[CVE-2026-90078][LOW] net/sched: act_skbmod: fix length calculations and avoid invalid header warnings [ Upstream

[CVE-2026-90078][LOW] net/sched: act_skbmod: fix length calculations and avoid invalid header warnings [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90078
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net/sched: act_skbmod: fix length calculations and avoid invalid header warnings [ Upstream
Commit: 985a37781ade19061400100c2ba0f43979dd4d63
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=985a37781ade19061400100c2ba0f43979dd4d63
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90078
Analysis date: Thu, 17 Sep 2026 13:39:01 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90078 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90078

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: LOW
Manual review required: NO

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

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

CVE-2026-90078	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90078 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';CWE-682;CWE-754;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '3.7';DESCR 'act_skbmod could calculate writable packet length incorrectly on TC ingress and L3 devices. The code may call skb_network_header_len() when the transport header is not set and may use a MAC header length when no MAC header exists. This can trigger kernel warnings and can also cause valid short IPv4 or IPv6 packets to be dropped because skb_ensure_writable is asked to cover more bytes than the packet actually contains. For the CVSS the PR:N is used in the paranoid network triggered interpretation because an external sender may only need packet reachability when a vulnerable TC skbmod action is already configured by an administrator. The issue is not a memory corruption primitive and there is no supported path to privilege escalation. Impact is limited denial of service through warnings or packet drops, with no confidentiality or integrity impact.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	REMOTE OOB DANGER NETWORK SKB HARDWARE IPV6 SYZBOT PACKET  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=1
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap changed the score.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Limited availability impact. The patch and report support kernel warnings and dropping of valid short packets due to incorrect writable length calculation.

Negative signals:

* No negative scoring applied. Host-admin-equivalent privilege is not positively established for every possible trigger model, so no -2 penalty is applied.

Important non-awarded signals:

* Remote reachable +2 not awarded. Network packets may exercise the path only if a vulnerable TC skbmod action is already configured.
* Reliable kernel crash +1 not awarded. Evidence shows WARN and packet drops, not panic or fatal host-wide crash.
* UAF or lifetime corruption not awarded. No stale pointer, refcount misuse, double release, or post-free access is established.
* Controlled corruption not awarded. skb_ensure_writable is given an incorrect length, but the demonstrated effect is failure or packet drop, not OOB write.
* Confidentiality and integrity signals not awarded. No disclosure, arbitrary write, protected object modification, or privilege escalation path is supported.
* Common networking core path +1 not awarded. This is a TC action path requiring specific configuration, not an always-on core packet-processing primitive.
* Manual-review preservation signals not awarded. There is no Class A or Class B primitive.

3. Reachability analysis

The vulnerable code is in `tcf_skbmod_act()` and is reached when the skbmod TC action is installed and processes packets. Creating or attaching TC actions normally requires `CAP_NET_ADMIN` in the relevant network namespace, but the supplied input does not fully establish whether the complete setup is host-admin-only or namespace-scoped in all practical cases.

If an administrator has already configured the affected skbmod action, packet input may trigger the warning or packet-drop behavior. That is prior privileged setup rather than direct unauthenticated remote exploitation. The issue is therefore not scored as directly network reachable. No special hardware is required. No race or timing condition is involved.

4. Primitive and severity interpretation

The exact issue is an incorrect header-length calculation and invalid header-state assumption. `skb_network_header_len()` assumes `transport_header` is set, which may be false at TC ingress. `skb_mac_header_len()` may also be invalid on L3 devices such as TUN where the MAC header is unset. The resulting length passed to `skb_ensure_writable()` can be larger than the real packet length, causing valid short packets to be dropped.

PrimitiveClass is `NONE` because no meaningful kernel corruption, ownership violation, lifetime bug, UAF, OOB write, authorization bypass, or security-boundary primitive is established. The demonstrated consequence is warning noise and limited packet-processing DoS. Speculative privilege escalation is not supported.

ImportantCandidate is `NO` because there is no unresolved corruption or boundary-bypass primitive. AutoDowngradeAllowed is `YES` because this is a low-impact validation and length-calculation bug with no established memory corruption.

5. One-sentence report phrase

Incorrect skbmod header length calculation on TC ingress or L3 devices can trigger kernel warnings and drop valid short packets, but the evidence supports only limited denial of service and no memory corruption or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the patch supports a warning and packet-drop issue only, with no UAF, repeated lifetime mutation, OOB write, disclosure, authorization bypass, or plausible Important-grade primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: net/sched: act_skbmod: fix length calculations and avoid invalid header warnings [ Upstream
Commit: 985a37781ade19061400100c2ba0f43979dd4d63
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=985a37781ade19061400100c2ba0f43979dd4d63

Changed files:
  include/linux/skbuff.h
  net/sched/act_skbmod.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=985a37781ade19061400100c2ba0f43979dd4d63

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

The original Linux kernel CVE announcement for CVE-2026-90078 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90078

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:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS score: 3.7

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: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).