[CVE-2026-63968][MODERATE REGULAR] ipv6: fix possible infinite loop in fib6_select_path() [ Upstream

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

CVE: CVE-2026-63968
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv6: fix possible infinite loop in fib6_select_path() [ Upstream
Commit: 3948a7d92f7678e89e1776bb2d169afcad63b1ae
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3948a7d92f7678e89e1776bb2d169afcad63b1ae
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63968
Analysis date: Sun, 19 Jul 2026 23:30:26 -0400
ActionableScore: 4
ActionableScore lower bound: 0
Actionable bucket: Borderline manual review recommended / DoS-only network-path race
Manual review required: YES

Summary:
An IPv6 routing race in `fib6_select_path()` can cause a softirq-side infinite loop during concurrent sibling route deletion, leading to CPU soft lockup and denial of service under high route churn.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63968 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63968

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-63968	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-362;*CWE-835;**CWE-400;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.9';DESCR 'An IPv6 routing race can make fib6_select_path loop indefinitely when an RCU reader walks a fib6_siblings ring while a writer removes the first sibling without waiting for readers. The stale next pointer can keep the softirq side walker inside the old ring and prevent it from reaching the expected list terminator, causing a CPU soft lockup. For the CVSS the PR:N is used in the paranoid score because packet processing can reach this path without local privileges if an attacker can send IPv6 traffic to a routing target during concurrent nexthop churn. The reliable base case is more restricted because creating the required route churn usually needs local routing control. The issue is network reachable only under specific routing and churn conditions, so AC:H is kept. Impact is denial of service only through CPU lockup and routing path stall.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / DoS-only network-path race (with actual score 2)	YES	REMOTE DEADLOCK SIMPLEFIX NETWORK IPV6  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended / DoS-only network-path race**

## 2. Signal breakdown

Paranoid score signals:

* Remote reachable / network-triggerable: +2
  `fib6_select_path()` is in the IPv6 routing packet path, so packet processing can reach the vulnerable walker if the target is forwarding or routing IPv6 traffic.
* Reliable kernel crash / strong DoS: +1
  The described failure is an infinite loop in softirq context, causing CPU soft lockup.
* Broad/default/common subsystem or broadly deployed exposure: +1
  IPv6 routing is a common kernel networking subsystem.
* Common networking core path or packet-processing path: +1
  The affected function is part of IPv6 route selection.
* Hard or unreliable race / special timing required: -1
  Triggering requires overlap between RCU-side route selection and concurrent sibling route deletion or high nexthop churn.

Conservative score signals:

* Reliable kernel crash / strong DoS: +1
* Broad/default/common subsystem or broadly deployed exposure: +1
* Common networking core path or packet-processing path: +1
* Hard or unreliable race / special timing required: -1
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2
  Reliable creation of the required route churn normally requires privileged routing control.

No memory-corruption, LPE, confidentiality, or integrity signals are counted. The stale list pointer is used as a loop-termination problem under RCU semantics, not as a demonstrated UAF, overwrite, object reuse, or disclosure primitive.

## 3. Reachability analysis

A local privileged actor with routing control can most reliably trigger the condition by creating high IPv6 nexthop or sibling route churn. A remote attacker may be able to exercise the affected packet path by sending IPv6 traffic to a router during concurrent route churn, but the attacker generally does not control the required writer-side deletion unless another management process is changing routes. Network namespaces can lower practical privilege in containerized setups if an untrusted actor has CAP_NET_ADMIN in a relevant namespace with IPv6 routing objects, but typical host-wide route manipulation is privileged. The path is common networking code, but realistic exploitation requires specific timing and multipath or sibling route activity.

## 4. Severity interpretation

This behaves like a DoS-only race rather than an Important-class memory corruption issue. The theoretical network angle makes it worth manual review, especially for routers or systems with frequent IPv6 route churn, but the patch does not support privilege escalation, data disclosure, arbitrary write, UAF reclaim, or type confusion. Conservative handling is ordinary Moderate or lower when only privileged local route control can create the condition. Paranoid handling is borderline actionable because the affected walker is in a packet-processing path and can soft-lock a CPU.

## 5. One-sentence report phrase

An IPv6 routing race in `fib6_select_path()` can cause a softirq-side infinite loop during concurrent sibling route deletion, leading to CPU soft lockup and denial of service under high route churn.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the bug is in a common IPv6 packet path and has a defensible network-reachable paranoid interpretation, but it is DoS-only and requires a hard race with concurrent route churn.

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

Patch: ipv6: fix possible infinite loop in fib6_select_path() [ Upstream
Commit: 3948a7d92f7678e89e1776bb2d169afcad63b1ae
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3948a7d92f7678e89e1776bb2d169afcad63b1ae

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=3948a7d92f7678e89e1776bb2d169afcad63b1ae

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

The original Linux kernel CVE announcement for CVE-2026-63968 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63968

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 0
  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).