bpf: Set mac_len in bpf_skb_change_head

[ Upstream commit 84316ca4e100d8cbfccd9f774e23817cb2059868 ]

The skb_change_head() helper did not set "skb->mac_len", which is
problematic when it's used in combination with skb_redirect_peer().
Without it, redirecting a packet from a L3 device such as wireguard to
the veth peer device will cause skb->data to point to the middle of the
IP header on entry to tcp_v4_rcv() since the L2 header is not pulled
correctly due to mac_len=0.

Fixes: 3a0af8fd61 ("bpf: BPF for lightweight tunnel infrastructure")
Signed-off-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210519154743.2554771-2-joamaki@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
Change-Id: If3a6c34ef7e6e56641e18964fdf30d8250a028d9

net/core/filter.c

@@ -2438,6 +2438,7 @@ BPF_CALL_3(bpf_skb_change_head, struct sk_buff *, skb, u32, head_room,
 		__skb_push(skb, head_room);
 		memset(skb->data, 0, head_room);
 		skb_reset_mac_header(skb);
+		skb_reset_mac_len(skb);
 	}
 
 	bpf_compute_data_end(skb);