CVE-2025-21629 addresses a vulnerability in the Linux kernel related to the handling of hardware checksum offload for IPv6 packets with extension headers, specifically for BIG TCP packets. A previous commit disabled hardware offload for IPv6 packets with extension headers on devices advertising NETIF_F_IPV6_CSUM, causing warnings and potential issues with packet transmission. This update re-enables the offload feature for BIG TCP packets on devices that support BIG TCP TSO, restoring previous behavior for extension headers that are not transmitted by physical devices.