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>
int ipv6_chk_home_addr(struct net *net, const struct in6_addr *addr)
{
	int ret = 0;
	struct inet6_ifaddr *ifp = NULL;
	unsigned int hash = inet6_addr_hash(addr);

	rcu_read_lock_bh();
	hlist_for_each_entry_rcu_bh(ifp, &inet6_addr_lst[hash], addr_lst) {
		if (!net_eq(dev_net(ifp->idev->dev), net))
			continue;
		if (ipv6_addr_equal(&ifp->addr, addr) &&
		    (ifp->flags & IFA_F_HOMEADDRESS)) {
			ret = 1;
			break;
		}
	}
	rcu_read_unlock_bh();
	return ret;
}

int ipv6_chk_home_addr(struct net *net, const struct in6_addr *addr)
{
	int ret = 0;
	struct inet6_ifaddr *ifp = NULL;
	unsigned int hash = inet6_addr_hash(addr);

	rcu_read_lock_bh();
	hlist_for_each_entry_rcu_bh(ifp, &inet6_addr_lst[hash], addr_lst) {
		if (!net_eq(dev_net(ifp->idev->dev), net))
			continue;
		if (ipv6_addr_equal(&ifp->addr, addr) &&
		    (ifp->flags & IFA_F_HOMEADDRESS)) {
			ret = 1;
			break;
		}
	}
	rcu_read_unlock_bh();
	return ret;
}
