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 void copy_from_user_state(struct xfrm_state *x, struct xfrm_usersa_info *p)
{
	memcpy(&x->id, &p->id, sizeof(x->id));
	memcpy(&x->sel, &p->sel, sizeof(x->sel));
	memcpy(&x->lft, &p->lft, sizeof(x->lft));
	x->props.mode = p->mode;
	x->props.replay_window = p->replay_window;
	x->props.reqid = p->reqid;
	x->props.family = p->family;
	memcpy(&x->props.saddr, &p->saddr, sizeof(x->props.saddr));
	x->props.flags = p->flags;

	if (!x->sel.family && !(p->flags & XFRM_STATE_AF_UNSPEC))
		x->sel.family = p->family;
}

static void copy_from_user_state(struct xfrm_state *x, struct xfrm_usersa_info *p)
{
	memcpy(&x->id, &p->id, sizeof(x->id));
	memcpy(&x->sel, &p->sel, sizeof(x->sel));
	memcpy(&x->lft, &p->lft, sizeof(x->lft));
	x->props.mode = p->mode;
	x->props.replay_window = p->replay_window;
	x->props.reqid = p->reqid;
	x->props.family = p->family;
	memcpy(&x->props.saddr, &p->saddr, sizeof(x->props.saddr));
	x->props.flags = p->flags;

	if (!x->sel.family && !(p->flags & XFRM_STATE_AF_UNSPEC))
		x->sel.family = p->family;
}
