xfrm: ipcomp: Call pskb_may_pull in ipcomp_input

If a malformed packet is received there may not be enough data
to pull.  This isn't a problem in practice because the caller
has already done xfrm_parse_spi which in effect does the same
thing.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

net/xfrm/xfrm_ipcomp.c

@@ -97,6 +97,9 @@ int ipcomp_input(struct xfrm_state *x, struct sk_buff *skb)
 	int err = -ENOMEM;
 	struct ip_comp_hdr *ipch;
 
+	if (!pskb_may_pull(skb, sizeof(*ipch)))
+		return -EINVAL;
+
 	if (skb_linearize_cow(skb))
 		goto out;