[CVE-2026-64514][LOW] userfaultfd: gate must_wait writability check on pte_present() [ Upstream

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

CVE: CVE-2026-64514
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: userfaultfd: gate must_wait writability check on pte_present() [ Upstream
Commit: a5700a4c1c9099ac2ac73fe8a09cf059972e0d2f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a5700a4c1c9099ac2ac73fe8a09cf059972e0d2f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64514
Analysis date: Sat, 25 Jul 2026 11:14:57 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A local userfaultfd write-protect race can misclassify a non-present swap or migration PTE as still WP-blocked, causing the faulting thread to wait indefinitely for UFFDIO_WAKE and resulting in local denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-64514 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64514

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
======================================================================

CVE-2026-64514	LOW	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:N/I:N/A:H';CWE-362;CWE-667;CWE-755;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'userfaultfd may incorrectly keep a faulting thread asleep because userfaultfd_must_wait() and userfaultfd_huge_must_wait() can apply pte_write() or huge_pte_write() to a non-present swap or migration PTE read without the page table lock. On such entries the write bit check decodes offset bits rather than valid present PTE flags, so the code can incorrectly decide that the fault is still write-protect blocked and wait for a UFFDIO_WAKE that may never arrive. For the CVSS the PR:L is used because a local process needs access to userfaultfd and must participate in the affected fault handling path. The issue is not network reachable and is triggered by local memory management activity involving userfaultfd plus concurrent swap-out or migration. Impact is denial of service through a stuck fault path. No direct confidentiality or integrity impact is justified because the patch shows an incorrect state check rather than UAF, OOB access, or arbitrary write.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	WRITE OOB DISK IMPROVEONLY LINUS MEMORY  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process using userfaultfd write-protect handling may reach this path for its own mappings, depending on product sysctl and userfaultfd policy.
* Availability impact realistic: +1. The described failure mode is an indefinite wait for UFFDIO_WAKE, causing a stuck faulting thread or affected process.
* Hard or timing-dependent race: -1. The issue requires swap-out or migration to replace a present PTE between fault delivery and the lockless must_wait re-check.

Paranoid interpretation:

* Score 2 if the race is considered realistically triggerable under memory pressure or migration-heavy workloads and the stuck task affects a service.
* No memory-corruption, LPE, confidentiality, or integrity signals are justified.

## 3. Reachability analysis

The bug is locally reachable through userfaultfd fault handling, especially UFFD write-protect flows. It is not network reachable. A local process must participate in the affected memory-management path and hit a concurrent swap-out or migration window. Namespace impact depends on whether userfaultfd is enabled or restricted in the target product, but the patch does not show a container escape or cross-boundary primitive.

Call-site confidence: high, because the changed functions and their direct behavior are included in the patch.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local DoS issue, not an Important-class vulnerability. The patch fixes an incorrect interpretation of non-present swap or migration PTE bits by pte_write() or huge_pte_write(). The realistic impact is a stuck fault path. There is no demonstrated UAF, OOB access, arbitrary write, object lifetime corruption, page-cache corruption, or privilege-escalation primitive.

## 5. One-sentence report phrase

A local userfaultfd write-protect race can misclassify a non-present swap or migration PTE as still WP-blocked, causing the faulting thread to wait indefinitely for UFFDIO_WAKE and resulting in local denial of service.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

The issue is timing-dependent and availability-only based on the patch. It does not expose memory corruption, privilege escalation, network reachability, or a security-boundary bypass.

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

Patch: userfaultfd: gate must_wait writability check on pte_present() [ Upstream
Commit: a5700a4c1c9099ac2ac73fe8a09cf059972e0d2f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a5700a4c1c9099ac2ac73fe8a09cf059972e0d2f

Changed files:
  fs/userfaultfd.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=a5700a4c1c9099ac2ac73fe8a09cf059972e0d2f

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

The original Linux kernel CVE announcement for CVE-2026-64514 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64514

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1
  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: 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).