[PATCH net] bridge: cfm: Fix race condition in peer_mep deletion

[Hyunwoo Kim <imv4bel@gmail.com>]

When a peer MEP is being deleted, cancel_delayed_work_sync() is called
on ccm_rx_dwork before freeing. However, br_cfm_frame_rx() runs in
softirq context under rcu_read_lock (without RTNL) and can re-schedule
ccm_rx_dwork via ccm_rx_timer_start() between cancel_delayed_work_sync()
returning and kfree_rcu() being called.

The following is a simple race scenario:

           cpu0                                     cpu1

mep_delete_implementation()
  cancel_delayed_work_sync(ccm_rx_dwork);
                                           br_cfm_frame_rx()
                                             // peer_mep still in hlist
                                             if (peer_mep->ccm_defect)
                                               ccm_rx_timer_start()
                                                 queue_delayed_work(ccm_rx_dwork)
  hlist_del_rcu(&peer_mep->head);
  kfree_rcu(peer_mep, rcu);
                                           ccm_rx_work_expired()
                                             // on freed peer_mep

To prevent this, cancel_delayed_work_sync() is replaced with
disable_delayed_work_sync() in both peer MEP deletion paths, so
that subsequent queue_delayed_work() calls from br_cfm_frame_rx()
are silently rejected.

The cc_peer_disable() helper retains cancel_delayed_work_sync()
because it is also used for the CC enable/disable toggle path where
the work must remain re-schedulable.

Fixes: dc32cbb3dbd7 ("bridge: cfm: Kernel space implementation of CFM. CCM frame RX added.")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
 net/bridge/br_cfm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/bridge/br_cfm.c b/net/bridge/br_cfm.c
index 2c70fe47de38..118c7ea48c35 100644
--- a/net/bridge/br_cfm.c
+++ b/net/bridge/br_cfm.c
@@ -576,7 +576,7 @@ static void mep_delete_implementation(struct net_bridge *br,
 
 	/* Empty and free peer MEP list */
 	hlist_for_each_entry_safe(peer_mep, n_store, &mep->peer_mep_list, head) {
-		cancel_delayed_work_sync(&peer_mep->ccm_rx_dwork);
+		disable_delayed_work_sync(&peer_mep->ccm_rx_dwork);
 		hlist_del_rcu(&peer_mep->head);
 		kfree_rcu(peer_mep, rcu);
 	}
@@ -732,7 +732,7 @@ int br_cfm_cc_peer_mep_remove(struct net_bridge *br, const u32 instance,
 		return -ENOENT;
 	}
 
-	cc_peer_disable(peer_mep);
+	disable_delayed_work_sync(&peer_mep->ccm_rx_dwork);
 
 	hlist_del_rcu(&peer_mep->head);
 	kfree_rcu(peer_mep, rcu);
-- 
2.43.0