[PATCH] usb: gadget: u_serial: Add null pointer check in gs_start_io

[Kuen-Han Tsai <khtsai@google.com>]

If gs_close has cleared port->port.tty and gs_start_io is called
afterwards, then the function tty_wakeup will attempt to access the value
of the pointer port->port.tty which will cause a null pointer
dereference error.

To avoid this, add a null pointer check to gs_start_io before attempting
to access the value of the pointer port->port.tty.

Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
---
 drivers/usb/gadget/function/u_serial.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
index 97f07757d19e..1115396b46a0 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -539,16 +539,20 @@ static int gs_alloc_requests(struct usb_ep *ep, struct list_head *head,
 static int gs_start_io(struct gs_port *port)
 {
 	struct list_head	*head = &port->read_pool;
-	struct usb_ep		*ep = port->port_usb->out;
+	struct usb_ep		*ep;
 	int			status;
 	unsigned		started;
 
+	if (!port->port_usb || !port->port.tty)
+		return -EIO;
+
 	/* Allocate RX and TX I/O buffers.  We can't easily do this much
 	 * earlier (with GFP_KERNEL) because the requests are coupled to
 	 * endpoints, as are the packet sizes we'll be using.  Different
 	 * configurations may use different endpoints with a given port;
 	 * and high speed vs full speed changes packet sizes too.
 	 */
+	ep = port->port_usb->out;
 	status = gs_alloc_requests(ep, head, gs_read_complete,
 		&port->read_allocated);
 	if (status)
-- 
2.41.0.rc0.172.g3f132b7071-goog