[PATCH] USB: serial: upd78f0730: fix NULL-deref on hangup

[Johan Hovold <johan@kernel.org>]

The port tty pointer is cleared on hangup before deasserting the modem
control signals and should not be used in the dtr_rts() callback.

Factor out a new upd78f0730_port_tiocmset() helper that can be used in
upd78f0730_dtr_rts() to avoid dereferencing a NULL-pointer on hangup
(e.g. on disconnect).

This issue was flagged by an LLM.

Fixes: ea534e0b4047 ("USB: serial: add uPD78F0730 USB to Serial Adaptor Driver")
Cc: stable@vger.kernel.org	# 4.11
Cc: Maksim Salau <maksim.salau@gmail.com>
Assisted-by: LLM
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/serial/upd78f0730.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/serial/upd78f0730.c b/drivers/usb/serial/upd78f0730.c
index 78c4d3c19a8e..f2189b994ed7 100644
--- a/drivers/usb/serial/upd78f0730.c
+++ b/drivers/usb/serial/upd78f0730.c
@@ -201,10 +201,9 @@ static int upd78f0730_tiocmget(struct tty_struct *tty)
 	return res;
 }
 
-static int upd78f0730_tiocmset(struct tty_struct *tty,
+static int upd78f0730_port_tiocmset(struct usb_serial_port *port,
 			unsigned int set, unsigned int clear)
 {
-	struct usb_serial_port *port = tty->driver_data;
 	struct upd78f0730_port_private *private;
 	struct upd78f0730_set_dtr_rts request;
 	struct device *dev = &port->dev;
@@ -238,6 +237,13 @@ static int upd78f0730_tiocmset(struct tty_struct *tty,
 	return res;
 }
 
+static int upd78f0730_tiocmset(struct tty_struct *tty, unsigned int set, unsigned int clear)
+{
+	struct usb_serial_port *port = tty->driver_data;
+
+	return upd78f0730_port_tiocmset(port, set, clear);
+}
+
 static int upd78f0730_break_ctl(struct tty_struct *tty, int break_state)
 {
 	struct upd78f0730_port_private *private;
@@ -267,7 +273,6 @@ static int upd78f0730_break_ctl(struct tty_struct *tty, int break_state)
 
 static void upd78f0730_dtr_rts(struct usb_serial_port *port, int on)
 {
-	struct tty_struct *tty = port->port.tty;
 	unsigned int set = 0;
 	unsigned int clear = 0;
 
@@ -276,7 +281,7 @@ static void upd78f0730_dtr_rts(struct usb_serial_port *port, int on)
 	else
 		clear = TIOCM_DTR | TIOCM_RTS;
 
-	upd78f0730_tiocmset(tty, set, clear);
+	upd78f0730_port_tiocmset(port, set, clear);
 }
 
 static speed_t upd78f0730_get_baud_rate(struct tty_struct *tty)
-- 
2.55.0