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 attach_one_algo(struct xfrm_algo **algpp, u8 *props,
			   struct xfrm_algo_desc *(*get_byname)(const char *, int),
			   struct nlattr *rta)
{
	struct xfrm_algo *p, *ualg;
	struct xfrm_algo_desc *algo;

	if (!rta)
		return 0;

	ualg = nla_data(rta);

	algo = get_byname(ualg->alg_name, 1);
	if (!algo)
		return -ENOSYS;
	*props = algo->desc.sadb_alg_id;

	p = kmemdup(ualg, xfrm_alg_len(ualg), GFP_KERNEL);
	if (!p)
		return -ENOMEM;

	strcpy(p->alg_name, algo->name);
	*algpp = p;
	return 0;
}

static int attach_one_algo(struct xfrm_algo **algpp, u8 *props,
			   struct xfrm_algo_desc *(*get_byname)(const char *, int),
			   struct nlattr *rta)
{
	struct xfrm_algo *p, *ualg;
	struct xfrm_algo_desc *algo;

	if (!rta)
		return 0;

	ualg = nla_data(rta);

	algo = get_byname(ualg->alg_name, 1);
	if (!algo)
		return -ENOSYS;
	*props = algo->desc.sadb_alg_id;

	p = kmemdup(ualg, xfrm_alg_len(ualg), GFP_KERNEL);
	if (!p)
		return -ENOMEM;

	strcpy(p->alg_name, algo->name);
	*algpp = p;
	return 0;
}
