CVE-2026-98852: net: extend IPv6 exthdr detection of tunneled packets
In the Linux kernel, the following vulnerability has been resolved:
net: extend IPv6 exthdr detection of tunneled packets
Commit c4336a07eb6b ("net: correctly handle tunneled traffic on IPV6CSUM GSO fallback") split skbgsohasextensionhdr() into mutually exclusive branches on skb->encapsulation. This did not yet address all paths:
1. With skb->encapsulation set, the outer header is not checked. IPv6 tunnels such as ip6gre and ip6tunnel add an outer Destination Options header by default (encaplimit). Their GSO packets skip software GSO, then skbcsumhwoffloadhelp() sees the outer extension header and calls skbchecksumhelp() on the GSO skb, which warns and drops it.
2. Tunnels over IPv6 without an outer transport header, such as ip6tunnel, leave skb->transportheader at the inner transport header. skbnetworkheaderlen() then spans the outer IPv6, tunnel and inner IP headers, a false positive.
3. UDP tunnels without an inner network header, such as SCTP-in-UDP or PSP, have no inner IPv6 header to check. Decide on the outer header alone.
4. Directly dereferencing inneriphdr(skb)->version without skbheaderpointer() is unsafe.
Instead, check ipv6exthdr(nexthdr) on the outer IPv6 header and, if set, on the inner IPv6 header. Read the headers with skbheaderpointer(). Keep the skbnetworkheaderlen() check when !skb->encapsulation to also catch encapsulated packets without skb->encapsulation (e.g., virtio).
Use the same helper in skbcsumhwoffloadhelp(). Its open coded test has the false positive of (2) and ignores the inner header.
Background: checksum offload of tunneled packets invariants:
Non-GSO skb: - If the inner packet is CHECKSUMPARTIAL, Local Checksum Offload computes the outer checksum in software and the device offloads only the inner L4 checksum. - If the inner packet is CHECKSUMNONE (e.g., SCTP-in-UDP, ESP-in-UDP, Remote Checksum Offload), the device offloads the outer UDP or GRE checksum instead.
GSO skb: - A device with NETIFFGSOUDPTUNNELCSUM or NETIFFGSOGRECSUM computes both inner and outer checksums per segment. The outer checksum is seeded with only the pseudo-header checksum.
A NETIFFIPV6CSUM device must parse through the outer headers to reach the inner ones.
Affected Software
Event History
Frequently Asked Questions
Which systems are most likely to encounter this issue?
Systems using IPv6 tunneling or encapsulated traffic paths are most relevant, particularly ip6_gre and ip6_tunnel. The issue also covers UDP tunnel cases without an inner network header, including SCTP-in-UDP or PSP.
What traffic conditions trigger the failure?
The described failure involves GSO packets on tunneled IPv6 traffic when extension-header handling is incomplete. For IPv6 tunnels that add an outer Destination Options header by default through encap_limit, packets can skip software GSO and then be warned about and dropped during checksum handling.
Is a default tunnel configuration implicated?
Yes. The description states that IPv6 tunnels such as ip6_gre and ip6_tunnel add an outer Destination Options header by default via encap_limit, making that traffic pattern relevant without requiring a separately added outer extension header.