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 int __net_init addrconf_init_net(struct net *net)
{
	int err = -ENOMEM;
	struct ipv6_devconf *all, *dflt;

	all = kmemdup(&ipv6_devconf, sizeof(ipv6_devconf), GFP_KERNEL);
	if (all == NULL)
		goto err_alloc_all;

	dflt = kmemdup(&ipv6_devconf_dflt, sizeof(ipv6_devconf_dflt), GFP_KERNEL);
	if (dflt == NULL)
		goto err_alloc_dflt;

	/* these will be inherited by all namespaces */
	dflt->autoconf = ipv6_defaults.autoconf;
	dflt->disable_ipv6 = ipv6_defaults.disable_ipv6;

	net->ipv6.devconf_all = all;
	net->ipv6.devconf_dflt = dflt;

#ifdef CONFIG_SYSCTL
	err = __addrconf_sysctl_register(net, "all", NULL, all);
	if (err < 0)
		goto err_reg_all;

	err = __addrconf_sysctl_register(net, "default", NULL, dflt);
	if (err < 0)
		goto err_reg_dflt;
#endif
	return 0;

#ifdef CONFIG_SYSCTL
err_reg_dflt:
	__addrconf_sysctl_unregister(all);
err_reg_all:
	kfree(dflt);
#endif
err_alloc_dflt:
	kfree(all);
err_alloc_all:
	return err;
}

static int __net_init addrconf_init_net(struct net *net)
{
	int err = -ENOMEM;
	struct ipv6_devconf *all, *dflt;

	all = kmemdup(&ipv6_devconf, sizeof(ipv6_devconf), GFP_KERNEL);
	if (all == NULL)
		goto err_alloc_all;

	dflt = kmemdup(&ipv6_devconf_dflt, sizeof(ipv6_devconf_dflt), GFP_KERNEL);
	if (dflt == NULL)
		goto err_alloc_dflt;

	/* these will be inherited by all namespaces */
	dflt->autoconf = ipv6_defaults.autoconf;
	dflt->disable_ipv6 = ipv6_defaults.disable_ipv6;

	net->ipv6.devconf_all = all;
	net->ipv6.devconf_dflt = dflt;

#ifdef CONFIG_SYSCTL
	err = __addrconf_sysctl_register(net, "all", NULL, all);
	if (err < 0)
		goto err_reg_all;

	err = __addrconf_sysctl_register(net, "default", NULL, dflt);
	if (err < 0)
		goto err_reg_dflt;
#endif
	return 0;

#ifdef CONFIG_SYSCTL
err_reg_dflt:
	__addrconf_sysctl_unregister(all);
err_reg_all:
	kfree(dflt);
#endif
err_alloc_dflt:
	kfree(all);
err_alloc_all:
	return err;
}
