[NETNS][IPV6]: Make mld_max_msf readonly in other namespaces.

The mld_max_msf protects the system with a maximum allowed multicast
source filters. Making this variable per namespace can be potentially
an problem if someone inside a namespace set it to a big value, that
will impact the whole system including other namespaces.

I don't see any benefits to have it per namespace for now, so in order
to keep a directory entry in a newly created namespace, I make it
read-only when we are not in the initial network namespace.
Signed-off-by: NDaniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/sysctl_net_ipv6.c

@@ -122,6 +122,12 @@ static int ipv6_sysctl_net_init(struct net *net)
 	ipv6_table[5].data = &net->ipv6.sysctl.frags.timeout;
 	ipv6_table[6].data = &net->ipv6.sysctl.frags.secret_interval;
 
+	/* We don't want this value to be per namespace, it should be global
+	   to all namespaces, so make it read-only when we are not in the
+	   init network namespace */
+	if (net != &init_net)
+		ipv6_table[7].mode = 0444;
+
 	net->ipv6.sysctl.table = register_net_sysctl_table(net, net_ipv6_ctl_path,
 							   ipv6_table);
 	if (!net->ipv6.sysctl.table)