[CVE-2026-90248][MODERATE REGULAR] net/sched: cls_api: fix teardown of an adopted proto on insert-race loss [ Upstream

[CVE-2026-90248][MODERATE REGULAR] net/sched: cls_api: fix teardown of an adopted proto on insert-race loss [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90248
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: cls_api: fix teardown of an adopted proto on insert-race loss [ Upstream
Commit: a68e664ddf16ecae6d769d6a2eb356f8abab75c9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a68e664ddf16ecae6d769d6a2eb356f8abab75c9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90248
Analysis date: Thu, 17 Sep 2026 13:05:13 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90248 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90248

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 REGULAR
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-2026-90248	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90248 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:N/I:H/A:L';CWE-362;CWE-667;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'tc_new_tfilter() can mishandle an insert race when two requests try to create a tcf_proto for the same chain and priority. The losing request may receive the already inserted proto from the winning request, but the old error path can still treat it as owned by the loser and call teardown on it. This can silently unlink an active classifier that was already advertised through RTM_NEWTFILTER. For the CVSS the PR:L is used for the paranoid score because CAP_NET_ADMIN can be delegated to container root, service accounts, or network namespace users in some deployments. The issue is not directly network reachable and is triggered through local tc netlink control plane operations rather than packet traffic. Impact is mainly integrity impact against traffic control policy and limited availability impact due to unexpected classifier removal. No direct memory corruption or privilege escalation primitive is evident from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	REMOTE DANGER RACE NETWORK LINUS QDISC  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap changed the conservative or paranoid score.

2. Signal breakdown

Conservative signals:

* +1 active control-plane ownership/state confusion: the losing `tc_new_tfilter()` request can treat another request's already inserted `tcf_proto` as its own.
* +1 integrity impact: the wrong errout path can silently unlink an active classifier already advertised through `RTM_NEWTFILTER`.
* +1 practical consequence chain: the patch-supported sequence is concrete: insert race loss -> adopted proto -> wrong teardown -> classifier unlink.
* +1 limited availability impact: unexpected classifier removal can disrupt traffic-control behavior or filtering policy.
* -1 privileged tc control-plane operation: creating or modifying tc filters normally requires `CAP_NET_ADMIN`, but host-vs-namespace scope is not fully resolved from the supplied patch.

Paranoid difference:

* +1 paranoid reachability interpretation: delegated or namespace-scoped `CAP_NET_ADMIN` may make this reachable by a reduced-privilege service account or container-style actor in some deployments. This supports 4 instead of 3, but not Important-class scoring.

Material non-awarded signals:

* RealUAF not awarded: the patch establishes wrong ownership/teardown of a live proto, not a concrete stale post-free dereference.
* Concrete repeated release/refcount not awarded: `tcf_proto_put()` appears to release the extra reference returned for the winner proto, and the supplied evidence does not establish a second lifetime mutation on the same dangling reference after object lifetime ended.
* Strong LPE not awarded: no controlled reclaim, arbitrary write, callback control, credential impact, or privilege-escalation path is shown.
* Remote reachability not awarded: this is local rtnetlink/tc control-plane activity, not packet-triggered data-plane processing.
* Extra availability not awarded: classifier removal is already counted as limited availability/policy disruption, with no host-wide crash shown.

Repeated-release checklist:

* First release can end lifetime: not established for the winner proto in the supplied flow.
* Exact same dangling reference survives: not established.
* Second operation is a real lifetime/refcount mutation on freed storage: not established.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The trigger is a local tc/rtnetlink control-plane race where two requests try to create a `tcf_proto` for the same chain, priority, and protocol. The attacker needs the ability to issue tc filter creation operations against the relevant qdisc or block.

Normal tc filter management requires `CAP_NET_ADMIN`. The supplied context does not prove whether the complete operation requires host/init namespace `CAP_NET_ADMIN` or whether it can be completed entirely inside an attacker-controlled network namespace. Therefore `HostAdminRequired=UNKNOWN` and `NamespaceScopedPrivilege=UNKNOWN`.

There is no direct network packet trigger. Incoming traffic may be affected after a classifier is removed, but packets do not directly exercise the vulnerable insertion race. No special hardware, firmware, boot option, or rare subsystem requirement is established. The race is real, but it is between repeatable local control-plane operations and is not treated as a one-shot uncontrollable timing penalty.

4. Primitive and severity interpretation

The exact primitive is an ownership/state confusion in the tc classifier insertion path. A request that loses `tcf_chain_tp_insert_unique()` receives another request's proto with an extra reference, but the old boolean state still makes the error path behave as if the current request owns that proto. The result is incorrect teardown of a live classifier object.

This is an active structured kernel object, but the demonstrated stale behavior is wrong unlink/teardown rather than memory corruption. There is no demonstrated UAF, no proven double free, no arbitrary read/write, and no controlled replacement of freed memory. The demonstrated consequence is silent removal of an active traffic-control classifier, which is mainly an integrity impact against network policy and a limited availability impact against traffic-control behavior.

`PrimitiveClass=CLASS_C` because the established behavior is invalid active-state teardown without an independently useful kernel corruption primitive. `ImportantCandidate=NO` because Important-grade exploitation leverage is not established. `AutoDowngradeAllowed=YES` because no unresolved UAF, repeated refcount mutation, authorization bypass, COW violation, controlled corruption, or LPE path is shown. Manual review is still recommended because silent unlink of active classifiers can have operational policy impact and because namespace/delegated `CAP_NET_ADMIN` reachability may matter for product triage.

5. One-sentence report phrase

A tc insert race can cause a losing `tc_new_tfilter()` request to tear down another request's active `tcf_proto`, silently unlinking an advertised classifier and causing traffic-control policy integrity impact with possible limited availability disruption.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended to confirm whether the affected product allows delegated or namespace-scoped `CAP_NET_ADMIN` to reach the complete tc filter creation path and whether silent classifier unlink has security-policy impact in the deployment.

7. Call-site confidence

Call-site confidence: medium

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

Patch: net/sched: cls_api: fix teardown of an adopted proto on insert-race loss [ Upstream
Commit: a68e664ddf16ecae6d769d6a2eb356f8abab75c9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a68e664ddf16ecae6d769d6a2eb356f8abab75c9

Changed files:
  net/sched/cls_api.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=a68e664ddf16ecae6d769d6a2eb356f8abab75c9

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

The original Linux kernel CVE announcement for CVE-2026-90248 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90248

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:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L
  The Best / paranoid CVSS score: 5.3

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: 3
  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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).