bonding: fix a race in IGMP handling

RCU conversion in IGMP code done in net-next-2.6 raised a race in
__bond_resend_igmp_join_requests().

It iterates in_dev->mc_list without appropriate protection (RTNL, or
read_lock on in_dev->mc_list_lock).

Another cpu might delete an entry while we use it and trigger a fault.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -878,8 +878,10 @@ static void __bond_resend_igmp_join_requests(struct net_device *dev)
 	rcu_read_lock();
 	in_dev = __in_dev_get_rcu(dev);
 	if (in_dev) {
+		read_lock(&in_dev->mc_list_lock);
 		for (im = in_dev->mc_list; im; im = im->next)
 			ip_mc_rejoin_group(im);
+		read_unlock(&in_dev->mc_list_lock);
 	}
 
 	rcu_read_unlock();