CVE-2026-74743

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

macvlan: inherit neededheadroom and neededtailroom from lowerdev

macvlan devices inherit hardheaderlen from lowerdev during macvlan_init(),

but leave neededheadroom and neededtailroom set to 0.

When the underlying lowerdev requires extra headroom or tailroom for

headers/trailers (e.g. macsec, ipsec, wireguard, tunnels, or veth with rx

headroom), upper layers calculating packet headroom and tailroom fail to

reserve sufficient space.

This can result in reallocation overhead, skb headroom underflows, or KASAN

slab-use-after-free crashes when devhardheader() / macvlanhardheader()

prepends header data or when lower devices append tailroom.

Fix this by:

1. Inheriting neededheadroom and neededtailroom from lowerdev in macvlan_init().
2. Propagating neededheadroom and neededtailroom updates to attached macvlans

   in macvlandeviceevent() when receiving NETDEVFEATCHANGE events.