[CVE-2026-98568][IMPORTANT] vxlan: use one headroom snapshot for neighbour replies [ Upstream

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

CVE: CVE-2026-98568
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: vxlan: use one headroom snapshot for neighbour replies [ Upstream
Commit: 45a67662bf8ce3aa5643accf98d328dfbb47eaf6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=45a67662bf8ce3aa5643accf98d328dfbb47eaf6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98568
Analysis date: Sun, 18 Oct 2026 22:10:19 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A reproducible TOCTOU race between VXLAN neighbour reply creation and link headroom updates causes a confirmed kernel heap out-of-bounds write, requiring manual review because adjacent-object corruption and below-host-admin namespace reachability remain unresolved.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98568 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98568

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
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CVE-2026-98568	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98568 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-362;CWE-367;*CWE-787;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A TOCTOU race in vxlan_na_create can cause an out of bounds write because LL_RESERVED_SPACE was sampled separately for skb allocation and headroom reservation. A concurrent vxlan_changelink operation can increase needed_headroom between these reads, after which skb_push and Ethernet header construction may write beyond the allocated skb head. KASAN confirmed a four byte write two bytes beyond a 704 byte skbuff_small_head allocation. For the CVSS the PR:L reflects that reliable triggering normally requires CAP_NET_ADMIN or equivalent control over a VXLAN interface in an accessible network namespace. AC:H is used because exploitation requires a timing race between neighbour reply generation and a link configuration update. The issue is not directly remotely exploitable because network traffic can initiate neighbour reply processing but cannot independently perform the required vxlan_changelink operation. Impact is at least a local denial of service through kernel memory corruption and a possible kernel crash. In the worst defensible case the heap out of bounds write may affect adjacent kernel data and could permit confidentiality loss, integrity impact, or privilege escalation, but this requires manual validation. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied. final reconciliation stage IMPORTANT to IMPORTANT because KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and V33_ACTIVE_STATE_BOUNDARY_CANDIDATE and IMPORTANT_CANDIDATE_MANUAL_REVIEW and KP_ONLY_DOWNGRADE_VETO and V42_LEGACY_CLASSA_FD_VETO and MANUAL_REVIEW_REQUIRED and TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	KASAN OOB SIMPLEFIX RACE NETWORK SKB LINUS IPV6 INCREASED_TO_HIGH_BASED_ON_GUESSCVSS V33_ACTIVE_STATE_BOUNDARY_CANDIDATE IMPORTANT_CANDIDATE_MANUAL_REVIEW KP_ONLY_DOWNGRADE_VETO V42_LEGACY_CLASSA_FD_VETO MANUAL_REVIEW_REQUIRED TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 5
* Raw paranoid score: 3
* Paranoid score: 5
* Applied manual-review safety floor: 3 -> 5 because a confirmed attacker-influenced kernel heap OOB write must not be automatically closed.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Strong corruption primitive. A TOCTOU race allows the second headroom value to exceed the allocation basis, and KASAN confirmed a four-byte heap OOB write.
* +1 Realistic kernel crash or strong DoS. Corruption occurs beyond an active `skbuff_small_head` allocation and can plausibly cause an Oops or panic, although a non-KASAN crash is not demonstrated.
* +0 Remote reachability. Network traffic can initiate neighbour-reply generation, but it cannot independently perform the required `vxlan_changelink()` operation.
* +0 Local unprivileged trigger. Complete below-host-admin reachability is not established.
* +0 Race penalty. The race was reproducible, and the evidence does not establish that it is a narrow one-shot or externally uncontrollable race.
* +0 Additional confidentiality or integrity consequence. The adjacent victim, useful overwritten field, disclosure path, and privilege-escalation chain are unresolved.
* +0 Additional availability. The possible crash is already counted once.
* +0 V33 active-state trust-boundary signal. A privileged downstream consumer of specifically corrupted adjacent state and a concrete guest or namespace boundary are not established.
* +0 NEW52 B5-R. There is no retry-accumulated index or offset. This is a two-snapshot TOCTOU allocation mismatch.
* +0 NEW53 B3-PENDING. No shared or file-backed storage write is established.
* +0 NEW54 B4-PENDING. No authorization decision or omitted security guard is involved.

No privilege, hardware, rare-subsystem, or race penalty is applied because the supplied evidence does not positively establish the required reducing conditions.

3. Reachability analysis

The complete trigger requires neighbour-reply generation in `vxlan_na_create()` concurrent with a `vxlan_changelink()` update that changes `needed_headroom`. The link update normally involves `CAP_NET_ADMIN`, but the supplied context does not resolve whether the complete operation requires host-level administration or can be performed entirely in an attacker-created user and network namespace.

Network traffic may drive the reply side of the race, but traffic alone cannot update link headroom. Therefore direct remote reachability is not established. No special hardware, boot option, or sysctl prerequisite is shown. The race is reproducible and both operations may be repeatable, so no hard-race deduction is justified.

4. Primitive and severity interpretation

The exact primitive is a confirmed heap out-of-bounds write caused by inconsistent headroom snapshots. Allocation uses the first `LL_RESERVED_SPACE()` value, while reservation formerly used a later value that could increase concurrently. The subsequent Ethernet-header construction then writes beyond the allocated skb head.

This is not a UAF, repeated release, shared-backing write, authorization bypass, or retry-accumulated B5-R case. It is CLASS_A because an attacker-influenced configuration race produces an actual kernel heap overwrite. The demonstrated consequence is four bytes written two bytes beyond a 704-byte allocation under KASAN. A kernel crash is plausible. Corruption of a useful adjacent object, information disclosure, arbitrary write, privilege escalation, and namespace-to-host escape remain unproven.

ImportantCandidate is YES because a concrete heap overwrite exists and its adjacent-object consequences require exploitation-leverage analysis. AutoDowngradeAllowed is NO because automatic closure would discard an established kernel-corruption primitive, even though Important severity and LPE are not proven.

5. One-sentence report phrase

A reproducible TOCTOU race between VXLAN neighbour reply creation and link headroom updates causes a confirmed kernel heap out-of-bounds write, requiring manual review because adjacent-object corruption and below-host-admin namespace reachability remain unresolved.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes an actual heap OOB write, while the affected adjacent object, payload leverage, complete capability boundary, and potential privilege-escalation consequences require source-level validation.

7. Call-site confidence

Call-site confidence: high

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

Patch: vxlan: use one headroom snapshot for neighbour replies [ Upstream
Commit: 45a67662bf8ce3aa5643accf98d328dfbb47eaf6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=45a67662bf8ce3aa5643accf98d328dfbb47eaf6

Changed files:
  drivers/net/vxlan/vxlan_core.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=45a67662bf8ce3aa5643accf98d328dfbb47eaf6

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

The original Linux kernel CVE announcement for CVE-2026-98568 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98568

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: unknown
  Paranoid ActionableScore: 5

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: NO
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).