ip6_tunnel: Clear IP6CB in ip6tunnel_xmit()

commit 23f4ffed upstream.

skb->cb may contain data from previous layers. In the observed scenario,
the garbage data were misinterpreted as IP6CB(skb)->frag_max_size, so
that small packets sent through the tunnel are mistakenly fragmented.

This patch unconditionally clears the control buffer in ip6tunnel_xmit(),
which affects ip6_tunnel, ip6_udp_tunnel and ip6_gre. Currently none of
these tunnels set IP6CB(skb)->flags, otherwise it needs to be done earlier.
Signed-off-by: Eli Cooper <elicooper@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: apply to ip6_tnl_xmit2()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/ipv6/ip6_tunnel.c

@@ -983,6 +983,7 @@ static int ip6_tnl_xmit2(struct sk_buff *skb,
 	ipv6_addr_copy(&ipv6h->saddr, &fl6->saddr);
 	ipv6_addr_copy(&ipv6h->daddr, &fl6->daddr);
 	nf_reset(skb);
+	memset(skb->cb, 0, sizeof(struct inet6_skb_parm));
 	pkt_len = skb->len;
 	err = ip6_local_out(skb);