xfrm4: fix the ports decode of sctp protocol

The SCTP pushed the skb data above the sctp chunk header, so the check
of pskb_may_pull(skb, xprth + 4 - skb->data) in _decode_session4() will
never return 0 because xprth + 4 - skb->data < 0, the ports decode of
sctp will always fail.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 1 deletions

diff --git a/net/ipv4/xfrm4_policy.c b/net/ipv4/xfrm4_policy.c
index 60d918c96a4f..0071ee6f441f 100644
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -136,7 +136,8 @@ _decode_session4(struct sk_buff *skb, struct flowi *fl, int reverse)
                 case IPPROTO_TCP:
                 case IPPROTO_SCTP:
                 case IPPROTO_DCCP:
-                        if (pskb_may_pull(skb, xprth + 4 - skb->data)) {
+                        if (xprth + 4 < skb->data ||
+                            pskb_may_pull(skb, xprth + 4 - skb->data)) {
                                 __be16 *ports = (__be16 *)xprth;
 
                                 fl->fl_ip_sport = ports[!!reverse];