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_policy(struct xfrm_policy *xp, struct xfrm_userpolicy_info *p)
{
	xp->priority = p->priority;
	xp->index = p->index;
	memcpy(&xp->selector, &p->sel, sizeof(xp->selector));
	memcpy(&xp->lft, &p->lft, sizeof(xp->lft));
	xp->action = p->action;
	xp->flags = p->flags;
	xp->family = p->sel.family;
	/* XXX xp->share = p->share; */
}

static void copy_from_user_policy(struct xfrm_policy *xp, struct xfrm_userpolicy_info *p)
{
	xp->priority = p->priority;
	xp->index = p->index;
	memcpy(&xp->selector, &p->sel, sizeof(xp->selector));
	memcpy(&xp->lft, &p->lft, sizeof(xp->lft));
	xp->action = p->action;
	xp->flags = p->flags;
	xp->family = p->sel.family;
	/* XXX xp->share = p->share; */
}
