usb: rndis_host: Secure rndis_query check against int overflow

Variables off and len typed as uint32 in rndis_query function
are controlled by incoming RNDIS response message thus their
value may be manipulated. Setting off to a unexpectetly large
value will cause the sum with len and 8 to overflow and pass
the implemented validation step. Consequently the response
pointer will be referring to a location past the expected
buffer boundaries allowing information leakage e.g. via
RNDIS_OID_802_3_PERMANENT_ADDRESS OID.

Fixes: ddda0862 ("USB: rndis_host, various cleanups")
Signed-off-by: NSzymon Heidrich <szymon.heidrich@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/usb/rndis_host.c

@@ -255,7 +255,8 @@ static int rndis_query(struct usbnet *dev, struct usb_interface *intf,
 
 	off = le32_to_cpu(u.get_c->offset);
 	len = le32_to_cpu(u.get_c->len);
-	if (unlikely((8 + off + len) > CONTROL_BUFFER_SIZE))
+	if (unlikely((off > CONTROL_BUFFER_SIZE - 8) ||
+		     (len > CONTROL_BUFFER_SIZE - 8 - off)))
 		goto response_error;
 
 	if (*reply_len != -1 && len != *reply_len)