xfrm_user: return error pointer instead of NULL

When dump_one_state() returns an error, e.g. because of a too small
buffer to dump the whole xfrm state, xfrm_state_netlink() returns NULL
instead of an error pointer. But its callers expect an error pointer
and therefore continue to operate on a NULL skbuff.

This could lead to a privilege escalation (execution of user code in
kernel context) if the attacker has CAP_NET_ADMIN and is able to map
address 0.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
static int copy_to_user_kmaddress(const struct xfrm_kmaddress *k, struct sk_buff *skb)
{
	struct xfrm_user_kmaddress uk;

	memset(&uk, 0, sizeof(uk));
	uk.family = k->family;
	uk.reserved = k->reserved;
	memcpy(&uk.local, &k->local, sizeof(uk.local));
	memcpy(&uk.remote, &k->remote, sizeof(uk.remote));

	return nla_put(skb, XFRMA_KMADDRESS, sizeof(uk), &uk);
}

static int copy_to_user_kmaddress(const struct xfrm_kmaddress *k, struct sk_buff *skb)
{
	struct xfrm_user_kmaddress uk;

	memset(&uk, 0, sizeof(uk));
	uk.family = k->family;
	uk.reserved = k->reserved;
	memcpy(&uk.local, &k->local, sizeof(uk.local));
	memcpy(&uk.remote, &k->remote, sizeof(uk.remote));

	return nla_put(skb, XFRMA_KMADDRESS, sizeof(uk), &uk);
}
