[PATCH net v3] net: xdp: account for layer 3 packets in generic skb handler

["Jason A. Donenfeld" <Jason@zx2c4.com>]

A user reported that packets from wireguard were possibly ignored by XDP
[1]. Apparently, the generic skb xdp handler path seems to assume that
packets will always have an ethernet header, which really isn't always
the case for layer 3 packets, which are produced by multiple drivers.
This patch fixes the oversight. If the mac_len is 0, then we assume
that it's a layer 3 packet, and in that case prepend a pseudo ethhdr to
the packet whose h_proto is copied from skb->protocol, which will have
the appropriate v4 or v6 ethertype. This allows us to keep XDP programs'
assumption correct about packets always having that ethernet header, so
that existing code doesn't break, while still allowing layer 3 devices
to use the generic XDP handler.

[1] https://lore.kernel.org/wireguard/M5WzVK5--3-2@tuta.io/

Reported-by: Adhipati Blambangan <adhipati@tuta.io>
Cc: David Ahern <dsahern@gmail.com>
Cc: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 net/core/dev.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 77c154107b0d..3bc9a96bc808 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4510,9 +4510,9 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	u32 metalen, act = XDP_DROP;
 	__be16 orig_eth_type;
 	struct ethhdr *eth;
+	u32 mac_len = ~0;
 	bool orig_bcast;
 	int hlen, off;
-	u32 mac_len;
 
 	/* Reinjected packets coming from act_mirred or similar should
 	 * not get XDP generic processing.
@@ -4544,6 +4544,12 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	 * header.
 	 */
 	mac_len = skb->data - skb_mac_header(skb);
+	if (!mac_len) {
+		eth = skb_push(skb, sizeof(struct ethhdr));
+		eth_zero_addr(eth->h_source);
+		eth_zero_addr(eth->h_dest);
+		eth->h_proto = skb->protocol;
+	}
 	hlen = skb_headlen(skb) + mac_len;
 	xdp->data = skb->data - mac_len;
 	xdp->data_meta = xdp->data;
@@ -4611,6 +4617,8 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 		kfree_skb(skb);
 		break;
 	}
+	if (!mac_len)
+		skb_pull(skb, sizeof(struct ethhdr));
 
 	return act;
 }
-- 
2.26.2