UBUNTU-CVE-2026-98852: Medium severity ubuntu/linux-hwe-edge vulnerability

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.