CVE-2025-21629: net: reenable NETIF_F_IPV6_CSUM offload for BIG TCP packets

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

net: reenable NETIFFIPV6CSUM offload for BIG TCP packets

The blamed commit disabled hardware offoad of IPv6 packets with extension headers on devices that advertise NETIFFIPV6CSUM, based on the definition of that feature in skbuff.h:

- %NETIFFIPV6CSUM - Driver (device) is only able to checksum plain TCP or UDP packets over IPv6. These are specifically unencapsulated packets of the form IPv6|TCP or IPv6|UDP where the Next Header field in the IPv6 header is either TCP or UDP. IPv6 extension headers are not supported with this feature. This feature cannot be set in features for a device with NETIFFHWCSUM also set. This feature is being DEPRECATED (see below).

The change causes skbwarnbadoffload to fire for BIG TCP packets.

[ 496.310233] WARNING: CPU: 13 PID: 23472 at net/core/dev.c:3129 skbwarnbadoffload+0xc4/0xe0

[ 496.310297] ? skbwarnbadoffload+0xc4/0xe0 [ 496.310300] skbchecksumhelp+0x129/0x1f0 [ 496.310303] skbcsumhwoffloadhelp+0x150/0x1b0 [ 496.310306] validatexmitskb+0x159/0x270 [ 496.310309] validatexmitskblist+0x41/0x70 [ 496.310312] schdirectxmit+0x5c/0x250 [ 496.310317] qdiscrun+0x388/0x620

BIG TCP introduced an IPV6TLVJUMBO IPv6 extension header to communicate packet length, as this is an IPv6 jumbogram. But, the feature is only enabled on devices that support BIG TCP TSO. The header is only present for PFPACKET taps like tcpdump, and not transmitted by physical devices.

For this specific case of extension headers that are not transmitted, return to the situation before the blamed commit and support hardware offload.

ipv6hashopoptjumbo() tests not only whether this header is present, but also that it is the only extension header before a terminal (L4) header.