[CVE-2026-53366][MODERATE 7.0] ipv4: account for fraggap on the paged allocation path [ Upstream

[CVE-2026-53366][MODERATE 7.0] ipv4: account for fraggap on the paged allocation path [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53366
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv4: account for fraggap on the paged allocation path [ Upstream
Commit: ce494707a9c07f27c219ca67f3e138061f53d9b3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ce494707a9c07f27c219ca67f3e138061f53d9b3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53366
Analysis date: Thu, 16 Jul 2026 01:51:59 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An IPv4 `__ip_append_data()` accounting bug can undersize the skb linear area on the paged allocation path when `fraggap` is present, causing at least local DoS and possibly memory corruption requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2026-53366 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53366

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-2026-53366	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-131;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'An accounting bug in the IPv4 __ip_append_data paged allocation path can make the skb linear area smaller than required when fraggap bytes are carried into the next skb. The later copy into the linear area may operate past the intended allocation boundary or trigger skb consistency checks, so the realistic impact is at least a local denial of service via kernel crash. For the CVSS the PR:L is used because a local process is required to generate the affected IPv4 send path and no remote packet processing trigger is shown. The issue is not directly network reachable from a remote attacker because it is in the local transmit construction path. For the paranoid score, the highest still defensible interpretation is used because the bug class is an undersized buffer before copy in kernel networking code, which is a plausible memory corruption candidate and may need review for privilege escalation potential.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	REMOTE SIMPLEFIX NETWORK SKB KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	guess

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

ActionableScore=7
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* +1 Local unprivileged trigger: a local process can plausibly reach `__ip_append_data()` through IPv4 transmit paths.
* +1 Constrained kernel buffer overwrite candidate: the linear skb area is undersized by `fraggap`, but target control is not demonstrated.
* +1 Memory layout invariant restoration: the patch restores consistency between `alloclen`, `pagedlen`, `datalen`, `transhdrlen`, and `fraggap`.
* +1 Reliable kernel crash / strong DoS: undersized skb allocation before copy can plausibly trigger kernel crash or skb consistency failures.
* +1 Broad/default/common subsystem: IPv4 transmit path is core networking code.
* +1 Common networking core path: this is in `net/ipv4/ip_output.c`.
* -1 Hard or special triggering conditions: requires paged allocation path plus `fraggap` conditions, likely involving zerocopy/SG/fragmentation behavior.
* -1 Constrained primitive uncertainty: no demonstrated attacker-selected overwrite target or clear reclaim/object-confusion primitive.

Paranoid additional interpretation:

* +2 Strong memory corruption primitive: undersized allocation followed by copy in kernel skb construction is a plausible OOB write class.
* +1 Weak LPE concern: local kernel heap/skb corruption may be exploitable, but no concrete privilege-escalation primitive is shown.

## 3. Reachability analysis

The bug is reachable from local IPv4 packet generation, not from remote packet receive processing. A remote attacker cannot directly trigger it merely by sending packets to the host. The realistic attacker is a local user or process that can create sockets and drive the affected transmit path.

Namespaces may make this more relevant in containerized environments because unprivileged or delegated network namespaces can still allow local packet generation, depending on product policy. No CAP_NET_ADMIN requirement is shown for the basic socket send path, so PR:L is the safer practical assumption.

The subsystem is default and broad because IPv4 networking is common, but the exact vulnerable path appears condition-dependent: paged allocation, `fraggap`, and likely zerocopy or scatter-gather related behavior must align.

Call-site confidence: high for the immediate allocation/copy mismatch because the relevant arithmetic and later copy are shown in the patch. Medium for practical exploitability because the exact user-controlled trigger constraints are not fully shown.

## 4. Severity interpretation

Realistically, this behaves at least like an actionable local DoS because the patch fixes an undersized skb linear allocation before a copy operation. The conservative score remains below Important because there is no demonstrated arbitrary write, type confusion, UAF reclaim, or privilege-escalation chain.

The paranoid interpretation is stronger because this is not a pure validation or resource leak bug. It is a kernel networking buffer size accounting bug that can create an undersized allocation before copy, which is a plausible memory corruption primitive. That makes it unsuitable for automatic closure and worthy of manual review for LPE potential.

## 5. One-sentence report phrase

An IPv4 `__ip_append_data()` accounting bug can undersize the skb linear area on the paged allocation path when `fraggap` is present, causing at least local DoS and possibly memory corruption requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch fixes a size/layout mismatch before a kernel copy in a core networking skb construction path. Conservative impact is DoS, but the paranoid memory-corruption interpretation is technically defensible.

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

Patch: ipv4: account for fraggap on the paged allocation path [ Upstream
Commit: ce494707a9c07f27c219ca67f3e138061f53d9b3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ce494707a9c07f27c219ca67f3e138061f53d9b3

Changed files:
  net/ipv4/ip_output.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=ce494707a9c07f27c219ca67f3e138061f53d9b3

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

The original Linux kernel CVE announcement for CVE-2026-53366 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53366

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:L/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:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 4
  Paranoid ActionableScore: 7

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).