[CVE-2026-63969][MODERATE REGULAR] ipv6: fix possible infinite loop in rt6_fill_node() [ Upstream

[CVE-2026-63969][MODERATE REGULAR] ipv6: fix possible infinite loop in rt6_fill_node() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63969
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv6: fix possible infinite loop in rt6_fill_node() [ Upstream
Commit: b014a63d2f2c2c767762b548381882dfb1655529
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b014a63d2f2c2c767762b548381882dfb1655529
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63969
Analysis date: Sun, 19 Jul 2026 22:26:36 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
An RCU race in IPv6 route dumping can make rt6_fill_node() walk a detached fib6 sibling ring indefinitely, causing a CPU stall or soft lockup during local routing control-plane activity.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-63969 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63969

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: MODERATE REGULAR
Manual review required: YES

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

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-63969	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;*CWE-835;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'An RCU race in IPv6 route dumping can make rt6_fill_node loop over an old fib6 sibling ring after a route has been detached with list_del_rcu. The reader can fail to reach its original list head and may spin until a CPU stall or soft lockup occurs. For the CVSS the PR:L is used for the paranoid score because a local user or delegated network namespace actor may be able to trigger route dumps while route churn is happening, although host route modification is often restricted. The issue is not directly network reachable and is triggered through local routing control plane activity rather than packet processing. Impact is denial of service via CPU stall only. No confidentiality or integrity impact is assigned because the observed primitive is an infinite loop in an RCU list walk, not a demonstrated UAF dereference or write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3)	YES	REMOTE DEADLOCK SIMPLEFIX NETWORK HARDWARE IPV6  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1
  The commit explicitly describes an infinite loop in rt6_fill_node() causing CPU stall / soft lockup.
* Broad/default/common subsystem: +1
  IPv6 routing is a broadly deployed kernel networking subsystem.
* Hard or unreliable race / special timing required: -1
  Triggering depends on concurrent RCU-side route dumping and route deletion / churn.
* Requires CAP_NET_ADMIN in typical deployments: -1
  Route churn normally requires network administration privileges, but the full -2 penalty is softened because delegated network namespaces or containerized network administration can lower practical privileges.

Conservative total: 2

Paranoid additional / adjusted signals:

* Local unprivileged or low-privileged trigger path: +1
  Route dumps may be reachable by local users, and delegated network namespaces can make route churn available to less trusted actors.
* Availability impact realistic: +1
  The failure mode is a CPU stall / soft lockup in kernel networking code, which can affect system responsiveness beyond a harmless warning.
* Hard race penalty not applied in paranoid score: +0 instead of -1
  The issue was reported by automated testing and resembles a previously fixed pattern, so under route churn it may be reproducible enough to justify manual review.

Paranoid total: 4

No memory-corruption signals are awarded. The patch shows stale RCU list traversal causing an infinite loop, not UAF dereference, write-after-free, object reuse, type confusion, arbitrary write, or information disclosure.

## 3. Reachability analysis

The affected path is IPv6 route dumping / route serialization in rt6_fill_node(). A realistic trigger requires a reader walking fib6_siblings while a writer removes a route with list_del_rcu(), leaving the reader on an old sibling ring.

In normal host deployments, modifying IPv6 routes usually requires CAP_NET_ADMIN, while route dumping may be available to less privileged local users. In container or delegated network namespace setups, an attacker with network administration rights inside a namespace may be able to create the required route churn without full host-root privileges. The issue is not directly network-triggerable by packets and should be treated as local control-plane DoS.

## 4. Severity interpretation

This behaves like a Borderline / actionable Moderate DoS issue, not an Important memory-corruption issue. The theoretical concern is an RCU race in core networking code, but the demonstrated primitive is an infinite loop / CPU stall only. There is no evidence in the patch of attacker-controlled reclaim, stale object reuse, writable stale access, callback dispatch, or privilege escalation.

The conservative score is low because reliable triggering requires route churn and timing. The paranoid score reaches manual-review range because IPv6 routing is common, the bug is an RCU race in networking code, and the availability impact can be system-wide.

## 5. One-sentence report phrase

An RCU race in IPv6 route dumping can make rt6_fill_node() walk a detached fib6 sibling ring indefinitely, causing a CPU stall or soft lockup during local routing control-plane activity.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is likely DoS-only, but it is an RCU race in IPv6 routing with kernel-wide availability impact and possible delegated namespace reachability.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: ipv6: fix possible infinite loop in rt6_fill_node() [ Upstream
Commit: b014a63d2f2c2c767762b548381882dfb1655529
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b014a63d2f2c2c767762b548381882dfb1655529

Changed files:
  net/ipv6/route.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=b014a63d2f2c2c767762b548381882dfb1655529

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

The original Linux kernel CVE announcement for CVE-2026-63969 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63969

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:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2
  Paranoid ActionableScore: 4

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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).