[BACKPORT 4.4.y 16/25] usb: gadget: Add the gserial port checking in gs_start_tx()

[Arnd Bergmann <arnd@arndb.de>]

From: Baolin Wang <baolin.wang@linaro.org>

When usb gadget is set gadget serial function, it will be crash in below
situation.

It will clean the 'port->port_usb' pointer in gserial_disconnect() function
when usb link is inactive, but it will release lock for disabling the endpoints
in this function. Druing the lock release period, it maybe complete one request
to issue gs_write_complete()--->gs_start_tx() function, but the 'port->port_usb'
pointer had been set NULL, thus it will be crash in gs_start_tx() function.

This patch adds the 'port->port_usb' pointer checking in gs_start_tx() function
to avoid this situation.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
(cherry picked from commit 511a36d2f357724312bb3776d2f6eed3890928b2)
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/usb/gadget/function/u_serial.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
index 4ea44f7122ee..d73618475664 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -361,10 +361,15 @@ __acquires(&port->port_lock)
 */
 {
 	struct list_head	*pool = &port->write_pool;
-	struct usb_ep		*in = port->port_usb->in;
+	struct usb_ep		*in;
 	int			status = 0;
 	bool			do_tty_wake = false;
 
+	if (!port->port_usb)
+		return status;
+
+	in = port->port_usb->in;
+
 	while (!port->write_busy && !list_empty(pool)) {
 		struct usb_request	*req;
 		int			len;
-- 
2.20.0