CVE-2026-80976

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

seg6: reset IP6CB after IPv6 decapsulation

decapandvalidate() pulls the outer SRv6 headers and makes the inner

packet the skb network header. The IPv6 control block still contains

values collected while parsing the outer packet, including nhoff and

extension-header flags.

End.DX6 and End.DT6 route the inner IPv6 packet directly to the IPv6

input path. An unprivileged user can reach End.DT6 from a user and net

namespace by installing a local SID and injecting an outer packet with

Hop-by-Hop and Destination Options headers followed by an SRH and a

minimal inner IPv6 packet.

The outer extension headers leave a large nhoff in IP6CB. After

decapsulation, ip6protocoldeliver_rcu() uses that stale offset on the

inner packet and reads beyond the skb head. KASAN reports:

  BUG: KASAN: slab-out-of-bounds in ip6protocoldeliver_rcu

  ip6protocoldeliver_rcu+0x1118/0x1450

  ip6inputfinish+0x11b/0x240

  seg6localinput_core+0xed/0x2e0

  lwtunnel_input+0x1e9/0x4e0

  ipv6rthdrrcv+0x525f/0x6c50

  ip6protocoldeliver_rcu+0xcb7/0x1450

Before clearing IP6CB for an inner IPv6 packet, save its incoming

interface index and L3 slave state. Restore both after the clear and set

nhoff to the inner IPv6 base-header nexthdr field.

Use IP6CB(skb)->iif rather than skb->skb_iif because VRF processing can

replace skb_iif with the L3 master while IP6CB keeps the receiving

interface. Preserve IP6SKB_L3SLAVE for the same reason.