net: ethernet: mediatek: avoid potential invalid memory access

Potential dangerous invalid memory might be accessed if invalid mac value
reflected from the forward port field in rxd4 caused by possible potential
hardware defects. So added a simple sanity checker to avoid the kind of
situation happening.
Signed-off-by: NSean Wang <sean.wang@mediatek.com>
Acked-by: NJohn Crispin <john@phrozen.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/mediatek/mtk_eth_soc.c

@@ -947,6 +947,10 @@ static int mtk_poll_rx(struct napi_struct *napi, int budget,
 		      RX_DMA_FPORT_MASK;
 		mac--;
 
+		if (unlikely(mac < 0 || mac >= MTK_MAC_COUNT ||
+			     !eth->netdev[mac]))
+			goto release_desc;
+
 		netdev = eth->netdev[mac];
 
 		if (unlikely(test_bit(MTK_RESETTING, &eth->state)))