ipv4: l2tp: fix a potential issue in l2tp_ip_recv

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

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 3 deletions

diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index ec22078b0914..42de4ccd159f 100644
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -123,12 +123,11 @@ static int l2tp_ip_recv(struct sk_buff *skb)
         struct l2tp_tunnel *tunnel = NULL;
         int length;
 
-        /* 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;
 
@@ -156,6 +155,9 @@ static int l2tp_ip_recv(struct sk_buff *skb)
                 if (!pskb_may_pull(skb, length))
                         goto discard;
 
+                /* Point to L2TP header */
+                optr = ptr = skb->data;
+                ptr += 4;
                 pr_debug("%s: ip recv\n", tunnel->name);
                 print_hex_dump_bytes("", DUMP_PREFIX_OFFSET, ptr, length);
         }