CVE-2026-45844: netfilter: arp_tables: fix IEEE1394 ARP payload parsing

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

netfilter: arptables: fix IEEE1394 ARP payload parsing

Weiming Shi says:

"arppacketmatch() unconditionally parses the ARP payload assuming two hardware addresses are present (source and target). However, IPv4-over-IEEE1394 ARP (RFC 2734) omits the target hardware address field, and arphdrlen() already accounts for this by returning a shorter length for ARPHRDIEEE1394 devices.

As a result, on IEEE1394 interfaces arppacketmatch() advances past a nonexistent target hardware address and reads the wrong bytes for both the target device address comparison and the target IP address. This causes arptables rules to match against garbage data, leading to incorrect filtering decisions: packets that should be accepted may be dropped and vice versa.

The ARP stack in net/ipv4/arp.c (arpcreate and arpprocess) already handles this correctly by skipping the target hardware address for ARPHRDIEEE1394. Apply the same pattern to arppacketmatch()."

Mangle the original patch to always return 0 (no match) in case user matches on the target hardware address which is never present in IEEE1394.

Note that this returns 0 (no match) for either normal and inverse match because matching in the target hardware address in ARPHRDIEEE1394 has never been supported by arptables. This is intentional, matching on the target hardware address should never evaluate true for ARPHRDIEEE1394.

Moreover, adjust arptmangle to drop the packet too as AI suggests:

In arptmangle, the logic assumes a standard ARP layout. Because IEEE1394 (FireWire) omits the target hardware address, the linear pointer arithmetic miscalculates the offset for the target IP address. This causes mangling operations to write to the wrong location, leading to packet corruption. To ensure safety, this patch drops packets (NFDROP) when mangling is requested for these fields on IEEE1394 devices, as the current implementation cannot correctly map the FireWire ARP payload.

This omits both mangling target hardware and IP address. Even if IP address mangling should be possible in IEEE1394, this would require to adjust arptmangle offset calculation, which has never been supported.

Based on patch from Weiming Shi <bestswngs@gmail.com>.