[PATCH v2 net] ipvlan: unregister upper devices outside pnodes_lock

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

syzbot reported the following circular locking dependency:

  xs->mutex -> netdev lock -> pnodes_lock -> net->xdp.lock -> xs->mutex

The pnodes_lock -> net->xdp.lock edge is recorded when
ipvlan_device_event(NETDEV_UNREGISTER) calls unregister_netdevice_many()
while holding pnodes_lock.  A nested NETDEV_UNREGISTER notification for
an IPvlan device enters xsk_notifier(), which acquires net->xdp.lock.

Keep pnodes_lock only while marking the upper devices as dying, removing
them from port->ipvlans, and queueing them for unregistration.  Once the
devices have been detached from the protected list, release pnodes_lock
before unregister_netdevice_many() invokes notifier callbacks.

The port remains alive across unregistration because
ipvlan_device_event() holds the reference acquired by ipvlan_port_get().
The dying flag prevents a concurrent ->dellink() callback from deleting a
queued device again.

Fixes: 35add1093e2f ("ipvlan: Protect ipvl_port.ipvlans with mutex.")
Reported-by: syzbot+aa48b5fe7bfda62d1682@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=aa48b5fe7bfda62d1682
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
---
v2: avoid list_empty() check as unregister_netdevice_many() already does
    it for us (Kuniyuki)
---
 drivers/net/ipvlan/ipvlan_main.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ipvlan/ipvlan_main.c b/drivers/net/ipvlan/ipvlan_main.c
index ee46a55f73d1..2d4340627ca6 100644
--- a/drivers/net/ipvlan/ipvlan_main.c
+++ b/drivers/net/ipvlan/ipvlan_main.c
@@ -846,7 +846,6 @@ static int ipvlan_device_event(struct notifier_block *unused,
 				__ipvlan_link_delete(net, ipvlan->dev, &lst_kill);
 		}
 
-		unregister_netdevice_many(&lst_kill);
 		break;
 	}
 	case NETDEV_FEAT_CHANGE:
@@ -895,6 +894,9 @@ static int ipvlan_device_event(struct notifier_block *unused,
 
 	mutex_unlock(&port->pnodes_lock);
 
+	/* Avoid invoking nested netdevice notifiers under pnodes_lock. */
+	unregister_netdevice_many(&lst_kill);
+
 	ipvlan_port_put(port);
 
 	return ret;
-- 
2.43.0