[CVE-2025-40255][MODERATE REGULAR] net: core: prevent NULL deref in generic_hwtstamp_ioctl_lower() [ Upstream

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

CVE: CVE-2025-40255
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: core: prevent NULL deref in generic_hwtstamp_ioctl_lower() [ Upstream
Commit: 8817f816ae41908e9625c0770c4af0dcdcc01238
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8817f816ae41908e9625c0770c4af0dcdcc01238
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-40255
Analysis date: Wed, 09 Sep 2026 02:41:36 -0400
ActionableScore: 2
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-2025-40255 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-40255

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: NO

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

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

CVE-2025-40255	MODERATE	https://www.kernelcve.org/list/?q=CVE-2025-40255 CHECK WITH IMPACT FROM ORIG NN MODERATE	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:H';**CWE-476;*CWE-20;CWE-754;BEST CVSS score: '5.5';DESCR 'generic_hwtstamp_ioctl_lower can dereference kernel_cfg ifr as NULL when the ethtool tsconfig Netlink path reaches the generic hardware timestamp helper through phylib and VLAN hwtstamp handling. A local process can trigger the bug by issuing the affected ethtool Netlink tsconfig request on a suitable network device stack, causing a kernel crash through NULL pointer dereference. For the CVSS the PR:L is used because the trigger requires local code or a local process able to send the Netlink request, and it may be reachable without full root privileges depending on the exact ethtool operation and namespace policy. The issue is not directly network reachable because it is triggered through a local Netlink control plane path rather than packet traffic. Impact is denial of service only via kernel crash, with no supported path to information disclosure or privilege escalation from the shown primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) 	MAYBE	REMOTE DANGER NULLPTR SIMPLEFIX LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2. No floor or cap applied.
* Raw paranoid score: 2
* Paranoid score: 2. No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Reliable kernel crash / strong DoS. The commit explicitly states that `generic_hwtstamp_ioctl_lower()` dereferences `kernel_cfg->ifr` when it is NULL, causing a system crash.
* +1 Local unprivileged or low-privilege trigger. The vulnerable path is reached through the ethtool tsconfig Netlink GET-style preparation path, and the supplied prior triage treats this as PR:L rather than host-admin-only.

Negative or non-awarded signals:

* Real UAF not awarded. The primitive is a NULL pointer dereference, not a stale post-lifetime object use.
* CLASS_A or CLASS_B not awarded. No memory corruption, object lifetime corruption, authorization bypass, COW violation, semantic reinterpretation, or repeated release/refcount mutation is established.
* Remote reachability not awarded. This is a local Netlink control-plane path, not packet-triggered network input.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* C/I impact not awarded. The evidence supports crash only, with no disclosure or unauthorized modification primitive.
* Extra Availability not awarded because the same kernel crash is already counted once.
* Race/timing penalty not applied. No race is involved.
* Host-admin penalty not applied. The supplied context and prior triage point to PR:L, not positively established host-admin-equivalent privilege.

3. Reachability analysis

The trigger is a local ethtool tsconfig Netlink request that reaches:

`tsconfig_prepare_data()` -> `dev_get_hwtstamp_phylib()` -> `vlan_hwtstamp_get()` -> `generic_hwtstamp_get_lower()` -> `generic_hwtstamp_ioctl_lower()`.

The evaluated attacker is a local process able to issue the affected Netlink request against a suitable network device stack. The supplied evidence does not show direct packet-triggered reachability from the network. It also does not establish that host root or host CAP_NET_ADMIN is required for the complete failing GET-style path, so HostAdminRequired is set to NO based on the provided PR:L hint and normal ethtool Netlink semantics.

Namespace scope remains UNKNOWN because the supplied patch does not fully prove whether all required device and ethtool operations are available inside attacker-created namespaces. Prior privileged setup may be needed in some deployments if the vulnerable stack requires a VLAN or PHY-backed device configuration, but that is not enough to subtract a host-admin penalty. No special rare hardware requirement is established beyond needing a suitable hwtstamp-capable or phylib/VLAN path.

4. Primitive and severity interpretation

The exact primitive is a NULL pointer dereference of `kernel_cfg->ifr` in `generic_hwtstamp_ioctl_lower()`.

This is not active structured UAF and not fixed stale storage. It is an invalid pointer dereference caused by missing input/state validation. There is no cleanup/destructor lifetime issue and no concrete repeated release/refcount operation.

PrimitiveClass is CLASS_C because the demonstrated behavior is crash/DoS only. The demonstrated consequence is a kernel crash. The speculative consequence would be broader kernel compromise, but no patch-supported mechanism exists for that: no UAF, no OOB write, no controlled object replacement, no arbitrary read/write, and no authorization bypass.

ImportantCandidate is NO because the primitive is a straightforward NULL dereference with no meaningful unresolved exploitation leverage beyond DoS. AutoDowngradeAllowed is YES because there is no concrete corruption or boundary primitive that would be lost by treating this as ordinary Moderate.

Likely operational severity is ordinary Moderate: relevant for systems where local users or services can issue the affected ethtool Netlink request, but not an Important-class kernel corruption candidate from the supplied evidence.

5. One-sentence report phrase

A local ethtool tsconfig Netlink request can reach generic_hwtstamp_ioctl_lower with kernel_cfg ifr set to NULL, causing a NULL pointer dereference and kernel crash without evidence of information disclosure or privilege escalation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: confirm the exact permission model for the affected ethtool tsconfig operation and whether the complete trigger is reachable by ordinary local users, namespace-scoped users, or only delegated network-management processes.

7. Call-site confidence

Call-site confidence: medium

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

Patch: net: core: prevent NULL deref in generic_hwtstamp_ioctl_lower() [ Upstream
Commit: 8817f816ae41908e9625c0770c4af0dcdcc01238
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8817f816ae41908e9625c0770c4af0dcdcc01238

Changed files:
  net/core/dev_ioctl.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=8817f816ae41908e9625c0770c4af0dcdcc01238

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

The original Linux kernel CVE announcement for CVE-2025-40255 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-40255

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2

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).