net: fix generic XDP to handle if eth header was mangled

XDP can modify (and resize) the Ethernet header in the packet.

There is a bug in generic-XDP, because skb->protocol and skb->pkt_type
are setup before reaching (netif_receive_)generic_xdp.

This bug was hit when XDP were popping VLAN headers (changing
eth->h_proto), as skb->protocol still contains VLAN-indication
(ETH_P_8021Q) causing invocation of skb_vlan_untag(skb), which corrupt
the packet (basically popping the VLAN again).

This patch catch if XDP changed eth header in such a way, that SKB
fields needs to be updated.

V2: on request from Song Liu, use ETH_HLEN instead of mac_len,
in __skb_push() as eth_type_trans() use ETH_HLEN in paired skb_pull_inline().

Fixes: d445516966 ("net: xdp: support xdp generic on virtual devices")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/core/dev.c

@@ -4258,6 +4258,9 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	struct netdev_rx_queue *rxqueue;
 	void *orig_data, *orig_data_end;
 	u32 metalen, act = XDP_DROP;
+	__be16 orig_eth_type;
+	struct ethhdr *eth;
+	bool orig_bcast;
 	int hlen, off;
 	u32 mac_len;
 
@@ -4298,6 +4301,9 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	xdp->data_hard_start = skb->data - skb_headroom(skb);
 	orig_data_end = xdp->data_end;
 	orig_data = xdp->data;
+	eth = (struct ethhdr *)xdp->data;
+	orig_bcast = is_multicast_ether_addr_64bits(eth->h_dest);
+	orig_eth_type = eth->h_proto;
 
 	rxqueue = netif_get_rxqueue(skb);
 	xdp->rxq = &rxqueue->xdp_rxq;
@@ -4321,6 +4327,14 @@ 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_bcast != is_multicast_ether_addr_64bits(eth->h_dest))) {
+		__skb_push(skb, ETH_HLEN);
+		skb->protocol = eth_type_trans(skb, skb->dev);
+	}
+
 	switch (act) {
 	case XDP_REDIRECT:
 	case XDP_TX: