[CVE-2026-68385][MODERATE REGULAR] s390/checksum: Fix csum_partial() without vector facility [ Upstream

[CVE-2026-68385][MODERATE REGULAR] s390/checksum: Fix csum_partial() without vector facility [ Upstream

[AL-KERNEL]

CVE: CVE-2026-68385
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: s390/checksum: Fix csum_partial() without vector facility [ Upstream
Commit: 5fc0a2a6eeb99cac991242bb48796c7749ce3261
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5fc0a2a6eeb99cac991242bb48796c7749ce3261
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68385
Analysis date: Mon, 10 Aug 2026 19:51:55 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
On s390 systems without the vector facility, csum_partial() may checksum address zero instead of the source buffer, allowing network-triggered checksum corruption and possible packet-processing disruption under affected hardware conditions.

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

The original Linux kernel CVE announcement for CVE-2026-68385 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68385

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: YES

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

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

CVE-2026-68385	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L';**CWE-476;CWE-125;CWE-682;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.6';DESCR 'On s390 systems without the vector facility, csum_partial can calculate a checksum from address zero instead of the source buffer because the fallback path passes dst to cksm while dst is NULL for the non copy case. This can corrupt checksum generation or validation and may cause packets to be rejected or accepted with an invalid checksum under specific network paths. For the CVSS the PR:N is used for the paranoid score because a remote peer can trigger checksum work through normal network traffic when the affected system is reachable. AC:H is used because the issue is architecture and hardware dependent and requires an s390 system without VX plus a reachable code path using this fallback. Impact is mainly network integrity and availability degradation. Limited confidentiality impact is kept only in the paranoid score because the returned checksum may depend on data read from address zero, but it is not a direct memory disclosure or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)	YES	OOB SIMPLEFIX HARDWARE LINUS  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

* `+2 Remote reachable / network-triggerable`: `csum_partial()` is an architecture checksum helper used by networking paths, so remote traffic can plausibly trigger checksum calculation when the affected system is reachable.
* `+1 Common networking core path or packet-processing path`: checksum helpers are part of packet processing, not an obscure ioctl-only path.
* `+1 Integrity impact plausible`: the bug can compute checksums from address zero instead of `src`, causing incorrect checksum generation or validation.
* `+1 Availability impact realistic`: incorrect checksums can cause packet rejection, protocol failures, or network degradation on affected systems.
* `+1 Confidentiality impact plausible, paranoid only`: the returned checksum may depend on data read from address zero, but this is only a weak checksum-oracle style concern, not direct memory disclosure.
* `-2 Rare hardware-only condition`: the bug is limited to s390 machines without the vector facility, which is a narrow hardware/configuration condition.
* `+0 Memory corruption`: this is an invalid read / wrong-buffer checksum calculation, not UAF, OOB write, double free, type confusion, or arbitrary write.
* `+0 Privilege escalation`: no credible LPE primitive is shown by the patch or commit context.

Call-site confidence: medium. The direct call sites are not included, but the helper name and commit message clearly tie this to `csum_partial()` checksum behavior.

## 3. Reachability analysis

A remote peer may influence packet data and cause checksum work through normal networking paths, so PR:N is defensible for CVSS-style paranoid reachability. However, exploitation depends on an s390 target without vector facility and on traffic paths that use this fallback. Namespaces and containers do not materially change the trigger requirement because this is below namespace policy in the architecture checksum helper. The path is common once the affected hardware condition is met, but the affected hardware condition is narrow.

## 4. Severity interpretation

This behaves like an actionable Moderate / borderline manual-review issue, not an Important-class kernel vulnerability. The realistic impact is incorrect checksum behavior with network integrity and availability effects. The theoretical confidentiality concern is limited because checksum output may depend on address-zero data, but there is no direct read primitive, no attacker-controlled overwrite, and no demonstrated privilege escalation path.

## 5. One-sentence report phrase

On s390 systems without the vector facility, csum_partial() may checksum address zero instead of the source buffer, allowing network-triggered checksum corruption and possible packet-processing disruption under affected hardware conditions.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network-triggerable in a common packet-processing helper on affected s390 systems and involves an invalid kernel read source, even though the currently supported impact looks limited to checksum integrity degradation and availability rather than strong memory corruption or LPE.

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

Patch: s390/checksum: Fix csum_partial() without vector facility [ Upstream
Commit: 5fc0a2a6eeb99cac991242bb48796c7749ce3261
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5fc0a2a6eeb99cac991242bb48796c7749ce3261

Changed files:
  arch/s390/lib/csum-partial.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=5fc0a2a6eeb99cac991242bb48796c7749ce3261

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

The original Linux kernel CVE announcement for CVE-2026-68385 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68385

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 5.6

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: 3
  Paranoid ActionableScore: 4

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).