net: handle 802.1P vlan 0 packets properly

When stack receives pkt: [802.1P vlan 0][802.1AD vlan 100][IPv4],
vlan_do_receive() returns false if it does not find vlan_dev. Later
__netif_receive_skb_core() fails to find packet type handler for
skb->protocol 801.1AD and drops the packet.

801.1P header with vlan id 0 should be handled as untagged packets.
This patch fixes it by checking if vlan_id is 0 and processes next vlan
header.
Signed-off-by: Govindarajulu Varadarajan <gvaradar@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -4923,8 +4923,36 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc,
 	}
 
 	if (unlikely(skb_vlan_tag_present(skb))) {
-		if (skb_vlan_tag_get_id(skb))
+check_vlan_id:
+		if (skb_vlan_tag_get_id(skb)) {
+			/* Vlan id is non 0 and vlan_do_receive() above couldn't
+			 * find vlan device.
+			 */
 			skb->pkt_type = PACKET_OTHERHOST;
+		} else if (skb->protocol == cpu_to_be16(ETH_P_8021Q) ||
+			   skb->protocol == cpu_to_be16(ETH_P_8021AD)) {
+			/* Outer header is 802.1P with vlan 0, inner header is
+			 * 802.1Q or 802.1AD and vlan_do_receive() above could
+			 * not find vlan dev for vlan id 0.
+			 */
+			__vlan_hwaccel_clear_tag(skb);
+			skb = skb_vlan_untag(skb);
+			if (unlikely(!skb))
+				goto out;
+			if (vlan_do_receive(&skb))
+				/* After stripping off 802.1P header with vlan 0
+				 * vlan dev is found for inner header.
+				 */
+				goto another_round;
+			else if (unlikely(!skb))
+				goto out;
+			else
+				/* We have stripped outer 802.1P vlan 0 header.
+				 * But could not find vlan dev.
+				 * check again for vlan id to set OTHERHOST.
+				 */
+				goto check_vlan_id;
+		}
 		/* Note: we might in the future use prio bits
 		 * and set skb->priority like in vlan_do_receive()
 		 * For the time being, just ignore Priority Code Point