[PATCH] rtnetlink: Synchronze net in rtnl_unregister()

[Dmitry Safonov <dima@arista.com>]

rtnl_unregister() unsets handler from table, which is protected
by rtnl_lock or RCU. At this moment only dump handlers access the table
with rcu_lock(). Every other user accesses under rtnl.

Callers may expect that rtnl_unregister() prevents any further handlers
calls, alike rtnl_unregister_all(). And they do expect it.

I've looked on in-tree caller uses:
br_mdb: safe, but in err-path br_netlink_init()
fib_rules: safe - err-path is very early in __init
ip6mr: safe - following unregister_pernet_subsys() calls internally rcu_barrier()
qrtr: safe - following sock_unregister() calls internally synchronize_rcu()

While it's possible to document that rtnl_unregister() requires
synchronize_net() afterwards - unlike rtnl_unregister_all(), I believe
the module exit is very much slow-path.

Issue seems to be very theoretical and unlikely, so I'm not Cc'ing
stable tree.

Fixes: 6853dd488119 ("rtnetlink: protect handler table with rcu")
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Florian Westphal <fw@strlen.de>
Cc: "Hannes Frederic Sowa" <hannes@stressinduktion.org>
Cc: netdev@vger.kernel.org
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
 net/core/rtnetlink.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 5ea1bed08ede..3db70da4f951 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -308,7 +308,9 @@ int rtnl_unregister(int protocol, int msgtype)
 	rcu_assign_pointer(tab[msgindex], NULL);
 	rtnl_unlock();
 
-	kfree_rcu(link, rcu);
+	synchronize_net();
+
+	kfree(link);
 
 	return 0;
 }
-- 
2.20.1