digitv: limit messages to buffer size

commit 821117dc upstream.

Return an error rather than memcpy()ing beyond the end of the buffer.
Internal callers use appropriate sizes, but digitv_i2c_xfer may not.
Signed-off-by: Alyssa Milburn <amilburn@zall.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/media/usb/dvb-usb/digitv.c

@@ -30,6 +30,10 @@ static int digitv_ctrl_msg(struct dvb_usb_device *d,
 {
 	int wo = (rbuf == NULL || rlen == 0); /* write-only */
 	u8 sndbuf[7],rcvbuf[7];
+
+	if (wlen > 4 || rlen > 4)
+		return -EIO;
+
 	memset(sndbuf,0,7); memset(rcvbuf,0,7);
 
 	sndbuf[0] = cmd;