net: Protect iterations over net::fib_notifier_ops in fib_seq_sum()

There is at least unlocked deletion of net->ipv4.fib_notifier_ops
from net::fib_notifier_ops:

ip_fib_net_exit()
  rtnl_unlock()
  fib4_notifier_exit()
    fib_notifier_ops_unregister(net->ipv4.notifier_ops)
      list_del_rcu(&ops->list)

So fib_seq_sum() can't use rtnl_lock() only for protection.

The possible solution could be to use rtnl_lock()
in fib_notifier_ops_unregister(), but this adds
a possible delay during net namespace creation,
so we better use rcu_read_lock() till someone
really needs the mutex (if that happens).

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/fib_notifier.c

@@ -34,12 +34,14 @@ static unsigned int fib_seq_sum(void)
 
 	rtnl_lock();
 	for_each_net(net) {
-		list_for_each_entry(ops, &net->fib_notifier_ops, list) {
+		rcu_read_lock();
+		list_for_each_entry_rcu(ops, &net->fib_notifier_ops, list) {
 			if (!try_module_get(ops->owner))
 				continue;
 			fib_seq += ops->fib_seq_read(net);
 			module_put(ops->owner);
 		}
+		rcu_read_unlock();
 	}
 	rtnl_unlock();