CVE-2025-21629

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

net: reenable NETIF_F_IPV6_CSUM offload for BIG TCP packets

The blamed commit disabled hardware offoad of IPv6 packets with extension headers on devices that advertise NETIF_F_IPV6_CSUM, based on the definition of that feature in skbuff.h:

• • • %NETIF_F_IPV6_CSUM

• - 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
  

•   NETIF_F_HW_CSUM also set. This feature is being
  

•   DEPRECATED (see below).
  

The change causes skb_warn_bad_offload to fire for BIG TCP packets.

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

[ 496.310297] ? skb_warn_bad_offload+0xc4/0xe0 [ 496.310300] skb_checksum_help+0x129/0x1f0 [ 496.310303] skb_csum_hwoffload_help+0x150/0x1b0 [ 496.310306] validate_xmit_skb+0x159/0x270 [ 496.310309] validate_xmit_skb_list+0x41/0x70 [ 496.310312] sch_direct_xmit+0x5c/0x250 [ 496.310317] __qdisc_run+0x388/0x620

BIG TCP introduced an IPV6_TLV_JUMBO 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 PF_PACKET 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.

ipv6_has_hopopt_jumbo() tests not only whether this header is present, but also that it is the only extension header before a terminal (L4) header.

This vulnerability is rated as Moderate severity because exploitation requires local access to generate network traffic with specific IPv6 hop-by-hop extension headers, resulting at most in a kernel warning and potential system disruption rather than arbitrary code execution or data leakage. The issue occurs only on network interfaces configured with specific hardware checksum offload capabilities handling BIG TCP traffic, which is not a standard configuration across enterprise deployments. Consequently, the threat is confined to local denial-of-service conditions under non-default networking scenarios.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.