From 83d51909f235d8a94e9f1491482d334fb82897a3 Mon Sep 17 00:00:00 2001
From: Eric Dumazet <edumazet@google.com>
Date: Fri, 27 Feb 2015 18:35:35 -0800
Subject: [PATCH] macvtap: make sure neighbour code can push ethernet header

[ Upstream commit 2f1d8b9e8afa5a833d96afcd23abcb8cdf8d83ab ]

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
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