media: hdpvr: Add device num check and handling

Add hdpvr device num check and error handling

We need to increment the device count atomically before we checkout a
device to make sure that we do not reach the max count, otherwise we get
out-of-bounds errors as reported by syzbot.

Reported-and-tested-by: syzbot+aac8d0d7205f112045d2@syzkaller.appspotmail.com
Signed-off-by: Luke Nowakowski-Krijger <lnowakow@eng.ucsd.edu>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/usb/hdpvr/hdpvr-core.c

@@ -271,6 +271,7 @@ static int hdpvr_probe(struct usb_interface *interface,
 #endif
 	size_t buffer_size;
 	int i;
+	int dev_num;
 	int retval = -ENOMEM;
 
 	/* allocate memory for our device state and initialize it */
@@ -368,8 +369,17 @@ static int hdpvr_probe(struct usb_interface *interface,
 	}
 #endif
 
+	dev_num = atomic_inc_return(&dev_nr);
+	if (dev_num >= HDPVR_MAX) {
+		v4l2_err(&dev->v4l2_dev,
+			 "max device number reached, device register failed\n");
+		atomic_dec(&dev_nr);
+		retval = -ENODEV;
+		goto reg_fail;
+	}
+
 	retval = hdpvr_register_videodev(dev, &interface->dev,
-				    video_nr[atomic_inc_return(&dev_nr)]);
+				    video_nr[dev_num]);
 	if (retval < 0) {
 		v4l2_err(&dev->v4l2_dev, "registering videodev failed\n");
 		goto reg_fail;