module *owner = driver->owner;
 
+	mutex_lock(&tty_mutex);
 	if (tty->ops->cleanup)
 		tty->ops->cleanup(tty);
 
 	tty->magic = 0;
+	mutex_unlock(&tty_mutex);
+
 	tty_driver_kref_put(driver);
 	module_put(owner);