serdev: ttyport: fix NULL-deref on hangup

Make sure to use a properly refcounted tty_struct in write_wake up to
avoid dereferencing a NULL-pointer when a port is being hung up.

Fixes: bed35c6dfa6a ("serdev: add a tty port controller driver")
Cc: stable <stable@vger.kernel.org>     # 4.11
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 9 insertions, 2 deletions

diff --git a/drivers/tty/serdev/serdev-ttyport.c b/drivers/tty/serdev/serdev-ttyport.c
index 09fbdd52a561..12cb9139073e 100644
--- a/drivers/tty/serdev/serdev-ttyport.c
+++ b/drivers/tty/serdev/serdev-ttyport.c
@@ -49,12 +49,19 @@ static void ttyport_write_wakeup(struct tty_port *port)
 {
         struct serdev_controller *ctrl = port->client_data;
         struct serport *serport = serdev_controller_get_drvdata(ctrl);
+        struct tty_struct *tty;
+
+        tty = tty_port_tty_get(port);
+        if (!tty)
+                return;
 
-        if (test_and_clear_bit(TTY_DO_WRITE_WAKEUP, &port->tty->flags) &&
+        if (test_and_clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags) &&
             test_bit(SERPORT_ACTIVE, &serport->flags))
                 serdev_controller_write_wakeup(ctrl);
 
-        wake_up_interruptible_poll(&port->tty->write_wait, POLLOUT);
+        wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
+
+        tty_kref_put(tty);
 }
 
 static const struct tty_port_client_operations client_ops = {