[CVE-2026-90111][IMPORTANT] ip6mr: do not clone dst in ip6mr_cache_report() [ Upstream

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

CVE: CVE-2026-90111
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ip6mr: do not clone dst in ip6mr_cache_report() [ Upstream
Commit: 4a674afaae4136c71943aa3e35d433676fd40e8f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4a674afaae4136c71943aa3e35d433676fd40e8f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90111
Analysis date: Thu, 17 Sep 2026 20:29:51 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90111 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90111

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
======================================================================

CVE-2026-90111	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-90111 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'An IPv6 multicast routing use after free can occur because an ingress skb may carry a non refcounted NOREF dst that is only safe during the receive side RCU window. When an MFC lookup misses, the skb can be queued on the unresolved multicast forwarding queue and can outlive that RCU window. If the underlying route is deleted and the queued entry is later resolved through a wrong parent interface, ip6mr_cache_report can operate on a freed dst and trigger a slab use after free. For the CVSS the PR:N is used because a remote packet sender does not need a local account on the target when IPv6 multicast routing is exposed. The realistic impact is at least denial of service via kernel crash. In the worst case, because this is a network reachable kernel UAF, confidentiality and integrity impact may also be possible and should be reviewed manually. Practical exploitation depends on IPv6 multicast routing being enabled and reachable, and usually requires access to an internal multicast routing domain rather than the public Internet.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 6)	YES	REMOTE UAF NETWORK SKB IPV6 KPANIC PACKET 	YES	NO	checked

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

ActionableScore=7
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap changed the score.
* Raw paranoid score: 7
* Paranoid score: 7. No floor or cap changed the score.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF / lifetime violation. A NOREF dst attached to an ingress skb can escape the receive-side RCU lifetime when the skb is queued on the unresolved IPv6 multicast forwarding queue, and later dst_clone(skb_dst(pkt)) can operate on a freed dst.
* +1 Reliable kernel crash / strong DoS. The commit explicitly describes a slab use-after-free, which realistically supports kernel crash or panic.
* +1 Concrete RCU referenced-object lifetime mismatch. The skb remains queued beyond the RCU-safe receive path while still carrying a reference-like pointer to a dst object whose lifetime was not extended.
* +1 Concrete post-lifetime refcount mutation. dst_clone() normally takes a reference via the dst lifetime/refcount machinery, so the stale operation is not only a passive read. It can mutate refcount/lifetime state through storage whose original dst lifetime may already have ended.

Paranoid additional signal:

* +2 Remote reachable / network-triggerable. In an exposed IPv6 multicast routing setup, attacker-controlled multicast traffic can reach the vulnerable MFC miss and report path without a local account on the target.

Important non-awarded signals:

* Strong exploitation leverage +2 not awarded. Controlled reclaim, attacker-shaped replacement contents, arbitrary write, callback control, or demonstrated LPE are not established.
* Structured stale-object leverage +1 not separately awarded. The meaningful stale operation already counted is the dst_clone refcount/lifetime mutation.
* Weak LPE +1 not separately awarded. It would double-count the same post-lifetime refcount mutation.
* Extra availability +1 not awarded. The demonstrated availability impact is the same kernel crash already counted.
* Confidentiality and integrity bonuses not awarded. C/I impact remains possible in a worst-case UAF analysis, but no disclosure, arbitrary write, or concrete privilege-escalation primitive is shown.
* Race/timing penalty not applied. The bug involves a lifetime/RCU window and several state conditions, but the evidence does not prove a narrow one-shot timing race that is externally uncontrollable.
* Rare subsystem penalty not applied. IPv6 multicast routing is configuration-dependent and usually internal, but that alone is not concrete evidence for a negative score.

Repeated/refcount checklist:

* First lifetime-ending event: route deletion can free the dst object because the skb holds only a NOREF dst pointer.
* Exact stale reference survives: yes, skb_dst(pkt) remains attached to the queued skb.
* Second lifetime/refcount mutation: yes, dst_clone(skb_dst(pkt)) can invoke dst refcount handling on the freed dst.
* Dedicated +1 awarded: yes, as concrete post-lifetime refcount mutation.
* Paranoid repeated-refcount +1 separately awarded: no. The refcount mutation is already counted once and not used to stack an additional speculative heap-reuse point.
* FLOOR 5 applied: not needed. Raw conservative score already reaches 5.

3. Reachability analysis

The vulnerable path is in IPv6 multicast routing. An ingress multicast packet that misses the MFC lookup can be queued by ip6mr_cache_unresolved(). If the underlying route is later deleted and the MFC entry is resolved with a wrong parent interface, ip6_mr_forward() can call ip6mr_cache_report(..., MRT6MSG_WRONGMIF), reaching dst_clone() on the stale dst.

The likely external attacker is a remote or adjacent packet sender inside an IPv6 multicast routing domain. No local user account is needed for the packet-sending stage when the multicast routing service is exposed. However, the complete sequence also depends on multicast routing being enabled and on route/MFC state transitions. The supplied evidence does not fully prove that a remote attacker alone controls every state transition, so HostAdminRequired is UNKNOWN rather than NO.

Namespace-scoped privilege is not the relevant model here. This is packet-path exposure, not a demonstrated namespaced CAP_NET_ADMIN control-plane trigger.

Prior privileged setup may exist in practice because IPv6 multicast routing and mroute userspace handling are normally configured by an administrator. This does not mean the attacker must be host admin after the service is exposed.

No special hardware is required. Race repeatability is state-dependent, but not proven to be a hard one-shot race.

4. Primitive and severity interpretation

The exact primitive is a real slab use-after-free of a dst object caused by carrying a non-refcounted NOREF dst pointer outside its RCU-safe lifetime. The stale object is an active structured routing object, not fixed stale storage or a passive buffer. The stale use is not merely cleanup-only: ip6mr_cache_report() calls dst_clone() on the stale dst, which can touch lifetime/refcount state.

This is CLASS_B. It is a real kernel lifetime/corruption primitive with meaningful unresolved exploitation questions, but the supplied evidence does not establish CLASS_A primitives such as controlled reclaim, arbitrary kernel read/write, callback control, or demonstrated privilege escalation.

The demonstrated consequence is kernel crash / DoS. The speculative consequence is stronger exploitation from a network-reachable kernel UAF, but that remains unresolved and should not be treated as proven C/I impact.

ImportantCandidate=YES because this is a network-influenced active-object UAF with post-lifetime refcount mutation, reported by ZDI, and the exploitation leverage beyond crash is unresolved. AutoDowngradeAllowed=NO because automatically closing it as ordinary Moderate would discard a concrete kernel lifetime primitive on a packet-processing path.

Likely operational severity: at least Actionable Moderate, and a Strong Important candidate for manual review in products where IPv6 multicast routing is exposed to untrusted or semi-trusted networks.

5. One-sentence report phrase

An IPv6 multicast routing UAF can occur when a queued unresolved multicast skb keeps a NOREF dst beyond its RCU lifetime and later ip6mr_cache_report() calls dst_clone() on the freed dst, allowing a network-adjacent attacker in an exposed multicast routing domain to trigger at least kernel crash DoS and requiring manual review for possible stronger UAF impact.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: concrete slab UAF in a network packet path, involving an active routing object and a post-lifetime refcount operation, with unresolved exploitation leverage beyond the demonstrated crash.

7. Call-site confidence

Call-site confidence: high

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

Patch: ip6mr: do not clone dst in ip6mr_cache_report() [ Upstream
Commit: 4a674afaae4136c71943aa3e35d433676fd40e8f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4a674afaae4136c71943aa3e35d433676fd40e8f

Changed files:
  net/ipv6/ip6mr.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=4a674afaae4136c71943aa3e35d433676fd40e8f

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

The original Linux kernel CVE announcement for CVE-2026-90111 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90111

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:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 5
  Paranoid ActionableScore: 7

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).