Re: [PATCH net v4] net: Fix Hole-196 vulnerability by dropping unicast ARP/IP in L2 BMC

[Julian Anastasov <ja@ssi.bg>]

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	Hello,

On Mon, 2 Nov 2026, Star Chang wrote:

> Hi Julian, Johannes,
> 
> Thank you both for the great discussion and perspectives!
> 
> Putting the two points together gives a very clear and coherent picture:
> 
> 1. Julian correctly pointed out the real security risk with Hole-196:
>    Attackers can abuse not only unicast ARP replies, but also broadcast GARP
>    announcements (sip == tip == gateway_ip) to poison peer caches in a
>    shared-key network.

	The GARP request is one of the possibilities, the problem
is that any broadcast request (not just GARP) can poison the cache,
so by setting the flag and skipping the neigh_event_ns and neigh_update
we will ignore any changes of the hw addr.

> 2. At the same time, as Johannes reminded, Linux already has a dedicated knob
>    for that specific broadcast behavior: `drop_gratuitous_arp`.
> 
> Looking at the existing architecture, the two knobs complement each other
> naturally:
> - `drop_unicast_in_l2_multicast`: strictly enforces the L3/L2 boundary by
>   dropping unicast IP and unicast ARP replies (sip != tip) inside L2 BMC.
> - `drop_gratuitous_arp`: drops broadcast GARP (sip == tip) without tangling
>   broadcast filtering logic into the unicast knob.
> 
> This means users defending against Hole-196 can achieve complete coverage
> by enabling both knobs on the wireless interface, without needing to overload
> `drop_unicast_in_l2_multicast` or change core neighbor learning behavior.

	The DROP_GRATUITOUS_ARP check catches sip == tip = GWIP but
not sip = GWIP, tip=VICTIM_OR_OTHER_IP broadcast request.

> 
> This also means [PATCH net v5] (which dropped IP changes and strictly filtered
> non-GARP replies) is architecturally aligned. We can document this dual-knob
> recommendation in ip-sysctl.rst so administrators know how to fully secure
> shared wireless links.
> 
> Julian, Johannes — what do you think about this unified direction?
> 
> Best regards,
> Star Chang

Regards

--
Julian Anastasov <ja@ssi.bg>