net: amd-xgbe: Fix skb data length underflow

There will be BUG_ON() triggered in include/linux/skbuff.h leading to
intermittent kernel panic, when the skb length underflow is detected.

Fix this by dropping the packet if such length underflows are seen
because of inconsistencies in the hardware descriptors.

Fixes: 622c36f143fc ("amd-xgbe: Fix jumbo MTU processing on newer hardware")
Suggested-by: Tom Lendacky <[e-mail]>
Signed-off-by: Shyam Sundar S K <[e-mail]>
Acked-by: Tom Lendacky <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/amd/xgbe/xgbe-drv.c

···
 			buf2_len = xgbe_rx_buf2_len(rdata, packet, len);
 			len += buf2_len;
 
+			if (buf2_len > rdata->rx.buf.dma_len) {
+				/* Hardware inconsistency within the descriptors
+				 * that has resulted in a length underflow.
+				 */
+				error = 1;
+				goto skip_data;
+			}
+
 			if (!skb) {
 				skb = xgbe_create_skb(pdata, napi, rdata,
 						      buf1_len);
···
 		if (!last || context_next)
 			goto read_again;
 
-		if (!skb)
+		if (!skb || error) {
+			dev_kfree_skb(skb);
 			goto next_packet;
+		}
 
 		/* Be sure we don't exceed the configured MTU */
 		max_len = netdev->mtu + ETH_HLEN;