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

Currently ncm->notify_req is used to flag when a request is in-flight.
ncm->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 NCM driver will
unconditionally free ncm->notify_req leading to a NULL pointer dereference.

Fixes: 40d133d7 ("usb: gadget: f_ncm: convert to new function interface with backward compatibility")
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_ncm.c

@@ -53,6 +53,7 @@ struct f_ncm {
 	struct usb_ep			*notify;
 	struct usb_request		*notify_req;
 	u8				notify_state;
+	atomic_t			notify_count;
 	bool				is_open;
 
 	const struct ndp_parser_opts	*parser_opts;
@@ -547,7 +548,7 @@ static void ncm_do_notify(struct f_ncm *ncm)
 	int				status;
 
 	/* notification already in flight? */
-	if (!req)
+	if (atomic_read(&ncm->notify_count))
 		return;
 
 	event = req->buf;
@@ -587,7 +588,8 @@ static void ncm_do_notify(struct f_ncm *ncm)
 	event->bmRequestType = 0xA1;
 	event->wIndex = cpu_to_le16(ncm->ctrl_id);
 
-	ncm->notify_req = NULL;
+	atomic_inc(&ncm->notify_count);
+
 	/*
 	 * In double buffering if there is a space in FIFO,
 	 * completion callback can be called right after the call,
@@ -597,7 +599,7 @@ static void ncm_do_notify(struct f_ncm *ncm)
 	status = usb_ep_queue(ncm->notify, req, GFP_ATOMIC);
 	spin_lock(&ncm->lock);
 	if (status < 0) {
-		ncm->notify_req = req;
+		atomic_dec(&ncm->notify_count);
 		DBG(cdev, "notify --> %d\n", status);
 	}
 }
@@ -632,17 +634,19 @@ static void ncm_notify_complete(struct usb_ep *ep, struct usb_request *req)
 	case 0:
 		VDBG(cdev, "Notification %02x sent\n",
 		     event->bNotificationType);
+		atomic_dec(&ncm->notify_count);
 		break;
 	case -ECONNRESET:
 	case -ESHUTDOWN:
+		atomic_set(&ncm->notify_count, 0);
 		ncm->notify_state = NCM_NOTIFY_NONE;
 		break;
 	default:
 		DBG(cdev, "event %02x --> %d\n",
 			event->bNotificationType, req->status);
+		atomic_dec(&ncm->notify_count);
 		break;
 	}
-	ncm->notify_req = req;
 	ncm_do_notify(ncm);
 	spin_unlock(&ncm->lock);
 }
@@ -1649,6 +1653,11 @@ static void ncm_unbind(struct usb_configuration *c, struct usb_function *f)
 	ncm_string_defs[0].id = 0;
 	usb_free_all_descriptors(f);
 
+	if (atomic_read(&ncm->notify_count)) {
+		usb_ep_dequeue(ncm->notify, ncm->notify_req);
+		atomic_set(&ncm->notify_count, 0);
+	}
+
 	kfree(ncm->notify_req->buf);
 	usb_ep_free_request(ncm->notify, ncm->notify_req);
 }