bridge: cfm: Fix race condition in peer_mep deletion

bridge: cfm: Fix race condition in peer_mep deletion

[ Upstream commit 3715a00855316066cdda69d43648336367422127 ]

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>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/abBgYT5K_FI9rD1a@v4bel
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>