netdev - [PATCH] net: Protect iterations over net::fib_notifier_ops in fib_seq

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Date:   Tue, 14 Nov 2017 16:51:56 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     jiri@...lanox.com, idosch@...lanox.com, vvs@...tuozzo.com,
        davem@...emloft.net, kernel@...r.kernel.org,
        netdev@...r.kernel.org, ktkhai@...tuozzo.com
Subject: [PATCH] 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@...tuozzo.com>
---
 net/core/fib_notifier.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/fib_notifier.c b/net/core/fib_notifier.c
index 4fc202dbdfb6..fd5c613aea56 100644
--- a/net/core/fib_notifier.c
+++ b/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();