ipv6: addrconf: validate new MTU before applying it

Currently we don't check if the new MTU is valid or not and this allows
one to configure a smaller than minimum allowed by RFCs or even bigger
than interface own MTU, which is a problem as it may lead to packet
drops.

If you have a daemon like NetworkManager running, this may be exploited
by remote attackers by forging RA packets with an invalid MTU, possibly
leading to a DoS. (NetworkManager currently only validates for values
too small, but not for too big ones.)

The fix is just to make sure the new value is valid. That is, between
IPV6_MIN_MTU and interface's MTU.

Note that similar check is already performed at
ndisc_router_discovery(), for when kernel itself parses the RA.

Signed-off-by: Marcelo Ricardo Leitner <mleitner@redhat.com>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
void inet6_ifa_finish_destroy(struct inet6_ifaddr *ifp)
{
	WARN_ON(!hlist_unhashed(&ifp->addr_lst));

#ifdef NET_REFCNT_DEBUG
	pr_debug("%s\n", __func__);
#endif

	in6_dev_put(ifp->idev);

	if (cancel_delayed_work(&ifp->dad_work))
		pr_notice("delayed DAD work was pending while freeing ifa=%p\n",
			  ifp);

	if (ifp->state != INET6_IFADDR_STATE_DEAD) {
		pr_warn("Freeing alive inet6 address %p\n", ifp);
		return;
	}
	ip6_rt_put(ifp->rt);

	kfree_rcu(ifp, rcu);
}

void inet6_ifa_finish_destroy(struct inet6_ifaddr *ifp)
{
	WARN_ON(!hlist_unhashed(&ifp->addr_lst));

#ifdef NET_REFCNT_DEBUG
	pr_debug("%s\n", __func__);
#endif

	in6_dev_put(ifp->idev);

	if (cancel_delayed_work(&ifp->dad_work))
		pr_notice("delayed DAD work was pending while freeing ifa=%p\n",
			  ifp);

	if (ifp->state != INET6_IFADDR_STATE_DEAD) {
		pr_warn("Freeing alive inet6 address %p\n", ifp);
		return;
	}
	ip6_rt_put(ifp->rt);

	kfree_rcu(ifp, rcu);
}
