[CVE-2026-64597][IMPORTANT] smb: client: fix double-free in SMB2_close() replay

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

CVE: CVE-2026-64597
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: smb: client: fix double-free in SMB2_close() replay
Commit: 037511726228aaf165c7067ff2bfc88eaecdf1f3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=037511726228aaf165c7067ff2bfc88eaecdf1f3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64597
Analysis date: Thu, 06 Aug 2026 04:11:33 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A network-influenced double-free in the SMB/CIFS client `SMB2_close()` replay path can occur when stale response buffer bookkeeping is reused across retry attempts, allowing a malicious or compromised SMB server to trigger kernel memory corruption and likely DoS.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64597 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64597

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-2026-64597	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-416;*CWE-664;Other CVSS 'AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A double-free can occur in the SMB/CIFS client SMB2_close() replay path because response buffer bookkeeping can remain stale across retry attempts. A response bearing attempt can return a replayable error and free its response buffer, then a later SMB2_close_init() failure before the next send can make cleanup free the same response again. For the CVSS the PR:N is used because the attacker does not need a local account on the victim when acting as a malicious SMB server or network endpoint. The issue is network reachable when the victim client connects to an attacker controlled or compromised SMB server, or when SMB traffic can be tampered with by a network attacker. Impact is at least denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because double-free is a memory corruption primitive.';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. The vulnerable SMB client path can be influenced by a malicious or compromised SMB server, or by tampering with SMB traffic after the victim connects.
* Memory corruption, strong corruption primitive: +2. The commit explicitly describes a double-free of a response buffer in `SMB2_close()` replay handling.
* Real lifetime corruption: +1. The stale `resp_buftype` and `rsp_iov` state can cause cleanup to free a buffer that was already released.
* Reliable kernel crash / strong DoS: +1. A double-free in kernel SMB client cleanup is a credible kernel crash condition.
* Broad deployed exposure: +1. SMB/CIFS client support is common in enterprise and workstation environments, although it requires an SMB mount or connection.
* Corruption primitive constrained or not fully demonstrated: -1. The patch proves double-free, but does not show attacker-controlled reclaim, controlled replacement, callback control, or arbitrary write.

Paranoid additional signals:

* Confidentiality impact plausible: +1. Kernel double-free may expose broader memory corruption risk even though no leak primitive is shown.
* Integrity impact plausible: +1. Kernel double-free may allow corruption beyond DoS in a worst-case allocator/reclaim scenario, but this remains unproven.

Not awarded:

* Strong LPE plausibility: +0. No attacker-controlled reclaim, object replacement, function pointer control, or arbitrary write is demonstrated.
* Availability impact realistic: +0 extra. Crash impact is already counted under strong DoS.

Call-site confidence: high. The patch and commit directly identify the affected function, stale response bookkeeping, replay path, and double-free condition.

## 3. Reachability analysis

The attacker is typically a malicious SMB server, a compromised SMB server, or a network attacker capable of tampering with SMB traffic. The victim must have an SMB/CIFS client session or otherwise initiate access to the attacker-influenced server, so this is not unsolicited Internet-wide reachability like a listening server bug.

No local account on the victim is required for the attacker in the malicious-server model. Namespaces and containers do not materially reduce the risk if the host kernel SMB client performs the mount or connection. If an untrusted container can trigger host CIFS mounts through delegated infrastructure, practical exposure may increase, but ordinary container users usually cannot mount CIFS without elevated privileges.

The path is not always default-active because SMB client mounts are configuration-dependent. However, SMB/CIFS is common enough that this should not be treated as a rare subsystem.

## 4. Severity interpretation

This is not an ordinary Moderate correctness issue. The demonstrated primitive is a double-free in a network-influenced kernel client path. Realistic impact is at least remote-triggered kernel DoS against systems that connect to a malicious or compromised SMB server.

The conservative score treats the bug as Actionable Moderate because exploitation conditions are specific and no controlled reclaim or code execution primitive is shown. The paranoid score reaches Strong Important candidate because double-free is a concrete memory corruption class and the affected path is influenced by network-provided SMB responses.

## 5. One-sentence report phrase

A network-influenced double-free in the SMB/CIFS client `SMB2_close()` replay path can occur when stale response buffer bookkeeping is reused across retry attempts, allowing a malicious or compromised SMB server to trigger kernel memory corruption and likely DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is network-influenced kernel memory corruption with an explicit double-free primitive. Even if practical exploitation is likely DoS-only, the allocator and replay-path details should be reviewed manually before downgrading.

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UPSTREAM PATCH SUMMARY
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Patch: smb: client: fix double-free in SMB2_close() replay
Commit: 037511726228aaf165c7067ff2bfc88eaecdf1f3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=037511726228aaf165c7067ff2bfc88eaecdf1f3

Commit description:

A response-bearing attempt can return a replayable error and free its
response buffer. If SMB2_close_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=037511726228aaf165c7067ff2bfc88eaecdf1f3

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

The original Linux kernel CVE announcement for CVE-2026-64597 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64597

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:L/I:L/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).