[CVE-2026-64384][IMPORTANT] smb: client: fix change notify replay double-free

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

CVE: CVE-2026-64384
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: smb: client: fix change notify replay double-free
Commit: 5821f9dbb8b5b24391850a13418e633edd0fb003
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5821f9dbb8b5b24391850a13418e633edd0fb003
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64384
Analysis date: Sat, 25 Jul 2026 14:12:14 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An SMB client change notify replay bug can double-free a stale response buffer when a replayable server error is followed by SMB2_notify_init failure, allowing a malicious or compromised SMB server to trigger kernel memory corruption and at least a denial of service.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64384 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64384

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-2026-64384	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:N/S:U/C:H/I:H/A:H';CWE-415;*CWE-672;CWE-703;Other CVSS 'AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'SMB client change notify replay can hit a double free because response buffer bookkeeping from a previous attempt can remain valid after that response was already freed. A malicious or compromised SMB server can influence the replayable error path and make later cleanup free the stale response again if SMB2_notify_init fails before the next send. For the CVSS the PR:N means the attacker does not need an account on the victim host, but must control or tamper with the SMB server path used by the client. The base impact is at least denial of service via kernel crash. For the paranoid score, this is treated as kernel memory corruption with possible confidentiality and integrity impact, so manual review is required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	DANGER DISK UAF SERVERTOCLIENT KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. A malicious or compromised SMB server can influence the SMB2 change notify replay error path seen by the client.
* Memory corruption, strong corruption primitive: +2. The patch and commit explicitly describe a response buffer double-free.
* Real lifetime corruption: +1. The stale response bookkeeping can cause cleanup to free a buffer that was already freed by a previous response-bearing attempt.
* Reliable kernel crash / strong DoS: +1. Double-free in kernel SMB client cleanup is a credible kernel crash path.
* Broad/default/common subsystem exposure: +1. SMB/CIFS client is a commonly deployed network filesystem client, though it requires the client to use an SMB share.
* Hard or unreliable race / special sequence required: -1. Triggering requires a replayable error plus SMB2_notify_init failure before the next send, so the sequence is non-trivial.

Paranoid additional signals:

* Confidentiality impact plausible: +1. Double-free is a kernel heap corruption primitive, and in worst case may allow more than a crash if allocator reuse is controllable.
* Integrity impact plausible: +1. Same reason. This is not proven, but is defensible for manual triage because the primitive is double-free in kernel network filesystem code.

## 3. Reachability analysis

The likely attacker is a malicious SMB server, compromised SMB server, or network attacker able to tamper with the SMB server path used by the client. The attacker does not need a local account on the victim host, but the victim must already connect to or be induced to use the affected SMB server path. This is not a generic Internet-exposed listener on the victim. It is server-to-client attack surface in the SMB client.

Namespaces do not materially reduce the concern if a container or workload can mount or access an SMB share through host kernel CIFS code, but mounting SMB filesystems is often privileged in typical deployments. Existing mounted shares or automount setups can make the client-side processing occur without additional local privileges from the remote server side.

Call-site confidence: medium. The patch context and commit message directly identify SMB2_change_notify replay cleanup and stale response buffer freeing, but the full cleanup call chain is not included.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup bug. The conservative interpretation is an Actionable Moderate because the primitive is a real double-free with remote server influence and likely kernel crash impact. The paranoid interpretation is a Strong Important candidate because double-free is a concrete memory corruption class, and SMB client code processes attacker-influenced network responses from a server.

Realistic demonstrated impact is DoS. Theoretical impact can include broader heap corruption consequences, but no arbitrary write, controlled reclaim, or working privilege escalation chain is shown in the patch.

## 5. One-sentence report phrase

An SMB client change notify replay bug can double-free a stale response buffer when a replayable server error is followed by SMB2_notify_init failure, allowing a malicious or compromised SMB server to trigger kernel memory corruption and at least a denial of service.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch fixes an explicit double-free in kernel SMB client code with network-influenced inputs. Even if the reliable impact is likely DoS, the memory corruption primitive is strong enough that this should not be auto-closed.

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UPSTREAM PATCH SUMMARY
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Patch: smb: client: fix change notify replay double-free
Commit: 5821f9dbb8b5b24391850a13418e633edd0fb003
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5821f9dbb8b5b24391850a13418e633edd0fb003

Commit description:

A response-bearing attempt can return a replayable error and free its
response buffer. If SMB2_notify_init() fails before the next send, cleanup
retains the previous buffer type and frees that response again.

Reset response bookkeeping before each attempt to prevent the stale free.

Fixes: 4f1fffa ("cifs: commands that are retried should have replay flag set")
Cc: stable@vger.kernel.org
Signed-off-by: Henrique Carvalho <henrique.carvalho@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/client/smb2pdu.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=5821f9dbb8b5b24391850a13418e633edd0fb003

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

The original Linux kernel CVE announcement for CVE-2026-64384 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64384

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:R/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 6
  Paranoid ActionableScore: 8

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).