[CVE-2026-64061][IMPORTANT] netfs: Fix early put of sink folio in netfs_read_gaps() [ Upstream

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

CVE: CVE-2026-64061
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfs: Fix early put of sink folio in netfs_read_gaps() [ Upstream
Commit: 2a39d49c8d97df8cb8fa80c10859bc1ba7358c6b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2a39d49c8d97df8cb8fa80c10859bc1ba7358c6b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64061
Analysis date: Sun, 19 Jul 2026 17:12:33 -0400
ActionableScore: 9
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A write use-after-free in `netfs_read_gaps()` can occur because a sink folio is released before async read completion, allowing CIFS socket receive data to be copied into freed memory and causing kernel crash or potential memory corruption.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64061 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64061

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: IMPORTANT
Manual review required: YES

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-2026-64061	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'A use after free write in the netfs buffered read path occurs because netfs_read_gaps drops the sink folio before waiting for the asynchronous read request to complete. The iterator can still reference that folio while CIFS/SMB socket receive code copies TCP data into the read iterator, so later _copy_to_iter can write into freed memory. For the CVSS the PR:N in the paranoid network interpretation means the attacker does not need an account on the client when acting as a malicious or compromised file server, although the client must already access or be induced to access that server. The likely impact is at least denial of service via KASAN report or kernel crash. Because this is a write use after free, worst case memory corruption may allow confidentiality or integrity impact and possible privilege escalation, so manual review is required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)  SKIP CVE-2026-64061 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: ee4cdf7ba857a894ad1650d6ab77669cbbfa329e YES  NO NO unknown 	MAYBE	REMOTE DANGER DISK UAF HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=9
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: **7**
* Paranoid score: **9**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* **Remote reachable / network-triggerable: +2**
  In the paranoid interpretation, a malicious or compromised CIFS/SMB server can influence the data received by the client over TCP after the client accesses the share.

* **Local unprivileged trigger: +1**
  A local unprivileged user may be able to trigger the read path by reading files from an already mounted CIFS/netfs-backed filesystem.

* **Memory corruption, strong corruption primitive: +2**
  KASAN reports a real **write use-after-free** in `_copy_to_iter()`, not just a NULL dereference or WARN.

* **Real lifetime corruption: +1**
  The sink folio is released before `netfs_wait_for_read()`, while the async read iterator can still reference it.

* **Reliable kernel crash / strong DoS: +1**
  The reported KASAN UAF write is a strong crash/DoS condition and may become an Oops or panic depending on kernel config.

* **Weak LPE concern: +1**
  This is a write-after-free into a freed folio, but the patch does not demonstrate attacker-controlled reclaim or a reliable replacement object. LPE is plausible enough for manual review, not proven.

* **Confidentiality impact plausible: +1**
  UAF write corruption may affect reused kernel/page objects in worst case, but no direct info leak is shown.

* **Integrity impact plausible: +1**
  The primitive is a write into freed memory, with data coming from the network filesystem receive path.

* **Important filesystem/storage path: +1**
  The bug is in netfs buffered reads and is observed through CIFS receive/read handling.

Conservative score uses local mounted-filesystem reachability and does not fully rely on a malicious remote server model. Paranoid score includes the malicious/compromised server influence and worst-case UAF corruption impact.

## 3. Reachability analysis

A local user can likely trigger the vulnerable path by reading from an already mounted CIFS or other netfs-backed filesystem. Creating the mount is usually privileged, but using an existing mounted share may be available to unprivileged users depending on system policy.

A remote attacker cannot generally trigger this against an arbitrary host without the host connecting to or using an affected network filesystem. However, a malicious or compromised SMB/CIFS server can influence the TCP data path once the client performs reads, so the network-assisted scenario is technically defensible.

Namespaces and containers matter mainly if a container or service has access to the mounted network filesystem. The bug is not a normal inbound network service exposure, but it is reachable through a common filesystem client data path.

Call-site confidence: **high**, because the commit includes the concrete KASAN stack from CIFS socket receive into `_copy_to_iter()` and the patch directly moves `folio_put(sink)` after `netfs_wait_for_read()`.

## 4. Severity interpretation

This behaves above ordinary Moderate because the failure mode is a confirmed **write use-after-free** in a filesystem/network receive path. The realistic demonstrated impact is kernel memory corruption leading to crash or KASAN detection. Reliable privilege escalation is not demonstrated, but the primitive is strong enough that dismissing it as DoS-only would risk under-triage.

Conservative handling: **Strong Actionable Moderate / Important candidate**.
Paranoid handling: **High-end Important candidate**, due to network-influenced write UAF and possible corruption of reused memory.

## 5. One-sentence report phrase

A write use-after-free in `netfs_read_gaps()` can occur because a sink folio is released before async read completion, allowing CIFS socket receive data to be copied into freed memory and causing kernel crash or potential memory corruption.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.**

Reason: confirmed KASAN write UAF, async lifetime bug, filesystem/network receive path, and plausible but unproven LPE or broader memory corruption impact.

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

Patch: netfs: Fix early put of sink folio in netfs_read_gaps() [ Upstream
Commit: 2a39d49c8d97df8cb8fa80c10859bc1ba7358c6b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2a39d49c8d97df8cb8fa80c10859bc1ba7358c6b

Changed files:
  fs/netfs/buffered_read.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=2a39d49c8d97df8cb8fa80c10859bc1ba7358c6b

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

The original Linux kernel CVE announcement for CVE-2026-64061 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64061

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 7
  Paranoid ActionableScore: 9

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).