USB: cdc-acm: fix potential null-pointer dereference on disconnect

drivers/usb/class/cdc-acm.c

 /*
  * cdc-acm.c
  *
  * Copyright (c) 1999 Armin Fuerst      <fuerst@in.tum.de>
  * Copyright (c) 1999 Pavel Machek      <pavel@suse.cz>
  * Copyright (c) 1999 Johannes Erdfelt  <johannes@erdfelt.com>
  * Copyright (c) 2000 Vojtech Pavlik    <vojtech@suse.cz>
  * Copyright (c) 2004 Oliver Neukum     <oliver@neukum.name>
  * Copyright (c) 2005 David Kubicek     <dave@awk.cz>
  *
@@ skipping 279 matching lines @@
                 dbg("%s - urb shutting down with status: %d", __func__, status);
                 return;
         default:
                 dbg("%s - nonzero urb status received: %d", __func__, status);
                 goto exit;
         }
 
         if (!ACM_READY(acm))
                 goto exit;
 
+        usb_mark_last_busy(acm->dev);
+
         data = (unsigned char *)(dr + 1);
         switch (dr->bNotificationType) {
         case USB_CDC_NOTIFY_NETWORK_CONNECTION:
                 dbg("%s network", dr->wValue ?
                                         "connected to" : "disconnected from");
                 break;
 
         case USB_CDC_NOTIFY_SERIAL_STATE:
                 tty = tty_port_tty_get(&acm->port);
                 newctrl = get_unaligned_le16(data);
@@ skipping 19 matching lines @@
                         acm->ctrlin & ACM_CTRL_OVERRUN ? '+' : '-');
                         break;
 
         default:
                 dbg("unknown notification %d received: index %d len %d data0 %d data1 %d",
                         dr->bNotificationType, dr->wIndex,
                         dr->wLength, data[0], data[1]);
                 break;
         }
 exit:
-        usb_mark_last_busy(acm->dev);
         retval = usb_submit_urb(urb, GFP_ATOMIC);
         if (retval)
                 dev_err(&urb->dev->dev, "%s - usb_submit_urb failed with "
                         "result %d", __func__, retval);
 }
 
 /* data interface returns incoming bytes, or we got unthrottled */
 static void acm_read_bulk(struct urb *urb)
 {
         struct acm_rb *buf;
@@ skipping 1360 matching lines @@
         put_tty_driver(acm_tty_driver);
 }
 
 module_init(acm_init);
 module_exit(acm_exit);
 
 MODULE_AUTHOR(DRIVER_AUTHOR);
 MODULE_DESCRIPTION(DRIVER_DESC);
 MODULE_LICENSE("GPL");
 MODULE_ALIAS_CHARDEV_MAJOR(ACM_TTY_MAJOR);