PT-2026-117071 · Linux · Linux
CVE-2026-98852
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Published
2026-10-18
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Updated
2026-10-18
None
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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 IPV6 CSUM
GSO fallback") split skb gso has extension hdr() into mutually exclusive
branches on skb->encapsulation. This did not yet address all paths:
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With skb->encapsulation set, the outer header is not checked. IPv6 tunnels such as ip6 gre and ip6 tunnel add an outer Destination Options header by default (encap limit). Their GSO packets skip software GSO, then skb csum hwoffload help() sees the outer extension header and calls skb checksum help() on the GSO skb, which warns and drops it.
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Tunnels over IPv6 without an outer transport header, such as ip6 tunnel, leave skb->transport header at the inner transport header. skb network header len() then spans the outer IPv6, tunnel and inner IP headers, a false positive.
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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.
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Directly dereferencing inner ip hdr(skb)->version without skb header pointer() is unsafe.
Instead, check ipv6 ext hdr(nexthdr) on the outer IPv6 header and, if
set, on the inner IPv6 header. Read the headers with skb header pointer().
Keep the skb network header len() check when !skb->encapsulation to also
catch encapsulated packets without skb->encapsulation (e.g., virtio).
Use the same helper in skb csum hwoffload help(). 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 CHECKSUM PARTIAL, Local Checksum Offload computes the outer checksum in software and the device offloads only the inner L4 checksum.
- If the inner packet is CHECKSUM NONE (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 NETIF F GSO UDP TUNNEL CSUM or NETIF F GSO GRE CSUM computes both inner and outer checksums per segment. The outer checksum is seeded with only the pseudo-header checksum.
A NETIF F IPV6 CSUM device must parse through the outer headers to
reach the inner ones.
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Related Identifiers
Affected Products
Linux