[media] uvcvideo: Fix open/close race condition

Maintaining the users count using an atomic variable makes sure that
access to the counter won't be racy, but doesn't serialize access to the
operations protected by the counter. This creates a race condition that
could result in the status URB being submitted multiple times.
Use a mutex to protect the users count and serialize access to the
status start and stop operations.

Reported-by: Shawn Nematbakhsh <[e-mail]>
Signed-off-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/usb/uvc/uvc_driver.c

···
 	INIT_LIST_HEAD(&dev->chains);
 	INIT_LIST_HEAD(&dev->streams);
 	atomic_set(&dev->nstreams, 0);
-	atomic_set(&dev->users, 0);
 	atomic_set(&dev->nmappings, 0);
+	mutex_init(&dev->lock);
 
 	dev->udev = usb_get_dev(udev);
 	dev->intf = usb_get_intf(intf);
···
 
 	/* Controls are cached on the fly so they don't need to be saved. */
 	if (intf->cur_altsetting->desc.bInterfaceSubClass ==
-	    UVC_SC_VIDEOCONTROL)
-		return uvc_status_suspend(dev);
+	    UVC_SC_VIDEOCONTROL) {
+		mutex_lock(&dev->lock);
+		if (dev->users)
+			uvc_status_stop(dev);
+		mutex_unlock(&dev->lock);
+		return 0;
+	}
 
 	list_for_each_entry(stream, &dev->streams, list) {
 		if (stream->intf == intf)
···
 
 	if (intf->cur_altsetting->desc.bInterfaceSubClass ==
 	    UVC_SC_VIDEOCONTROL) {
-		if (reset) {
-			int ret = uvc_ctrl_resume_device(dev);
+		int ret = 0;
 
+		if (reset) {
+			ret = uvc_ctrl_resume_device(dev);
 			if (ret < 0)
 				return ret;
 		}
 
-		return uvc_status_resume(dev);
+		mutex_lock(&dev->lock);
+		if (dev->users)
+			ret = uvc_status_start(dev, GFP_NOIO);
+		mutex_unlock(&dev->lock);
+
+		return ret;
 	}
 
 	list_for_each_entry(stream, &dev->streams, list) {

drivers/media/usb/uvc/uvc_status.c

···
 	uvc_input_cleanup(dev);
 }
 
-int uvc_status_start(struct uvc_device *dev)
+int uvc_status_start(struct uvc_device *dev, gfp_t flags)
 {
 	if (dev->int_urb == NULL)
 		return 0;
 
-	return usb_submit_urb(dev->int_urb, GFP_KERNEL);
+	return usb_submit_urb(dev->int_urb, flags);
 }
 
 void uvc_status_stop(struct uvc_device *dev)
 {
 	usb_kill_urb(dev->int_urb);
 }
-
-int uvc_status_suspend(struct uvc_device *dev)
-{
-	if (atomic_read(&dev->users))
-		usb_kill_urb(dev->int_urb);
-
-	return 0;
-}
-
-int uvc_status_resume(struct uvc_device *dev)
-{
-	if (dev->int_urb == NULL || atomic_read(&dev->users) == 0)
-		return 0;
-
-	return usb_submit_urb(dev->int_urb, GFP_NOIO);
-}
-

drivers/media/usb/uvc/uvc_v4l2.c

···
 		return -ENOMEM;
 	}
 
-	if (atomic_inc_return(&stream->dev->users) == 1) {
-		ret = uvc_status_start(stream->dev);
+	mutex_lock(&stream->dev->lock);
+	if (stream->dev->users == 0) {
+		ret = uvc_status_start(stream->dev, GFP_KERNEL);
 		if (ret < 0) {
-			atomic_dec(&stream->dev->users);
+			mutex_unlock(&stream->dev->lock);
 			usb_autopm_put_interface(stream->dev->intf);
 			kfree(handle);
 			return ret;
 		}
 	}
+
+	stream->dev->users++;
+	mutex_unlock(&stream->dev->lock);
 
 	v4l2_fh_init(&handle->vfh, stream->vdev);
 	v4l2_fh_add(&handle->vfh);
···
 	kfree(handle);
 	file->private_data = NULL;
 
-	if (atomic_dec_return(&stream->dev->users) == 0)
+	mutex_lock(&stream->dev->lock);
+	if (--stream->dev->users == 0)
 		uvc_status_stop(stream->dev);
+	mutex_unlock(&stream->dev->lock);
 
 	usb_autopm_put_interface(stream->dev->intf);
 	return 0;

drivers/media/usb/uvc/uvcvideo.h

···
 	char name[32];
 
 	enum uvc_device_state state;
-	atomic_t users;
+	struct mutex lock;		/* Protects users */
+	unsigned int users;
 	atomic_t nmappings;
 
 	/* Video control interface */
···
 /* Status */
 extern int uvc_status_init(struct uvc_device *dev);
 extern void uvc_status_cleanup(struct uvc_device *dev);
-extern int uvc_status_start(struct uvc_device *dev);
+extern int uvc_status_start(struct uvc_device *dev, gfp_t flags);
 extern void uvc_status_stop(struct uvc_device *dev);
-extern int uvc_status_suspend(struct uvc_device *dev);
-extern int uvc_status_resume(struct uvc_device *dev);
 
 /* Controls */
 extern const struct v4l2_subscribed_event_ops uvc_ctrl_sub_ev_ops;