macvtap: make sure neighbour code can push ethernet header

Brian reported crashes using IPv6 traffic with macvtap/veth combo.

I tracked the crashes in neigh_hh_output()

-> memcpy(skb->data - HH_DATA_MOD, hh->hh_data, HH_DATA_MOD);

Neighbour code assumes headroom to push Ethernet header is
at least 16 bytes.

It appears macvtap has only 14 bytes available on arches
where NET_IP_ALIGN is 0 (like x86)

Effect is a corruption of 2 bytes right before skb->head,
and possible crashes if accessing non existing memory.

This fix should also increase IPv4 performance, as paranoid code
in ip_finish_output2() wont have to call skb_realloc_headroom()

Reported-by: Brian Rak <brak@vultr.com>
Tested-by: Brian Rak <brak@vultr.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macvtap.c

@@ -654,11 +654,14 @@ static void macvtap_skb_to_vnet_hdr(struct macvtap_queue *q,
 	} /* else everything is zero */
 }
 
+/* Neighbour code has some assumptions on HH_DATA_MOD alignment */
+#define MACVTAP_RESERVE HH_DATA_OFF(ETH_HLEN)
+
 /* Get packet from user space buffer */
 static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
 				struct iov_iter *from, int noblock)
 {
-	int good_linear = SKB_MAX_HEAD(NET_IP_ALIGN);
+	int good_linear = SKB_MAX_HEAD(MACVTAP_RESERVE);
 	struct sk_buff *skb;
 	struct macvlan_dev *vlan;
 	unsigned long total_len = iov_iter_count(from);
@@ -722,7 +725,7 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
 			linear = macvtap16_to_cpu(q, vnet_hdr.hdr_len);
 	}
 
-	skb = macvtap_alloc_skb(&q->sk, NET_IP_ALIGN, copylen,
+	skb = macvtap_alloc_skb(&q->sk, MACVTAP_RESERVE, copylen,
 				linear, noblock, &err);
 	if (!skb)
 		goto err;