[CVE-2026-53250][MODERATE 7.0] xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata() [ Upstream

[CVE-2026-53250][MODERATE 7.0] xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53250
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata() [ Upstream
Commit: 0dfe05b938435892875e07771170051346412df9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0dfe05b938435892875e07771170051346412df9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53250
Analysis date: Thu, 25 Jun 2026 09:09:05 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local AF_XDP userspace process can race writable UMEM TX checksum metadata between validation and use, causing out-of-bounds access in the kernel transmit checksum path and at least a kernel crash, with limited but review-worthy memory corruption concern.

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

The original Linux kernel CVE announcement for CVE-2026-53250 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53250

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-2026-53250	MODERATE	CHECK	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-367;CWE-362;*CWE-787;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A TOCTOU race in xsk_skb_metadata can occur because csum_start and csum_offset are read from mmap writable UMEM metadata for bounds validation and then read again for skb assignment. A malicious local userspace application can change these fields between the two reads, causing the validated values and the used values to differ. This can bypass the checksum bounds check and cause out of bounds memory access during checksum computation in the transmit path. For the CVSS the PR:L is used because the attacker needs local code execution and access to AF_XDP or an already usable XSK/UMEM transmit path, but does not necessarily need full host administrator privileges in all deployments. The issue is not directly network reachable as AV:N because packet traffic alone does not trigger it without local control of the UMEM metadata. Impact is at least local denial of service via kernel crash, and in the paranoid interpretation may allow confidentiality or integrity impact due to memory corruption risk.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	WRITE OOB SIMPLEFIX RACE SKB IMPROVEONLY KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged or low-privilege trigger: +1
  A local userspace process controlling AF_XDP UMEM metadata can race the kernel reads. In many deployments AF_XDP access is restricted, but this is not equivalent to full host root in all cases.

* Memory corruption, Weak/indirect corruption candidate: +1
  The patch describes attacker-controlled TOCTOU leading to out-of-bounds memory access in the checksum transmit path. The primitive is real, but the patch does not demonstrate arbitrary write, object replacement, type confusion, or controlled target selection.

* Weak LPE concern: +1 paranoid only
  OOB access in a kernel network TX path from userspace-controlled metadata is enough to require manual review, but practical privilege escalation is not demonstrated.

* Reliable kernel crash / strong DoS: +1
  The described consequence can be kernel crash through invalid checksum metadata used after validation.

* Local high-control kernel data-plane API: +1
  AF_XDP UMEM is mmap writable and gives userspace fine-grained control over packet buffers and TX metadata.

* Hard race / timing window: -1
  Exploitation requires changing csum_start or csum_offset between validation and later assignment, so the race window matters.

## 3. Reachability analysis

The bug is triggered by a local process that can use AF_XDP TX metadata and writable UMEM. It is not directly network-triggerable because incoming or outgoing packets alone do not control the metadata race without local userspace participation. Namespace and container setups may matter: if AF_XDP or related capabilities are delegated to a container or service account, the practical PR is closer to low privilege than full administrator. In default general-purpose systems, AF_XDP usage is less common and may require elevated networking capabilities or a preconfigured AF_XDP application.

## 4. Severity interpretation

This is more than an ordinary Moderate because the patch explicitly fixes a userspace-controlled TOCTOU that can bypass bounds validation and cause OOB access in kernel TX checksum handling. Realistic demonstrated impact is local DoS. Theoretical higher impact exists because the OOB access is in kernel networking code and comes from attacker-controlled metadata, but there is no demonstrated arbitrary write, reclaim primitive, or object confusion. Therefore the conservative result is borderline manual-review, while the paranoid result is Actionable Moderate.

## 5. One-sentence report phrase

A local AF_XDP userspace process can race writable UMEM TX checksum metadata between validation and use, causing out-of-bounds access in the kernel transmit checksum path and at least a kernel crash, with limited but review-worthy memory corruption concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

The issue involves attacker-controlled TOCTOU and OOB kernel memory access, not just a warning or resource leak. It should not be auto-closed even though direct privilege escalation is not proven.

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

Patch: xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata() [ Upstream
Commit: 0dfe05b938435892875e07771170051346412df9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0dfe05b938435892875e07771170051346412df9

Changed files:
  net/xdp/xsk.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=0dfe05b938435892875e07771170051346412df9

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

The original Linux kernel CVE announcement for CVE-2026-53250 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53250

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: 5

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).