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>
static void addrconf_dev_config(struct net_device *dev)
{
	struct inet6_dev *idev;

	ASSERT_RTNL();

	if ((dev->type != ARPHRD_ETHER) &&
	    (dev->type != ARPHRD_FDDI) &&
	    (dev->type != ARPHRD_ARCNET) &&
	    (dev->type != ARPHRD_INFINIBAND) &&
	    (dev->type != ARPHRD_IEEE802154) &&
	    (dev->type != ARPHRD_IEEE1394) &&
	    (dev->type != ARPHRD_TUNNEL6) &&
	    (dev->type != ARPHRD_6LOWPAN)) {
		/* Alas, we support only Ethernet autoconfiguration. */
		return;
	}

	idev = addrconf_add_dev(dev);
	if (IS_ERR(idev))
		return;

	addrconf_addr_gen(idev, false);
}

static void addrconf_dev_config(struct net_device *dev)
{
	struct inet6_dev *idev;

	ASSERT_RTNL();

	if ((dev->type != ARPHRD_ETHER) &&
	    (dev->type != ARPHRD_FDDI) &&
	    (dev->type != ARPHRD_ARCNET) &&
	    (dev->type != ARPHRD_INFINIBAND) &&
	    (dev->type != ARPHRD_IEEE802154) &&
	    (dev->type != ARPHRD_IEEE1394) &&
	    (dev->type != ARPHRD_TUNNEL6) &&
	    (dev->type != ARPHRD_6LOWPAN)) {
		/* Alas, we support only Ethernet autoconfiguration. */
		return;
	}

	idev = addrconf_add_dev(dev);
	if (IS_ERR(idev))
		return;

	addrconf_addr_gen(idev, false);
}
