net: usb: lan78xx: Limit packet length to skb->len

Packet length retrieved from descriptor may be larger than
the actual socket buffer length. In such case the cloned
skb passed up the network stack will leak kernel memory contents.

Additionally prevent integer underflow when size is less than
ETH_FCS_LEN.

Fixes: 55d7de9d ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet device driver")
Signed-off-by: Szymon Heidrich <szymon.heidrich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/lan78xx.c

@@ -3579,13 +3579,29 @@ static int lan78xx_rx(struct lan78xx_net *dev, struct sk_buff *skb,
 		size = (rx_cmd_a & RX_CMD_A_LEN_MASK_);
 		align_count = (4 - ((size + RXW_PADDING) % 4)) % 4;
 
+		if (unlikely(size > skb->len)) {
+			netif_dbg(dev, rx_err, dev->net,
+				  "size err rx_cmd_a=0x%08x\n",
+				  rx_cmd_a);
+			return 0;
+		}
+
 		if (unlikely(rx_cmd_a & RX_CMD_A_RED_)) {
 			netif_dbg(dev, rx_err, dev->net,
 				  "Error rx_cmd_a=0x%08x", rx_cmd_a);
 		} else {
-			u32 frame_len = size - ETH_FCS_LEN;
+			u32 frame_len;
 			struct sk_buff *skb2;
 
+			if (unlikely(size < ETH_FCS_LEN)) {
+				netif_dbg(dev, rx_err, dev->net,
+					  "size err rx_cmd_a=0x%08x\n",
+					  rx_cmd_a);
+				return 0;
+			}
+
+			frame_len = size - ETH_FCS_LEN;
+
 			skb2 = napi_alloc_skb(&dev->napi, frame_len);
 			if (!skb2)
 				return 0;