[CVE-2026-98099][MODERATE REGULAR] ipv6: mcast: use rcu_assign_pointer() for __rcu list updates [ Upstream

[CVE-2026-98099][MODERATE REGULAR] ipv6: mcast: use rcu_assign_pointer() for __rcu list updates [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98099
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv6: mcast: use rcu_assign_pointer() for __rcu list updates [ Upstream
Commit: 2e46b0c9fcf7e10b64ce1630b925f47918d1f7f6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e46b0c9fcf7e10b64ce1630b925f47918d1f7f6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98099
Analysis date: Fri, 25 Sep 2026 15:24:17 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98099 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98099

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-2026-98099	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98099 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-662;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'IPv6 multicast list links were updated with raw pointer assignments while concurrent readers could traverse the same lists without locking under RCU. This violates the required RCU publication and access rules and may expose readers to inconsistent link visibility during multicast join, drop, socket close, or device group removal. The supplied code does not establish a concrete UAF, OOB access, information leak, or write primitive because the existing RCU deferred free mechanism is not removed. Impact is therefore expected to be incorrect multicast state or availability loss, with a kernel crash treated as the highest still defensible outcome. For the CVSS the PR:L reflects that a local process is needed to modify IPv6 multicast membership while a concurrent reader traverses the list. AC:H is used because successful triggering depends on a race window between the update and lockless traversal. Remote traffic may contribute to the reader side of the race, but network access alone does not normally provide control over the required local list update.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) 	MAYBE	REMOTE NETWORK IPV6  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +1 Local unprivileged trigger. An ordinary process can join, leave, and close IPv6 multicast sockets, reaching the affected socket membership list updates without host-admin privilege.
* +1 Common networking packet-processing path. Lockless traversal in `inet6_mc_check()` can occur during normal IPv6 multicast processing.
* Total: 2.

Paranoid-only signal:

* +1 Plausible kernel crash. Unsynchronized publication of an RCU list link may expose an inconsistent pointer to a lockless reader, although no actual crash is documented.
* Total: 3.

Not awarded:

* Real UAF was not awarded. The patch does not show an object being freed before the applicable RCU grace period.
* No RCU referenced-object lifetime mismatch was established.
* No controlled reclaim, arbitrary write, disclosure, repeated release, refcount mutation, or structured post-free reuse is demonstrated.
* No remote-reachability bonus was awarded. Remote packets can exercise the reader, but a local operation is needed to modify the membership list.
* No race penalty was applied. The patch establishes concurrency but does not show a narrow, one-shot, or materially uncontrollable race.
* No additional availability point was awarded because there is no distinct persistent effect beyond the paranoid crash hypothesis.
* No confidentiality, integrity, LPE, or security-boundary signal was awarded.

3. Reachability analysis

The socket-level paths are reachable by an ordinary local user through IPv6 multicast membership operations and socket close. These operations do not normally require host-admin-equivalent capability, so `HostAdminRequired=NO`.

No namespace-scoped capability is required for this attacker path, so `NamespaceScopedPrivilege=NO`. Some device-level multicast management paths may have stronger requirements, but they are not necessary to reach the affected socket-list update.

IPv6 networking and a usable interface are required, but there is no special hardware, unusual boot mode, or separate privileged setup established. Incoming traffic can contribute to concurrent lockless traversal, but network access alone does not perform the required writer-side list update. Repeated local join, drop, and close operations are inexpensive, so no hard-race penalty is justified.

4. Primitive and severity interpretation

The exact defect is inconsistent use of RCU access and publication primitives for IPv6 multicast list links. Writers used raw assignments while readers could traverse the lists locklessly.

The patch does not establish a lifetime violation. In particular, it does not show that `ipv6_mc_socklist` or `ifmcaddr6` objects are released before readers complete their RCU critical sections. A reader observing either the old or new valid link is therefore not by itself a UAF.

This is classified as `CLASS_C` because the supported behavior is an RCU synchronization defect with possible invalid-state or availability consequences, not an independently useful corruption primitive. The demonstrated evidence does not support Important-grade exploitation leverage. Automatic downgrade remains allowed, but manual review is recommended to verify the surrounding deferred-free paths and confirm that all removed objects remain alive through the relevant grace periods.

5. One-sentence report phrase

Raw updates to RCU-protected IPv6 multicast list links can race with lockless readers and potentially cause inconsistent traversal or a kernel crash, but the supplied code does not establish a UAF or another exploitable memory-corruption primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Verify that every removed multicast list object is released only after the correct RCU grace period and that no unshown path performs immediate destruction.

7. Call-site confidence

Call-site confidence: medium

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

Patch: ipv6: mcast: use rcu_assign_pointer() for __rcu list updates [ Upstream
Commit: 2e46b0c9fcf7e10b64ce1630b925f47918d1f7f6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e46b0c9fcf7e10b64ce1630b925f47918d1f7f6

Changed files:
  net/ipv6/mcast.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=2e46b0c9fcf7e10b64ce1630b925f47918d1f7f6

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

The original Linux kernel CVE announcement for CVE-2026-98099 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98099

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 3

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).