[CVE-2026-64385][IMPORTANT] smb: client: fix double-free in SMB2_ioctl() replay

[CVE-2026-64385][IMPORTANT] smb: client: fix double-free in SMB2_ioctl() replay

[AL-KERNEL]

CVE: CVE-2026-64385
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: smb: client: fix double-free in SMB2_ioctl() replay
Commit: 0be4bc64882edaefaaee8d1e27d083643eb778e6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0be4bc64882edaefaaee8d1e27d083643eb778e6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64385
Analysis date: Sat, 25 Jul 2026 12:55:59 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A double-free in the SMB client SMB2_ioctl() replay path may allow a malicious SMB server to trigger stale response-buffer cleanup after a replayable error, causing at least a kernel crash and requiring manual review for potential memory-corruption impact beyond DoS.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64385 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64385

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-2026-64385	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-415;*CWE-416;CWE-703;Other CVSS 'AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'A double free in the SMB client SMB2_ioctl() replay path can occur when a response bearing attempt returns a replayable error and frees its response buffer, but later cleanup still uses stale response bookkeeping after SMB2_ioctl_init() fails before the next send. A malicious or compromised SMB server may be able to drive the client into this state through crafted server responses during an ioctl replay sequence, although reliable triggering depends on a specific error and retry ordering. For the CVSS the PR:N because the attacker does not need a local account on the victim once the victim client communicates with an attacker controlled SMB server. UI:R is used because the victim normally has to mount or access the SMB share or otherwise initiate client communication. Impact is at least remote denial of service via kernel crash, and in the paranoid interpretation may include confidentiality and integrity impact because kernel double free is a memory corruption primitive that can sometimes be shaped into privilege escalation.';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=7

## 1. ActionableScore

* Conservative score: **7**
* Paranoid score: **8**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Remote/network-triggerable: **+2**. The vulnerable SMB client path can be influenced by responses from a malicious or compromised SMB server after the victim client connects.
* Memory corruption, strong corruption primitive: **+2**. The commit explicitly describes a **double-free** of a response buffer.
* Real lifetime corruption: **+1**. The bug is stale response bookkeeping across replay attempts, causing a freed buffer to be freed again.
* Reliable kernel crash / strong DoS: **+1**. A kernel double-free in the SMB client cleanup path is a realistic crash condition.
* Privilege escalation plausible: **+1 conservative / +2 paranoid**. Conservative scoring treats this as a serious but not demonstrated LPE concern because no reclaim or controlled replacement primitive is shown. Paranoid scoring raises it because kernel double-free bugs can sometimes be shaped into memory corruption beyond DoS.
* Broad/common subsystem exposure: **+1**. SMB/CIFS client support is widely deployed, although exploitation requires the client to connect to an attacker-controlled or compromised server.
* Hard or specific sequencing required: **-1**. Triggering depends on a response-bearing replayable error followed by SMB2_ioctl_init() failure before the next send.

## 3. Reachability analysis

The attacker most plausibly controls or compromises an SMB server and waits for the victim Linux SMB client to connect and execute the affected SMB2_ioctl() replay path. This is not a server-side listening attack against every host on the network. It is a client-side network-triggered issue with user or system interaction, such as mounting or accessing an SMB share.

Privileges on the victim are not required for the remote attacker once the victim communicates with the malicious server. Local mounting may require CAP_SYS_ADMIN or helper-mediated policy, but that is a victim-side precondition rather than attacker PR. Containers and namespaces do not substantially reduce the remote attacker requirement, but they may affect who can initiate SMB mounts locally.

Realistic exploitation requires a specific replay/error ordering, so exploitation is harder than a simple malformed packet crash. Still, the primitive is a kernel double-free in a network filesystem client path, so it should not be treated as ordinary Moderate.

## 4. Severity interpretation

This behaves like an **Important candidate**, or at minimum an **Actionable Moderate that should not be auto-closed**. The realistic demonstrated impact is kernel crash or DoS. The theoretical concern is stronger because the patch fixes a real double-free, which is a memory lifetime corruption primitive and can sometimes become exploitable beyond DoS.

There is no demonstrated arbitrary write, controlled reclaim, or public exploit path in the provided material. Therefore the conservative score does not assume reliable privilege escalation. The paranoid score keeps the issue elevated because double-free in kernel network-client code is historically risky and requires manual review.

## 5. One-sentence report phrase

A double-free in the SMB client SMB2_ioctl() replay path may allow a malicious SMB server to trigger stale response-buffer cleanup after a replayable error, causing at least a kernel crash and requiring manual review for potential memory-corruption impact beyond DoS.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is explicit kernel double-free memory corruption reachable through SMB server responses, with realistic DoS and plausible but unproven escalation potential.

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

Patch: smb: client: fix double-free in SMB2_ioctl() replay
Commit: 0be4bc64882edaefaaee8d1e27d083643eb778e6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0be4bc64882edaefaaee8d1e27d083643eb778e6

Commit description:

A response-bearing attempt can return a replayable error and free its
response buffer. If SMB2_ioctl_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=0be4bc64882edaefaaee8d1e27d083643eb778e6

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

The original Linux kernel CVE announcement for CVE-2026-64385 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64385

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: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: 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).