net, xdp: Update pkt_type if generic XDP changes unicast MAC

If a generic XDP program changes the destination MAC address from/to
multicast/broadcast, the skb->pkt_type is updated to properly handle
the packet when passed up the stack. When changing the MAC from/to
the NICs MAC, PACKET_HOST/OTHERHOST is not updated, though, making
the behavior different from that of native XDP.

Remember the PACKET_HOST/OTHERHOST state before calling the program
in generic XDP, and update pkt_type accordingly if the destination
MAC address has changed. As eth_type_trans() assumes a default
pkt_type of PACKET_HOST, restore that before calling it.

The use case for this is when a XDP program wants to push received
packets up the stack by rewriting the MAC to the NICs MAC, for
example by cluster nodes sharing MAC addresses.

Fixes: 29724956 ("net: fix generic XDP to handle if eth header was mangled")
Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210419141559.8611-1-martin@strongswan.org

net/core/dev.c

@@ -4723,10 +4723,10 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	void *orig_data, *orig_data_end, *hard_start;
 	struct netdev_rx_queue *rxqueue;
 	u32 metalen, act = XDP_DROP;
+	bool orig_bcast, orig_host;
 	u32 mac_len, frame_sz;
 	__be16 orig_eth_type;
 	struct ethhdr *eth;
-	bool orig_bcast;
 	int off;
 
 	/* Reinjected packets coming from act_mirred or similar should
@@ -4773,6 +4773,7 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	orig_data_end = xdp->data_end;
 	orig_data = xdp->data;
 	eth = (struct ethhdr *)xdp->data;
+	orig_host = ether_addr_equal_64bits(eth->h_dest, skb->dev->dev_addr);
 	orig_bcast = is_multicast_ether_addr_64bits(eth->h_dest);
 	orig_eth_type = eth->h_proto;
 
@@ -4800,8 +4801,11 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	/* check if XDP changed eth hdr such SKB needs update */
 	eth = (struct ethhdr *)xdp->data;
 	if ((orig_eth_type != eth->h_proto) ||
+	    (orig_host != ether_addr_equal_64bits(eth->h_dest,
+						  skb->dev->dev_addr)) ||
 	    (orig_bcast != is_multicast_ether_addr_64bits(eth->h_dest))) {
 		__skb_push(skb, ETH_HLEN);
+		skb->pkt_type = PACKET_HOST;
 		skb->protocol = eth_type_trans(skb, skb->dev);
 	}