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 validate_tmpl(int nr, struct xfrm_user_tmpl *ut, u16 family)
{
	int i;

	if (nr > XFRM_MAX_DEPTH)
		return -EINVAL;

	for (i = 0; i < nr; i++) {
		/* We never validated the ut->family value, so many
		 * applications simply leave it at zero.  The check was
		 * never made and ut->family was ignored because all
		 * templates could be assumed to have the same family as
		 * the policy itself.  Now that we will have ipv4-in-ipv6
		 * and ipv6-in-ipv4 tunnels, this is no longer true.
		 */
		if (!ut[i].family)
			ut[i].family = family;

		switch (ut[i].family) {
		case AF_INET:
			break;
#if IS_ENABLED(CONFIG_IPV6)
		case AF_INET6:
			break;
#endif
		default:
			return -EINVAL;
		}
	}

	return 0;
}

static int validate_tmpl(int nr, struct xfrm_user_tmpl *ut, u16 family)
{
	int i;

	if (nr > XFRM_MAX_DEPTH)
		return -EINVAL;

	for (i = 0; i < nr; i++) {
		/* We never validated the ut->family value, so many
		 * applications simply leave it at zero.  The check was
		 * never made and ut->family was ignored because all
		 * templates could be assumed to have the same family as
		 * the policy itself.  Now that we will have ipv4-in-ipv6
		 * and ipv6-in-ipv4 tunnels, this is no longer true.
		 */
		if (!ut[i].family)
			ut[i].family = family;

		switch (ut[i].family) {
		case AF_INET:
			break;
#if IS_ENABLED(CONFIG_IPV6)
		case AF_INET6:
			break;
#endif
		default:
			return -EINVAL;
		}
	}

	return 0;
}
