CVE-2026-68338: net/packet: avoid fanout hook re-registration after unregister

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

net/packet: avoid fanout hook re-registration after unregister

packetsetring() temporarily detaches a socket from packet delivery while reconfiguring its ring. It records the previous running state, clears po->num, unregisters the protocol hook when needed, drops po->bindlock, and later restores po->num and re-registers the hook from the saved wasrunning value.

That unlocked window can race with NETDEVUNREGISTER. The notifier can observe the socket as not running, skip unregisterprothook(), and invalidate the per-socket binding by setting po->ifindex to -1 and clearing po->prothook.dev. A one-member fanout group can still retain its shared fanout hook device pointer. When packetsetring() resumes, re-registering solely from the stale wasrunning state can re-add the fanout hook after the device has been unregistered.

Treat po->ifindex == -1 as an invalidated binding after reacquiring po->bindlock. This is distinct from ifindex 0, the normal unbound/wildcard state: ifindex -1 marks an existing device binding that was invalidated when the device was unregistered. Restore po->num as before, but do not re-register the hook if device unregister already detached the socket.