ipv6: fix a potential use after free in sit.c

pskb_may_pull() maybe change skb->data and make iph pointer oboslete,
fix it by geting ip header length directly.

Fixes: ca15a078 (sit: generate icmpv6 error when receiving icmpv4 error)
Cc: Oussama Ghorbel <ghorbel@pivasoftware.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 3 deletions

diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
index 6eab37cf5345..58e5b4710127 100644
--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c
@@ -485,11 +485,11 @@ static void ipip6_tunnel_uninit(struct net_device *dev)
  */
 static int ipip6_err_gen_icmpv6_unreach(struct sk_buff *skb)
 {
-        const struct iphdr *iph = (const struct iphdr *) skb->data;
+        int ihl = ((const struct iphdr *)skb->data)->ihl*4;
         struct rt6_info *rt;
         struct sk_buff *skb2;
 
-        if (!pskb_may_pull(skb, iph->ihl * 4 + sizeof(struct ipv6hdr) + 8))
+        if (!pskb_may_pull(skb, ihl + sizeof(struct ipv6hdr) + 8))
                 return 1;
 
         skb2 = skb_clone(skb, GFP_ATOMIC);
@@ -498,7 +498,7 @@ static int ipip6_err_gen_icmpv6_unreach(struct sk_buff *skb)
                 return 1;
 
         skb_dst_drop(skb2);
-        skb_pull(skb2, iph->ihl * 4);
+        skb_pull(skb2, ihl);
         skb_reset_network_header(skb2);
 
         rt = rt6_lookup(dev_net(skb->dev), &ipv6_hdr(skb2)->saddr, NULL, 0, 0);