[CVE-2026-90135][MODERATE 7.0] net: add missing ref_tracker_dir_exit() to alloc_netdev_mqs() [ Upstream

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

CVE: CVE-2026-90135
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: add missing ref_tracker_dir_exit() to alloc_netdev_mqs() [ Upstream
Commit: 2775bf36be7872834a7f8c0a0d9681a1a3e1cd4c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2775bf36be7872834a7f8c0a0d9681a1a3e1cd4c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90135
Analysis date: Fri, 18 Sep 2026 02:42:17 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
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-2026-90135 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90135

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 7.0
Manual review required: YES

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

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

CVE-2026-90135	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90135 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:L/I:L/A:H';*CWE-416;*CWE-672;CWE-459;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A use-after-free can occur in the net device allocation failure path because alloc_netdev_mqs can free the net_device object without destroying its ref_tracker directory state. If alloc_percpu or dev_addr_init fails after ref_tracker setup, stale debugfs/ref_tracker state can remain and a later read of /sys/kernel/debug/ref_tracker/* may dereference freed memory and crash the kernel. For the CVSS the PR:L is used for the paranoid score because some environments may delegate network device creation or debugfs access to reduced capability users, container root, or service accounts, even though normal systems often require administrator privileges. The issue is not network reachable and requires local control of the allocation path and debugfs access. Impact is at least local denial of service via kernel crash and in worst case may allow limited confidentiality or integrity impact due to the UAF class, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	REMOTE DANGER INIT SIMPLEFIX HARDWARE LINUS KPANIC 	YES	NO	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 4
* Applied conservative floor/cap: Real UAF floor 4 applied because a stale ref_tracker object is actually used after net_device free and host-admin-equivalent reachability is not fully proven from supplied evidence.
* Raw paranoid score: 4
* Paranoid score: 4
* Applied paranoid floor/cap: none.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +2 Real UAF / lifetime violation: alloc_netdev_mqs() can free the net_device object without calling ref_tracker_dir_exit(), leaving stale ref_tracker debugfs state that later references freed storage.
* +1 Reliable kernel crash / strong DoS: supplied report says reading /sys/kernel/debug/ref_tracker/* causes a use-after-free crash.
* +1 Paranoid concrete persistent resource/reference cleanup defect: missing ref_tracker_dir_exit() leaves ref_tracker/debugfs lifetime state behind after the normal allocation failure cleanup path.

Negative signals:

* No -2 host-admin penalty applied because the complete trigger boundary is not fully established. debugfs access and netdev creation are commonly privileged, but the prompt requires positive proof for HostAdminRequired=YES.
* No -1 race penalty applied. This is an ordered allocation failure cleanup bug, not a timing race.
* No rare subsystem penalty applied. ref_tracker/debugfs is diagnostic, but the bug is in common net_device allocation cleanup.

Non-awarded material signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity impact not awarded merely because UAF exists.
* Structured stale-object leverage not awarded because the demonstrated stale use is a diagnostic debugfs read leading to crash, not a meaningful post-reuse field consumption primitive.
* Weak LPE not awarded because no exploit-relevant reuse, callback, object replacement, or security-boundary effect is shown.
* Extra Availability not awarded because the crash is already counted.
* Concrete repeated release/refcount signal not awarded. The patch shows missing ref_tracker cleanup before kvfree(), not a second put/release/refcount mutation on the same dangling object.
* FLOOR 5 not applied because there is no concrete repeated lifetime mutation and no meaningful exploitation surface beyond the debugfs stale dereference.

Repeated-release/refcount check:

* First release can end lifetime: kvfree(dev) ends net_device storage lifetime.
* Exact same reference survives into a second lifetime mutation: not established.
* Second operation is a real lifetime/refcount mutation: not established.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The reported trigger has two stages: first, alloc_netdev_mqs() must fail after ref_tracker setup, for example at alloc_percpu() or dev_addr_init(); second, a reader must access /sys/kernel/debug/ref_tracker/*. Creating or driving net_device allocation paths often requires CAP_NET_ADMIN or a driver/control-plane path, and debugfs access is commonly restricted to root or administrator-like contexts. However, the supplied evidence does not fully prove that host-admin-equivalent privilege is required in every reachable configuration, so HostAdminRequired is UNKNOWN rather than YES.

Namespace-scoped privilege is also UNKNOWN. Some virtual network device creation paths may be namespace-aware, but the complete chain also involves debugfs/ref_tracker access and allocation failure conditions. The input does not establish that all required stages can be performed inside attacker-created namespaces below host admin.

There is no direct network reachability. Packet traffic alone does not reach the vulnerable debugfs stale read. No special hardware is required by the patch itself, but practical reproduction may require fault injection, memory pressure, or a specific failing allocation path. There is no race repeatability issue because the bug is not primarily timing-based.

4. Primitive and severity interpretation

The exact primitive is a real stale lifetime bug: ref_tracker state remains live or externally reachable after the associated struct net_device storage has been freed on an alloc_netdev_mqs() error path. The stale object is connected to an active structured kernel object, but the demonstrated consuming path is debugfs/ref_tracker diagnostic reading. The observed consequence is a use-after-free crash.

This is CLASS_B because a real kernel lifetime violation is established. It is not CLASS_A because there is no controlled reclaim, no attacker-shaped replacement object, no directed kernel write, no callback/function-pointer control, no demonstrated disclosure, and no privilege-escalation or security-boundary bypass.

The case is closer to a constrained crash-only UAF than to an Important-preserving active-object exploitation primitive. ImportantCandidate is NO because the supplied evidence does not show meaningful exploitation leverage beyond the crash. AutoDowngradeAllowed is YES because the demonstrated behavior is local diagnostic stale dereference DoS, with no concrete repeated refcount mutation or unresolved boundary primitive.

Likely operational severity is Borderline / Moderate-like. It should not be treated as urgent Important without additional evidence, but manual review is still reasonable because the primitive is a real UAF in core net_device lifetime cleanup.

5. One-sentence report phrase

A missing ref_tracker_dir_exit() call in the alloc_netdev_mqs() failure path can leave stale ref_tracker debugfs state after net_device storage is freed, so later reads of /sys/kernel/debug/ref_tracker/* may dereference freed memory and crash the kernel.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a real UAF, but the currently established path is constrained to local debugfs/ref_tracker access after an allocation failure and shows crash-only impact without controlled reuse or a repeated lifetime/refcount mutation.

7. Call-site confidence

Call-site confidence: medium

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

Patch: net: add missing ref_tracker_dir_exit() to alloc_netdev_mqs() [ Upstream
Commit: 2775bf36be7872834a7f8c0a0d9681a1a3e1cd4c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2775bf36be7872834a7f8c0a0d9681a1a3e1cd4c

Changed files:
  net/core/dev.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=2775bf36be7872834a7f8c0a0d9681a1a3e1cd4c

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

The original Linux kernel CVE announcement for CVE-2026-90135 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90135

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 4

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).