bridge: cfm: Fix race condition in peer_mep deletion

In the Linux kernel, the following vulnerability has been resolved:

bridge: cfm: Fix race condition in peer_mep deletion

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.