[CVE-2026-98102][MODERATE REGULAR] ipv6: mcast: fix RCU list diversion in ip6_mc_del1_src() [ Upstream

[CVE-2026-98102][MODERATE REGULAR] ipv6: mcast: fix RCU list diversion in ip6_mc_del1_src() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98102
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv6: mcast: fix RCU list diversion in ip6_mc_del1_src() [ Upstream
Commit: 4352737297b262e8367675c19a7dc6a9f53c97af
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4352737297b262e8367675c19a7dc6a9f53c97af
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98102
Analysis date: Fri, 25 Sep 2026 07:08:15 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98102 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98102

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: NO

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

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

CVE-2026-98102	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98102 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:L/A:L';CWE-362;CWE-284;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '3.6';DESCR 'An RCU race in IPv6 multicast source filter deletion can redirect a lockless reader from the active source list to the tombstone list before the RCU grace period has elapsed. A local process able to create and remove IPv6 multicast source filters may trigger incorrect source specific multicast filtering decisions, causing remaining active sources to be skipped and deleted sources to be examined. For the CVSS the PR:L is used because a local process must control multicast membership or source filter operations. The issue is not directly network reachable because network traffic can exercise the reader path but cannot by itself perform the required local filter deletion. Impact is primarily limited policy enforcement failure and possible multicast service disruption. No use after free, arbitrary write, information leak, or plausible privilege escalation primitive is shown by the patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	REMOTE SIMPLEFIX NETWORK IPV6  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS	YES	NO	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap was applied.

2. Signal breakdown

* +1 Local unprivileged trigger. IPv6 multicast membership and source filter operations are normally performed through socket multicast APIs and do not inherently require host administrator privileges. A local process can plausibly create and remove its own IPv6 source filters.
* +0 RCU race. The issue is a synchronization and list-linkage race, but a race alone does not receive a score increase.
* +0 No real UAF. The active `psf` object is not freed before the RCU reader can use it. The pre-fix bug changes `psf->sf_next` while readers may still traverse the object. The fix retires the original object through `kfree_rcu`.
* +0 No controlled reclaim, arbitrary write, refcount mutation, callback control, or attacker-controlled post-free object reuse is established.
* +0 No direct remote trigger. Network traffic can execute the lockless lookup path, but a local source-filter deletion operation is required to create the inconsistent list transition.
* +0 No reliable kernel crash, panic, memory corruption, disclosure, privilege escalation, or host-wide availability impact is demonstrated.
* +0 Integrity impact was not awarded. Incorrect multicast source-filter decisions are possible, but the supplied patch does not establish unauthorized modification of protected kernel state, another principals data, credentials, or policy objects.
* +0 Availability impact was not awarded. The visible result is incorrect multicast filtering behavior, not a demonstrated service-wide or kernel-wide denial of service.
* -0 No host-admin penalty. Host-admin-equivalent privilege is not required for normal socket-level multicast membership operations.
* -0 No hard-race penalty. The patch establishes concurrent RCU traversal, but does not establish that the timing window is unusually narrow, one-shot, externally uncontrollable, or difficult to retry.

3. Reachability analysis

The evaluated attacker is a local process that can create and remove IPv6 multicast source filters through normal socket operations. The relevant state transition is deletion of a source filter while concurrent IPv6 multicast traffic executes `ipv6_chk_mcast_addr` or another RCU-protected traversal of `pmc->mca_sources`.

The issue is not directly remotely triggerable. A remote sender may cause multicast receive-path lookups, but cannot independently perform the required local source-filter deletion unless it also controls a local process or another local control-plane interface.

HostAdminRequired is NO because source-specific multicast membership operations are normally socket operations available below host-admin privilege. NamespaceScopedPrivilege is UNKNOWN because the supplied patch does not establish that the complete source-filter setup, concurrent traffic path, and relevant interface configuration can all be created by an attacker solely within an attacker-controlled namespace.

No special hardware, boot option, privileged firmware state, or prior host-created device topology is indicated. Race repeatability is plausible through repeated filter add and remove activity plus multicast traffic, but the patch does not establish a stronger attacker-controlled corruption primitive.

4. Primitive and severity interpretation

The exact primitive is an RCU list-diversion logic error. A reader legitimately traversing `pmc->mca_sources` can reach an unlinked but still RCU-visible `psf` object. Before the RCU grace period ends, the writer overwrites `psf->sf_next` so that the reader follows the tombstone list rather than the remaining active-source list.

This is not a real use-after-free. The object remains alive during the RCU reader lifetime and is only retired through `kfree_rcu` in the fixed code. There is no first release followed by a second release, no stale refcount mutation, no controlled object replacement, and no post-free structured-field consumption.

The behavior is best classified as CLASS_C because it produces an invalid filtering state and potentially incorrect source-specific multicast decisions. The demonstrated consequence is that active sources can be skipped and deleted tombstone entries can be examined. A practical consequence may be incorrect delivery or rejection of multicast packets for affected memberships.

ImportantCandidate is NO because no meaningful kernel corruption, active-object UAF, authorization bypass, ownership violation, controlled memory access, or privilege-escalation primitive is established. AutoDowngradeAllowed is YES because the patch-supported behavior is constrained to RCU list traversal diversion and incorrect multicast filter semantics. It does not leave unresolved lifetime corruption leverage that would be lost through automatic downgrade.

Likely operational severity is low. Manual review remains useful to verify whether a specific product uses IPv6 source-specific multicast filtering as an access-control boundary between tenants, services, or security domains. Such a product-specific deployment could change the practical impact, but that consequence is not established by the patch alone.

5. One-sentence report phrase

An RCU list-linkage race in IPv6 multicast source filter deletion can cause lockless readers to traverse tombstone entries instead of remaining active source filters, resulting in incorrect multicast source-filter decisions without demonstrated memory corruption or privilege escalation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review whether the affected deployment uses IPv6 source-specific multicast filtering as a meaningful tenant, service, or traffic-admission boundary. The upstream evidence alone supports a correctness and filtering-policy issue, not an Important-class kernel-corruption primitive.

7. Call-site confidence

Call-site confidence: medium

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

Patch: ipv6: mcast: fix RCU list diversion in ip6_mc_del1_src() [ Upstream
Commit: 4352737297b262e8367675c19a7dc6a9f53c97af
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4352737297b262e8367675c19a7dc6a9f53c97af

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=4352737297b262e8367675c19a7dc6a9f53c97af

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

The original Linux kernel CVE announcement for CVE-2026-98102 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98102

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:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS score: 3.6

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: unknown
  Paranoid ActionableScore: 1

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).