[CVE-2026-80840][IMPORTANT] ipv6: seg6: clear IPv4 control block on IPIP decapsulation

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-80840
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ipv6: seg6: clear IPv4 control block on IPIP decapsulation
Commit: 10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80840
Analysis date: Fri, 04 Sep 2026 13:06:35 -0400
ActionableScore: 10
ActionableScore lower bound: 9
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A remote packet can cause stale IPv6 skb control-block fields to be interpreted as IPv4 option state after SRv6 IPIP decapsulation, leading to an attacker-influenced slab out-of-bounds write in `__ip_options_echo()` and a kernel crash with plausible broader memory-corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80840 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80840

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: IMPORTANT
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-80840	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-80840 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-665;CWE-843;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '9.8';DESCR 'An IPv6 SRv6 End.DX4 or End.DT4 decapsulation path can pass an inner IPv4 packet to IPv4 processing without clearing the shared skb control block. Stale IP6CB fields from the outer IPv6 packet can be interpreted as IPCB IPv4 option state, causing tcp_v4_route_req and __ip_options_echo to copy attacker controlled option data past a small option buffer. A remote sender can trigger this with a crafted IPv6 packet that reaches a configured SRv6 local action and contains an IPIP inner TCP SYN. For the CVSS the PR:N because the attacker does not need a local account or authenticated access on the target, only network reachability to the affected SRv6 endpoint. Impact is kernel memory corruption with a demonstrated slab out of bounds write, so denial of service is immediate and confidentiality, integrity, or privilege escalation impact is plausible in worst case. SRv6 deployments are usually limited to provider, data center, or controlled routing domains rather than the public Internet, but exposed or reachable SRv6 endpoints should be treated as high risk.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 10)	YES	REMOTE WRITE KASAN OOB NETWORK SKB LINUS IPV6 KPANIC PACKET 	NO	NO	guess

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ACTIONABLESCORE ANALYSIS
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ActionableScore=10
ActionableScoreLower=9

## 1. ActionableScore

* Conservative score: 9
* Paranoid score: 10
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2. A crafted IPv6 packet can reach the SRv6 End.DX4 or End.DT4 decapsulation path when such an endpoint is configured.
* Memory corruption, strong corruption primitive: +2. The patch context describes a demonstrated KASAN slab-out-of-bounds write in `__ip_options_echo()`, with a 255-byte copy into a 40-byte option-data area.
* Strong semantic reinterpretation primitive: +2. Attacker-influenced IPv6 control-block state is later consumed as IPv4 IPCB option state because `skb->cb` is shared between IP6CB and IPCB.
* Paranoid semantic/lifetime impact: +1. The reinterpreted fields affect active IPv4 option processing and memory-copy length behavior.
* Reliable kernel crash / strong DoS: +1. Reproducers for both End.DX4 and End.DT4 produced slab OOB writes and a kernel crash condition.
* Confidentiality impact plausible: +1. Kernel heap corruption can plausibly lead to disclosure in worst-case analysis, although no direct leak is demonstrated.
* Integrity impact plausible: +1. The OOB write is a concrete kernel memory corruption primitive.
* Common networking packet-processing path: +1. The vulnerable path is in packet decapsulation and TCP IPv4 request processing, although gated by SRv6 configuration.
* Rare AND difficult-to-reach subsystem/configuration: -1. Practical exposure requires SRv6 local actions such as End.DX4 or End.DT4.

Conservative total: 9. Paranoid total: 10.

## 3. Reachability analysis

The bug is triggerable by a remote sender if the target is configured as an SRv6 endpoint using End.DX4, End.DT4, or the IPv4 arm of End.DT46. No local account or authentication on the victim is required once the affected SRv6 path is reachable. This is not a default exposure for ordinary Linux hosts, and SRv6 deployments are usually limited to provider, data-center, or controlled routing domains. However, in an exposed SRv6 deployment the trigger is packet-driven and does not require a local namespace or container privilege boundary.

Call-site confidence: high. The supplied input includes the relevant End.DX4 and End.DT4 call paths down to `__ip_options_echo()` and a concrete KASAN OOB write result.

## 4. Severity interpretation

This behaves like an Important-class kernel networking memory-corruption issue, not an ordinary Moderate DoS. The realistic demonstrated impact is remote kernel crash through slab out-of-bounds write. The theoretical worst case includes confidentiality and integrity impact because the primitive is an attacker-influenced kernel heap overwrite in a packet-processing path. Exploitability is constrained by the need for SRv6 local action configuration, but exposed SRv6 endpoints should not be auto-closed as low-risk.

## 5. One-sentence report phrase

A remote packet can cause stale IPv6 skb control-block fields to be interpreted as IPv4 option state after SRv6 IPIP decapsulation, leading to an attacker-influenced slab out-of-bounds write in `__ip_options_echo()` and a kernel crash with plausible broader memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: this is a remotely reachable kernel OOB write with demonstrated KASAN evidence, attacker-influenced semantic reinterpretation of shared skb control-block data, and plausible C/I/A impact beyond simple DoS in exposed SRv6 deployments.

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UPSTREAM PATCH SUMMARY
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Patch: ipv6: seg6: clear IPv4 control block on IPIP decapsulation
Commit: 10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667

Commit description:

End.DX4 and End.DT4 decapsulate an IPv4 packet through
decap_and_validate() and send it directly to IPv4 routing. The inner
packet therefore bypasses ip_rcv_core(), which normally clears IPCB
before IPv4 interprets skb->cb.

The skb instead retains IP6CB data from the outer packet. IP6CB and
IPCB use the same skb->cb storage, so IP6CB(skb)->lastopt overlaps
IPCB(skb)->opt.optlen and srr, while IP6CB(skb)->nhoff overlaps rr and
ts.

The sender can make the stale optlen byte nonzero with a valid outer
extension-header chain. The reproducers put an eight-byte Destination
Options header immediately after the 40-byte IPv6 header and before the
Segment Routing Header. ipv6_destopt_rcv() records the sender-controlled
Destination Options offset in both lastopt and nhoff, setting them to
40. On the reproduced little-endian x86-64 kernel, IPv4 therefore sees
optlen = 40 and rr = 40.

Both tcp_v4_save_options() and __ip_options_echo() skip option copying
when optlen is zero. Here optlen is 40, so the TCP SYN path allocates
room for 40 bytes of option data and calls __ip_options_echo(). The
stale rr value makes that function read inner packet byte 41 as the
Record Route option length. The reproducers set that sender-controlled
byte to 255, so __ip_options_echo() copies 255 bytes into the 40-byte
option-data area.

Separate End.DX4 and End.DT4 reproducers on the unpatched v7.2-rc5
kernel both produced:

  BUG: KASAN: slab-out-of-bounds in __ip_options_echo()
  Write of size 255

The relevant End.DX4 call path is:

  __ip_options_echo
  tcp_v4_route_req
  tcp_conn_request
  tcp_v4_conn_request
  tcp_rcv_state_process
  tcp_v4_do_rcv
  tcp_v4_rcv
  ip_protocol_deliver_rcu
  ip_local_deliver_finish
  ip_local_deliver
  input_action_end_dx4_finish
  input_action_end_dx4

The relevant End.DT4 call path is:

  __ip_options_echo
  tcp_v4_route_req
  tcp_conn_request
  tcp_v4_conn_request
  tcp_rcv_state_process
  tcp_v4_do_rcv
  tcp_v4_rcv
  ip_protocol_deliver_rcu
  ip_local_deliver_finish
  ip_local_deliver
  input_action_end_dt4

tcp_v4_save_options() is inlined into the tcp_v4_route_req() path, so
it does not appear as a separate frame.

When decap_and_validate() handles IPPROTO_IPIP, save the ingress
interface from IP6CB, clear IPCB, and restore the saved value. Doing
this in the common decapsulation path covers End.DX4, End.DT4, and
End.DT46's IPv4 arm.

Use IP6CB(skb)->iif rather than skb->skb_iif. These actions run after
l3mdev processing, which can replace skb_iif with the L3 master;
IP6CB iif still records the receiving interface set at IPv6 ingress.

Fixes: 891ef8d ("ipv6: sr: implement additional seg6local actions")
Cc: stable@vger.kernel.org
Suggested-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Co-developed-by: David Lee <david.lee@trailofbits.com>
Signed-off-by: David Lee <david.lee@trailofbits.com>
Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>

[Commit description truncated; see the upstream URL below]

Changed files:
  net/ipv6/seg6_local.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-80840 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80840

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 9.8

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 9
  Paranoid ActionableScore: 10

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).