[PATCH 3.2 111/115] ipv4: l2tp: fix a potential issue in l2tp_ip_recv

[Ben Hutchings]

3.2.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Haishuang Yan <yanhaishu...@cmss.chinamobile.com>

[ Upstream commit 5745b8232e942abd5e16e85fa9b27cc21324acf0 ]

pskb_may_pull() can change skb->data, so we have to load ptr/optr at the
right place.

Signed-off-by: Haishuang Yan <yanhaishu...@cmss.chinamobile.com>
Signed-off-by: David S. Miller <da...@davemloft.net>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
 net/l2tp/l2tp_ip.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -129,12 +129,11 @@ static int l2tp_ip_recv(struct sk_buff *
        int length;
        int offset;
 
-       /* Point to L2TP header */
-       optr = ptr = skb->data;
-
        if (!pskb_may_pull(skb, 4))
                goto discard;
 
+       /* Point to L2TP header */
+       optr = ptr = skb->data;
        session_id = ntohl(*((__be32 *) ptr));
        ptr += 4;
 
@@ -162,6 +161,9 @@ static int l2tp_ip_recv(struct sk_buff *
                if (!pskb_may_pull(skb, length))
                        goto discard;
 
+               /* Point to L2TP header */
+               optr = ptr = skb->data;
+               ptr += 4;
                printk(KERN_DEBUG "%s: ip recv: ", tunnel->name);
 
                offset = 0;