ip_tunnel: Fix a memory corruption in ip_tunnel_xmit

[ Upstream commit 3e08f4a7 ]

We might extend the used aera of a skb beyond the total
headroom when we install the ipip header. Fix this by
calling skb_cow_head() unconditionally.

Bug was introduced with commit c5441932
("GRE: Refactor GRE tunneling code.")

Cc: Pravin Shelar <pshelar@nicira.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/ipv4/ip_tunnel.c

@@ -659,14 +659,14 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 
 	max_headroom = LL_RESERVED_SPACE(tdev) + sizeof(struct iphdr)
 					       + rt->dst.header_len;
-	if (max_headroom > dev->needed_headroom) {
+	if (max_headroom > dev->needed_headroom)
 		dev->needed_headroom = max_headroom;
+
 	if (skb_cow_head(skb, dev->needed_headroom)) {
 		dev->stats.tx_dropped++;
 		dev_kfree_skb(skb);
 		return;
 	}
-	}
 
 	skb_dst_drop(skb);
 	skb_dst_set(skb, &rt->dst);