[PATCH v2 2/2] bonding: fix igmp_retrans type and two related races

[nikolay@redhat.com]

From: Nikolay Aleksandrov <nikolay@redhat.com>

First the type of igmp_retrans (which is the actual counter of
igmp_resend parameter) is changed to u8 to be able to store values up
to 255 (as per documentation). There are two races that were hidden
there and which are easy to trigger after the previous fix, the first is
between bond_resend_igmp_join_requests and bond_change_active_slave
where igmp_retrans is set and can be altered by the periodic. The second
race condition is between multiple running instances of the periodic
(upon execution it can be scheduled again for immediate execution which
can cause the counter to go < 0 which in the unsigned case leads to
unnecessary igmp retransmissions).
Since in bond_change_active_slave bond->lock is held for reading and
curr_slave_lock for writing, we use curr_slave_lock for mutual
exclusion. We can't drop them as there're cases where RTNL is not held
when bond_change_active_slave is called. RCU is unlocked in
bond_resend_igmp_join_requests before getting curr_slave_lock since we
don't need it there and it's pointless to delay.
The decrement is moved inside the "if" block because if we decrement
unconditionally there's still a possibility for a race condition although
it is much more difficult to hit (many changes have to happen in
a very short period in order to trigger) which in the case of 3 parallel
running instances of this function and igmp_retrans == 1
(with check bond->igmp_retrans-- > 1) is:
f1 passes, doesn't re-schedule, but decrements - igmp_retrans = 0
f2 then passes, doesn't re-schedule, but decrements - igmp_retrans = 255
f3 does the unnecessary retransmissions.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
---
v2: updated commit message

 drivers/net/bonding/bond_main.c | 15 +++++++++++----
 drivers/net/bonding/bonding.h   |  2 +-
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 473633a..02d9ae7 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -764,8 +764,8 @@ static void bond_resend_igmp_join_requests(struct bonding *bond)
 	struct net_device *bond_dev, *vlan_dev, *upper_dev;
 	struct vlan_entry *vlan;
 
-	rcu_read_lock();
 	read_lock(&bond->lock);
+	rcu_read_lock();
 
 	bond_dev = bond->dev;
 
@@ -787,12 +787,19 @@ static void bond_resend_igmp_join_requests(struct bonding *bond)
 		if (vlan_dev)
 			__bond_resend_igmp_join_requests(vlan_dev);
 	}
+	rcu_read_unlock();
 
-	if (--bond->igmp_retrans > 0)
+	/* We use curr_slave_lock to protect against concurrent access to
+	 * igmp_retrans from multiple running instances of this function and
+	 * bond_change_active_slave
+	 */
+	write_lock_bh(&bond->curr_slave_lock);
+	if (bond->igmp_retrans > 1) {
+		bond->igmp_retrans--;
 		queue_delayed_work(bond->wq, &bond->mcast_work, HZ/5);
-
+	}
+	write_unlock_bh(&bond->curr_slave_lock);
 	read_unlock(&bond->lock);
-	rcu_read_unlock();
 }
 
 static void bond_resend_igmp_join_requests_delayed(struct work_struct *work)
diff --git a/drivers/net/bonding/bonding.h b/drivers/net/bonding/bonding.h
index 2baec24..f989e15 100644
--- a/drivers/net/bonding/bonding.h
+++ b/drivers/net/bonding/bonding.h
@@ -225,7 +225,7 @@ struct bonding {
 	rwlock_t curr_slave_lock;
 	u8	 send_peer_notif;
 	s8	 setup_by_slave;
-	s8       igmp_retrans;
+	u8       igmp_retrans;
 #ifdef CONFIG_PROC_FS
 	struct   proc_dir_entry *proc_entry;
 	char     proc_file_name[IFNAMSIZ];
-- 
1.8.1.4