usb: gadget: f_ecm: Use atomic_t to track in-flight request

Currently ecm->notify_req is used to flag when a request is in-flight.
ecm->notify_req is set to NULL and when a request completes it is
subsequently reset.

This is fundamentally buggy in that the unbind logic of the ECM driver will
unconditionally free ecm->notify_req leading to a NULL pointer dereference.

Fixes: da741b8c56 ("usb ethernet gadget: split CDC Ethernet function")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_ecm.c

@@ -52,6 +52,7 @@ struct f_ecm {
 	struct usb_ep			*notify;
 	struct usb_request		*notify_req;
 	u8				notify_state;
+	atomic_t			notify_count;
 	bool				is_open;
 
 	/* FIXME is_open needs some irq-ish locking
@@ -380,7 +381,7 @@ static void ecm_do_notify(struct f_ecm *ecm)
 	int				status;
 
 	/* notification already in flight? */
-	if (!req)
+	if (atomic_read(&ecm->notify_count))
 		return;
 
 	event = req->buf;
@@ -420,10 +421,10 @@ static void ecm_do_notify(struct f_ecm *ecm)
 	event->bmRequestType = 0xA1;
 	event->wIndex = cpu_to_le16(ecm->ctrl_id);
 
-	ecm->notify_req = NULL;
+	atomic_inc(&ecm->notify_count);
 	status = usb_ep_queue(ecm->notify, req, GFP_ATOMIC);
 	if (status < 0) {
-		ecm->notify_req = req;
+		atomic_dec(&ecm->notify_count);
 		DBG(cdev, "notify --> %d\n", status);
 	}
 }
@@ -448,17 +449,19 @@ static void ecm_notify_complete(struct usb_ep *ep, struct usb_request *req)
 	switch (req->status) {
 	case 0:
 		/* no fault */
+		atomic_dec(&ecm->notify_count);
 		break;
 	case -ECONNRESET:
 	case -ESHUTDOWN:
+		atomic_set(&ecm->notify_count, 0);
 		ecm->notify_state = ECM_NOTIFY_NONE;
 		break;
 	default:
 		DBG(cdev, "event %02x --> %d\n",
 			event->bNotificationType, req->status);
+		atomic_dec(&ecm->notify_count);
 		break;
 	}
-	ecm->notify_req = req;
 	ecm_do_notify(ecm);
 }
 
@@ -907,6 +910,11 @@ static void ecm_unbind(struct usb_configuration *c, struct usb_function *f)
 
 	usb_free_all_descriptors(f);
 
+	if (atomic_read(&ecm->notify_count)) {
+		usb_ep_dequeue(ecm->notify, ecm->notify_req);
+		atomic_set(&ecm->notify_count, 0);
+	}
+
 	kfree(ecm->notify_req->buf);
 	usb_ep_free_request(ecm->notify, ecm->notify_req);
 }