Re: [PATCH] net: fix fanout UAF in packet_release() via NETDEV_UP race

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Yochai Eisenrich wrote:
> `packet_release()` has a race window where `NETDEV_UP` can re-register a
> socket into a fanout group's `arr[]` array. The re-registration is not
> cleaned up by `fanout_release()`, leaving a dangling pointer in the fanout
> array. 
> `packet_release()` does NOT zero `po->num` in its `bind_lock` section.
> After releasing `bind_lock`, `po->num` is still non-zero and `po->ifindex`
> still matches the bound device. A concurrent `packet_notifier(NETDEV_UP)`
> that already found the socket in `sklist` can re-register the hook.
> For fanout sockets, this re-registration calls `__fanout_link(sk, po)`
> which adds the socket back into `f->arr[]` and increments `f->num_members`,
> but does NOT increment `f->sk_ref`.
> 
> The fix sets `po->num` to zero in `packet_release` while `bind_lock` is
> held to prevent NETDEV_UP from linking, preventing the race window.
> 
> This bug was found following an additional audit with Claude Code based on
> CVE-2025-38617.
> Link: https://blog.calif.io/p/a-race-within-a-race-exploiting-cve
> 
> Fixes: ce06b03e60fc ("packet: Add helpers to register/unregister ->prot_hook")
> 
> Signed-off-by: Yochai Eisenrich <echelonh@gmail.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>