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 xfrm_notify_policy(struct xfrm_policy *xp, int dir, const struct km_event *c)
{
	int len = nla_total_size(sizeof(struct xfrm_user_tmpl) * xp->xfrm_nr);
	struct net *net = xp_net(xp);
	struct xfrm_userpolicy_info *p;
	struct xfrm_userpolicy_id *id;
	struct nlmsghdr *nlh;
	struct sk_buff *skb;
	int headlen, err;

	headlen = sizeof(*p);
	if (c->event == XFRM_MSG_DELPOLICY) {
		len += nla_total_size(headlen);
		headlen = sizeof(*id);
	}
	len += userpolicy_type_attrsize();
	len += nla_total_size(sizeof(struct xfrm_mark));
	len += NLMSG_ALIGN(headlen);

	skb = nlmsg_new(len, GFP_ATOMIC);
	if (skb == NULL)
		return -ENOMEM;

	nlh = nlmsg_put(skb, c->pid, c->seq, c->event, headlen, 0);
	err = -EMSGSIZE;
	if (nlh == NULL)
		goto out_free_skb;

	p = nlmsg_data(nlh);
	if (c->event == XFRM_MSG_DELPOLICY) {
		struct nlattr *attr;

		id = nlmsg_data(nlh);
		memset(id, 0, sizeof(*id));
		id->dir = dir;
		if (c->data.byid)
			id->index = xp->index;
		else
			memcpy(&id->sel, &xp->selector, sizeof(id->sel));

		attr = nla_reserve(skb, XFRMA_POLICY, sizeof(*p));
		err = -EMSGSIZE;
		if (attr == NULL)
			goto out_free_skb;

		p = nla_data(attr);
	}

	copy_to_user_policy(xp, p, dir);
	err = copy_to_user_tmpl(xp, skb);
	if (!err)
		err = copy_to_user_policy_type(xp->type, skb);
	if (!err)
		err = xfrm_mark_put(skb, &xp->mark);
	if (err)
		goto out_free_skb;

	nlmsg_end(skb, nlh);

	return nlmsg_multicast(net->xfrm.nlsk, skb, 0, XFRMNLGRP_POLICY, GFP_ATOMIC);

out_free_skb:
	kfree_skb(skb);
	return err;
}

static int xfrm_notify_policy(struct xfrm_policy *xp, int dir, const struct km_event *c)
{
	int len = nla_total_size(sizeof(struct xfrm_user_tmpl) * xp->xfrm_nr);
	struct net *net = xp_net(xp);
	struct xfrm_userpolicy_info *p;
	struct xfrm_userpolicy_id *id;
	struct nlmsghdr *nlh;
	struct sk_buff *skb;
	int headlen, err;

	headlen = sizeof(*p);
	if (c->event == XFRM_MSG_DELPOLICY) {
		len += nla_total_size(headlen);
		headlen = sizeof(*id);
	}
	len += userpolicy_type_attrsize();
	len += nla_total_size(sizeof(struct xfrm_mark));
	len += NLMSG_ALIGN(headlen);

	skb = nlmsg_new(len, GFP_ATOMIC);
	if (skb == NULL)
		return -ENOMEM;

	nlh = nlmsg_put(skb, c->pid, c->seq, c->event, headlen, 0);
	err = -EMSGSIZE;
	if (nlh == NULL)
		goto out_free_skb;

	p = nlmsg_data(nlh);
	if (c->event == XFRM_MSG_DELPOLICY) {
		struct nlattr *attr;

		id = nlmsg_data(nlh);
		memset(id, 0, sizeof(*id));
		id->dir = dir;
		if (c->data.byid)
			id->index = xp->index;
		else
			memcpy(&id->sel, &xp->selector, sizeof(id->sel));

		attr = nla_reserve(skb, XFRMA_POLICY, sizeof(*p));
		err = -EMSGSIZE;
		if (attr == NULL)
			goto out_free_skb;

		p = nla_data(attr);
	}

	copy_to_user_policy(xp, p, dir);
	err = copy_to_user_tmpl(xp, skb);
	if (!err)
		err = copy_to_user_policy_type(xp->type, skb);
	if (!err)
		err = xfrm_mark_put(skb, &xp->mark);
	if (err)
		goto out_free_skb;

	nlmsg_end(skb, nlh);

	return nlmsg_multicast(net->xfrm.nlsk, skb, 0, XFRMNLGRP_POLICY, GFP_ATOMIC);

out_free_skb:
	kfree_skb(skb);
	return err;
}
