[CVE-2026-98526][IMPORTANT] llc: reserve device headroom for allocated frames

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

CVE: CVE-2026-98526
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: llc: reserve device headroom for allocated frames
Commit: 018020f5a48cc86fdd6dc2d380f6c70443ae7639
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=018020f5a48cc86fdd6dc2d380f6c70443ae7639
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98526
Analysis date: Sat, 24 Oct 2026 08:21:42 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
Crafted adjacent LLC traffic can trigger an skb heap underwrite when a VLAN response needs more link-layer headroom than `llc_alloc_frame()` reserved.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98526 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98526

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-2026-98526	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98526 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | 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:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-124;CWE-131;*CWE-787;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.8';DESCR 'An LLC response allocated for a stacked Ethernet device can reserve only the ordinary Ethernet header size even though a VLAN device needs additional link layer headroom. vlan_dev_hard_header can then push a VLAN header before eth_header pushes the lower Ethernet header, causing an skb headroom underflow and an out of bounds kernel write. A link adjacent attacker may trigger the response with crafted LLC traffic sent to an affected VLAN interface, although practical reachability requires LLC processing on that interface and the traffic is not normally routable. For the CVSS the PR:N value is used because no local account or prior privileges are required once the attacker can reach the affected layer 2 segment. Impact is at least a kernel crash and host denial of service. Since the primitive is a real kernel buffer underwrite, corruption of adjacent memory and privilege escalation remain plausible worst cases but are not demonstrated as reliable exploitation. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 8.8 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 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 V59_LEGACY_CLASSA_KP_VETO and MANUAL_REVIEW_REQUIRED and TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 3.1 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	REMOTE OOB SIMPLEFIX SKB HARDWARE LINUS V59_LEGACY_CLASSA_KP_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=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 5.
* Conservative score: 5; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; stronger confidentiality, integrity, or LPE consequences are not proven.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 — Strong corruption primitive: allocation reserves only `sizeof(struct ethhdr)`, while a stacked VLAN header plus lower Ethernet header can be pushed, producing a confirmed heap-backed skb headroom underwrite.
* +2 — Clearly network-reachable adjacent trigger: supplied evidence grounds crafted LLC traffic from an L2-adjacent, unauthenticated actor against an affected VLAN interface.
* +1 — Reliable kernel crash/strong DoS is plausible from writing before the skb allocation.
* No extra integrity/confidentiality point: no named adjacent object, protected state change, disclosure, controlled target, or LPE is demonstrated.
* No extra availability point: it would duplicate the same crash.
* No privilege, race, rarity, or hardware penalty is supported.
* NEW52/B5-R does not apply: this is an initial headroom-sizing error, not an offset accumulated across retries.
* NEW53 and B4-PENDING do not apply: neither shared backing nor an authorization decision is involved.

3. Reachability analysis

An L2-adjacent attacker can send crafted LLC traffic without a local account, UID 0, or effective host-administrator capability. An affected stacked VLAN interface and LLC processing must already exist, so deployment configuration is a prerequisite, but the supplied evidence does not show that the later packet trigger itself requires privilege. No namespace capability is part of the demonstrated path. There is no timing race or special hardware requirement.

4. Primitive and severity reading

The primitive is a real out-of-bounds write before an skb’s allocated data area: `vlan_dev_hard_header()` consumes extra headroom, after which `eth_header()` writes the lower Ethernet header beyond the reserved boundary. This is fixed skb storage, not a UAF or cleanup-path lifetime defect. The overwrite location is structurally determined and arbitrary target selection is not shown, but an attacker-triggerable heap underwrite is independently sufficient for CLASS_A review. Crash/host DoS is plausible; adjacent-object corruption, LPE, and C/I impact remain unproven. ImportantCandidate is YES and automatic downgrade is disallowed because the concrete kernel write primitive would otherwise be discarded.

5. One-sentence report phrase

Crafted adjacent LLC traffic can trigger an skb heap underwrite when a VLAN response needs more link-layer headroom than `llc_alloc_frame()` reserved.

6. Manual review advice

MANUAL CHECK REQUIRED

The confirmed network-triggerable heap OOB write warrants exploit-leverage and affected-configuration review despite the absence of demonstrated arbitrary write or LPE.

7. Call-site confidence

Call-site confidence: medium

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UPSTREAM PATCH SUMMARY
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Patch: llc: reserve device headroom for allocated frames
Commit: 018020f5a48cc86fdd6dc2d380f6c70443ae7639
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=018020f5a48cc86fdd6dc2d380f6c70443ae7639

Commit description:

llc_alloc_frame() reserves link-layer headroom using the device type.
This is insufficient for stacked Ethernet devices such as VLAN devices,
where vlan_dev_hard_header() pushes a VLAN header before the lower
device's Ethernet header. An LLC response on such a device can
therefore underflow skb headroom in eth_header().

Use LL_RESERVED_SPACE() to account for the device's actual required
headroom while preserving the existing LLC device-type check.

Fixes: bf9ae53 ("llc: use dev_hard_header")
Cc: stable@vger.kernel.org
Reported-by: VEGA <vega@nebusec.ai>
Signed-off-by: Zixuan Chai <petalzu987@gmail.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260924012613.2533934-1-weir@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/llc/llc_sap.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=018020f5a48cc86fdd6dc2d380f6c70443ae7639

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

The original Linux kernel CVE announcement for CVE-2026-98526 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98526

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:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.8

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).