vnet shutdown / ifnet_departure_event

[Kristof Provost]

Hi Bjoern,

I’m running into an issue with vnet shutdown. It manifests 
consistently with pfsync, but if I understand the problem fully it’s 
not really related to pfsync.

The issue is that we end up with a use-after-free of the struct ifnet of 
the pfsync interface.
When the jail shuts down the pfsync interface is destroyed, but because 
this is during vnet shutdown we skip a lot of the cleanup.
Including the `EVENTHANDLER_INVOKE(ifnet_departure_event, ifp);`, which 
means pf doesn’t get notified that the interface went away, so it 
keeps its struct pfi_kif for that interface, which it tries to clean up 
when we get round to doing the vnet shutdown for pf. At that point it 
tries to clear the if_pf_kif and pfg_pf_kif pointers, for an ifp which 
has already been freed.

Invoking the event handler from the ‘if (shutdown)’ code in 
if_detach_internal() fixes the problem, but I’m not totally confident 
that won’t have any unexpected side effects.

Best regards,
Kristof