[CVE-2026-99027][IMPORTANT] net: team: stop reusing skb after queue override [ Upstream

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

CVE: CVE-2026-99027
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: net: team: stop reusing skb after queue override [ Upstream
Commit: b21f20320fa2f3c683e3334d70de6bfd172e544a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b21f20320fa2f3c683e3334d70de6bfd172e544a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99027
Analysis date: Mon, 26 Oct 2026 22:32:56 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
The team queue-override path can reuse an skb after qdisc ownership transfer and free, causing a second drop/free operation and a confirmed four-byte slab UAF write.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-99027 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99027

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-2026-99027	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-99027 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=3 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:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'The team queue override path could continue using an skb after dev_queue_xmit had transferred ownership and a lower qdisc had dropped and freed it. The demonstrated primitive is a slab use-after-free with a kernel write in sk_skb_reason_drop, so impact includes kernel panic and a plausible path to broader memory corruption. For the CVSS the PR:L value reflects that a local process may trigger transmission through an existing team interface, while creation of the team queue override and qdisc configuration normally requires administrative setup. Reliable exploitation also depends on a nonzero queue mapping and forcing the selected override port qdisc to drop the skb. Direct remote reachability is not established by this patch. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 7.8 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. No severity changing production threshold or specialist override was recorded. 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 TEMP_KPANIC_MERGE_IMPORTANT_WINS. The independent weaker Aegis shadow evaluated CVSS 1.7 and ActionableScore 3 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	DANGER UAF NETWORK SKB LINUS QDISC KPANIC  TEMP_KPANIC_MERGE_IMPORTANT_WINS	-	-	checked

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

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

1. ActionableScore

* Raw base score: 4.
* Conservative score: 5; REAL UAF FLOOR 5 applies because the same skb survives ownership transfer/free and reaches a second lifetime-mutating release path.
* Raw paranoid score: 5; includes the defensible unknown leverage of a write-like operation through freed slab storage.
* Paranoid score: 5; no additional floor or cap.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 real UAF: `dev_queue_xmit()` takes ownership and the qdisc frees the skb, but `team_xmit()` subsequently passes that stale skb to the mode transmit operation.
* +1 concrete repeated release/lifetime mutation: the exact skb remains reachable; `ab_transmit()` calls the drop/free path, and `sk_skb_reason_drop()` writes four bytes through the freed object.
* +1 reliable kernel crash/DoS: KASAN reports the slab UAF and the supplied trace reaches panic.
* Paranoid +1: the second operation writes lifetime/drop state into storage whose skb lifetime has ended. This is not scored as arbitrary or directed writing.
* No privilege, race, rarity, or hardware penalty is supported.
* Controlled reclaim, attacker-selected target/payload, disclosure, arbitrary write, and LPE are not shown. No extra availability or integrity point is awarded.
* NEW52/B5-R does not apply: there is no retry-accumulated heap OOB write. B3-PENDING and B4-PENDING also do not apply.

3. Reachability analysis

An administrator normally configures the team queue override, override port, and dropping qdisc. A local process may later send through an existing configured team interface, but the evidence does not establish that a below-host-admin actor controls the complete nonzero-queue-mapping/drop sequence. The applicable capability namespace and whether all setup can be delegated through a user/net namespace are unresolved. No remote-only trigger, special hardware, or timing race is shown.

4. Primitive and severity reading

This is an active structured skb UAF, not fixed stale storage. The stale use is principally a second drop/free operation rather than broad structured-field processing. The first transmit handoff can free the skb; the same pointer then reaches a second operation that actively writes drop metadata and releases it.

That concrete repeated lifetime mutation is stronger than an ordinary cleanup dereference, but remains CLASS_B because controlled reclaim, useful replacement, and directed corruption are absent. ImportantCandidate is YES and automatic downgrade is disallowed because post-free write leverage remains materially unknown.

5. One-sentence report phrase

The team queue-override path can reuse an skb after qdisc ownership transfer and free, causing a second drop/free operation and a confirmed four-byte slab UAF write.

6. Manual review advice

MANUAL CHECK REQUIRED

The confirmed post-free write and repeated lifetime operation require exploit-leverage and privilege-boundary review despite the currently demonstrated panic outcome.

7. Call-site confidence

Call-site confidence: high

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

Patch: net: team: stop reusing skb after queue override [ Upstream
Commit: b21f20320fa2f3c683e3334d70de6bfd172e544a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b21f20320fa2f3c683e3334d70de6bfd172e544a

Changed files:
  net/core/skbuff.c
  net/core/dev.c
  drivers/net/team/team_mode_activebackup.c
  drivers/net/team/team_core.c
  net/sched/sch_generic.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=b21f20320fa2f3c683e3334d70de6bfd172e544a

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

The original Linux kernel CVE announcement for CVE-2026-99027 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99027

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:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 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).