[CVE-2026-90160][LOW] lwt_bpf: Restore reserved headroom after xmit program [ Upstream

[CVE-2026-90160][LOW] lwt_bpf: Restore reserved headroom after xmit program [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90160
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: lwt_bpf: Restore reserved headroom after xmit program [ Upstream
Commit: 753e5cdcca5474d230d62bb3489e5168ab27c272
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=753e5cdcca5474d230d62bb3489e5168ab27c272
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90160
Analysis date: Fri, 18 Sep 2026 07:23:52 -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-2026-90160 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90160

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-2026-90160	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90160 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';CWE-754;*CWE-20;CWE-703;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '3.3';DESCR 'An LWT_XMIT BPF program can reduce skb headroom after ip_finish_output2 reserved space for the link layer header. The post BPF check used dev hard_header_len instead of LL_RESERVED_SPACE(dev), so Ethernet could pass with 15 bytes of headroom even though the neighbour cached header path needs 16 bytes. This reaches the neigh_hh_output headroom warning and drops the packet, causing traffic denial of service for affected routes. For the CVSS the PR:L means the attacker is assumed to have delegated network administration or BPF route control, such as inside a container or service account that can attach an LWT_XMIT BPF program. The issue is not directly network reachable by packet input alone. Impact is denial of service only and there is no supported UAF, OOB access, or privilege escalation primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	REMOTE BPF SIMPLEFIX SKB HARDWARE 	NO	NO	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
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
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap changed the score.

2. Signal breakdown

Positive signals:

* +1 Availability impact realistic. The bug can cause packet drops for affected LWT_BPF routes after neigh_hh_output hits the headroom warning.

Negative or non-awarded signals:

* +0 Remote reachable. Incoming packets alone do not reach the vulnerable condition without prior LWT_XMIT BPF route setup.
* +0 Local unprivileged trigger. Ordinary unprivileged reachability is not established.
* +0 Local unprivileged high-control API. BPF route control is not established as ordinary unprivileged in the supplied evidence.
* +0 Reliable kernel crash. The demonstrated result is warning and packet drop, not kernel panic.
* +0 UAF or lifetime corruption. No stale object, double free, refcount misuse, or post-lifetime access is shown.
* +0 OOB or arbitrary write. The patch fixes a headroom size check mismatch, not a memory overwrite.
* +0 Confidentiality and integrity. No disclosure, unauthorized modification, or privilege escalation primitive is supported.
* +0 BPF strong leverage. The issue involves BPF control plane state, but the primitive is packet drop only.
* +0 Extra Availability. The same packet drop is already counted once and no distinct persistent host-wide availability mechanism is shown.

3. Reachability analysis

The practical trigger requires an LWT_XMIT BPF program that can reduce skb headroom and still return BPF_OK. It also requires route or LWT configuration that sends traffic through this program. Loading and attaching such BPF programs and configuring lightweight tunnel routing are privilege-gated operations, normally involving CAP_NET_ADMIN and BPF-related permissions.

Host-admin-equivalent privilege is not positively proven from the supplied patch alone, so HostAdminRequired is UNKNOWN rather than YES. Below-host-admin reachability is also not positively established, so HostAdminRequired is not NO. NamespaceScopedPrivilege is UNKNOWN because the supplied evidence does not prove that the complete chain, including BPF load, attach, route setup, and trigger traffic, is fully available inside attacker-created namespaces.

There is no direct network trigger from packet input alone. A remote sender might experience or amplify packet loss only after a privileged or delegated local actor has installed the affected LWT_BPF path.

4. Primitive and severity interpretation

The exact primitive is an insufficient post-BPF skb headroom validation. The old check used dev hard_header_len, while the neighbour cached-header path can require LL_RESERVED_SPACE(dev), for example 16 bytes on Ethernet instead of hard_header_len 14. This lets an skb with insufficient aligned L2 headroom pass the post-BPF check and later be dropped by neigh_hh_output.

This is not an active structured UAF, not fixed stale storage, and not a cleanup lifetime bug. No concrete repeated release or refcount mutation exists. The demonstrated consequence is route-specific packet loss or traffic denial of service. The speculative consequence does not extend to memory corruption or privilege escalation based on the supplied patch.

PrimitiveClass is CLASS_C because the behavior is primarily DoS/invalid-state packet handling without an independently useful corruption primitive. ImportantCandidate is NO because there is no memory corruption, authorization bypass, ownership violation, or unresolved exploitation leverage. AutoDowngradeAllowed is YES because the issue is constrained to packet drop behavior and does not require manual Important-vs-Moderate preservation.

5. One-sentence report phrase

An LWT_XMIT BPF program can reduce skb headroom so that the post BPF check accepts an skb with too little aligned L2 header space, causing neigh_hh_output to warn and drop packets on affected routes.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch supports packet-drop DoS only, with no UAF, OOB access, authorization bypass, privilege escalation, or direct remote trigger.

7. Call-site confidence

Call-site confidence: high

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

Patch: lwt_bpf: Restore reserved headroom after xmit program [ Upstream
Commit: 753e5cdcca5474d230d62bb3489e5168ab27c272
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=753e5cdcca5474d230d62bb3489e5168ab27c272

Changed files:
  net/core/lwt_bpf.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=753e5cdcca5474d230d62bb3489e5168ab27c272

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

The original Linux kernel CVE announcement for CVE-2026-90160 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90160

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:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS score: 3.3

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: 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).