[CVE-2026-64590][LOW] dma-buf/udmabuf: skip redundant cpu sync to fix cacheline EEXIST warning [ Upstream

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

CVE: CVE-2026-64590
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: dma-buf/udmabuf: skip redundant cpu sync to fix cacheline EEXIST warning [ Upstream
Commit: 0db56e7eae932f8e2f3eb44ad1a63633d8f504f8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0db56e7eae932f8e2f3eb44ad1a63633d8f504f8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64590
Analysis date: Thu, 06 Aug 2026 04:22:59 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A local udmabuf to DRM import path can trigger a spurious DMA API debug EEXIST warning due to redundant CPU sync handling, but the patch shows no memory corruption or privilege escalation primitive and the practical impact is at most low availability on debug or panic_on_warn systems.

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

The original Linux kernel CVE announcement for CVE-2026-64590 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64590

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64590	LOW	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-703;CWE-755;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:N';BEST CVSS score: '3.3';DESCR 'udmabuf can trigger a spurious DMA API debug warning because the same pages may be mapped by udmabuf and by a DRM importer while CPU cache sync handling is already performed through explicit dma-buf CPU access hooks. The observed symptom is a cacheline tracking EEXIST warning under CONFIG_DMA_API_DEBUG_SG, not a demonstrated memory corruption primitive. For the CVSS the PR:L is used for the paranoid score because a local user or local process with access to udmabuf and DRM import paths may be able to trigger the warning. The issue is not network reachable and there is no evidence of confidentiality impact, integrity impact, privilege escalation, UAF, OOB access, or arbitrary write. Impact is at most low availability impact on systems where warnings are treated as fatal or where debug warning storms are operationally disruptive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)	YES	DMAorINTERRUPT HARDWARE MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=1
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: **0**
* Paranoid score: **1**
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Local trigger possible: **+1**. A local process using udmabuf and a DRM importer such as amdgpu may reach the path.
* Legacy / non-default debug condition: **-1**. The visible failure depends on `CONFIG_DMA_API_DEBUG_SG`, which is a debug configuration and not typical production behavior.
* Memory corruption: **+0**. The patch does not show UAF, OOB access, double free, arbitrary write, type confusion, or refcount misuse.
* Reliable kernel crash / strong DoS: **+0**. The described effect is a spurious DMA API warning, not a demonstrated panic or crash.
* Confidentiality / integrity impact: **+0 / +0**. No leak, overwrite, ownership bypass, or file-backed/page-cache corruption primitive is shown.

Paranoid extra signal:

* Low availability concern: **+1**. On systems configured with fatal warnings, debug warning storms, or strict `panic_on_warn` policy, this could become a local low-impact availability issue.

## 3. Reachability analysis

The path is locally reachable by a process that can create or use `udmabuf` objects and import them into a DRM driver path such as amdgpu. This is not network reachable. Containers or namespaces may affect access to DRM render nodes or udmabuf device nodes, but the bug still requires local device/API access, so PR is not treated as remote or unauthenticated. The practical symptom requires `CONFIG_DMA_API_DEBUG_SG`, and the commit explicitly describes the warning as spurious because CPU synchronization is already handled by dma-buf CPU access hooks.

Call-site confidence: **high**, because the commit includes the relevant call chain and the changed map/unmap call sites.

## 4. Severity interpretation

This behaves like a **Low-like / ordinary Moderate correctness issue**, not an Important-class vulnerability. The patch suppresses a false DMA API debug overlap warning and avoids redundant CPU cache synchronization. There is no realistic evidence of privilege escalation, memory corruption, DMA ownership corruption, information disclosure, or attacker-controlled overwrite. The theoretical worst case is limited availability impact on debug or warning-fatal systems.

## 5. One-sentence report phrase

A local udmabuf to DRM import path can trigger a spurious DMA API debug EEXIST warning due to redundant CPU sync handling, but the patch shows no memory corruption or privilege escalation primitive and the practical impact is at most low availability on debug or panic_on_warn systems.

## 6. Manual review recommendation

**NO MANUAL CHECK NEEDED**

This can be auto-closed for security triage unless external evidence shows a real crash, memory corruption, or production-impacting warning-to-panic configuration.

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

Patch: dma-buf/udmabuf: skip redundant cpu sync to fix cacheline EEXIST warning [ Upstream
Commit: 0db56e7eae932f8e2f3eb44ad1a63633d8f504f8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0db56e7eae932f8e2f3eb44ad1a63633d8f504f8

Changed files:
  kernel/dma/debug.c
  drivers/dma-buf/udmabuf.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=0db56e7eae932f8e2f3eb44ad1a63633d8f504f8

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

The original Linux kernel CVE announcement for CVE-2026-64590 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64590

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:L/UI:N/S:U/C:N/I:N/A:N
  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: 0
  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).