[XFRM]: Fix possible overflow of sock->sk_policy

Spotted by, and original patch by, Balazs Scheidler.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
index ecade48..8da3e25 100644
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -1350,6 +1350,9 @@
 	if (nr > XFRM_MAX_DEPTH)
 		return NULL;
 
+	if (p->dir > XFRM_POLICY_OUT)
+		return NULL;
+
 	xp = xfrm_policy_alloc(GFP_KERNEL);
 	if (xp == NULL) {
 		*dir = -ENOBUFS;