[CVE-2026-64387][IMPORTANT] smb: client: fix query directory replay double-free

[CVE-2026-64387][IMPORTANT] smb: client: fix query directory replay double-free

[AL-KERNEL]

CVE: CVE-2026-64387
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: smb: client: fix query directory replay double-free
Commit: 3409aedf3c81a810243da94164f6621c9d205c98
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3409aedf3c81a810243da94164f6621c9d205c98
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64387
Analysis date: Sat, 25 Jul 2026 09:47:28 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious or compromised SMB server may trigger a replay-path double-free in SMB2_query_directory() by causing stale response buffer bookkeeping to be reused after a replayable error, leading at minimum to kernel crash DoS and requiring manual review for possible memory-corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64387 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64387

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-2026-64387	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-672;Other CVSS 'AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'SMB2_query_directory() can double-free a response buffer across a replay path because resp_buftype and rsp_iov can retain stale response bookkeeping from a previous response-bearing attempt. A malicious or compromised SMB server can potentially trigger this when a replayable error frees the first response and the next query-directory initialization fails before a new send, causing cleanup to free the stale response again. For the CVSS the PR:N is used because the attacker does not need privileges on the vulnerable client if they control the SMB server or can tamper with SMB traffic, although the victim client must connect to or use that SMB share. The issue is network-reachable in a server-to-client direction and the attack complexity is high because it depends on a replayable error path and an initialization failure window. Impact is at least kernel crash DoS and in the paranoid case may include confidentiality or integrity impact because a kernel 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. A malicious or compromised SMB server can influence SMB client responses after the victim connects to the share.
* Generic memory corruption, strong corruption primitive: +2. The commit explicitly describes a double-free of a response buffer.
* Real lifetime corruption: +1. The stale response bookkeeping can cause cleanup to free an already freed response buffer.
* Reliable kernel crash / strong DoS: +1. A kernel double-free in the SMB client cleanup path can realistically crash the client system.
* Broad/common deployed exposure: +1. SMB/CIFS client is a common enterprise filesystem client path.
* Hard or unreliable replay/error condition: -1. Triggering depends on a replayable error followed by SMB2_query_directory_init() failure before the next send.

Paranoid additional signals:

* Weak LPE concern: +1. Double-free is a kernel memory corruption primitive, but the patch does not show controlled reclaim, callback control, or arbitrary write.
* Confidentiality impact plausible: +1. Kernel heap corruption can plausibly create read or disclosure consequences in worst case.
* Integrity impact plausible: +1. Kernel heap corruption can plausibly corrupt kernel state in worst case.

Call-site confidence: high. The commit message directly identifies the stale buffer state, replay path, cleanup behavior, and double-free condition.

## 3. Reachability analysis

The attacker normally needs to control a malicious SMB server, compromise the SMB server, or tamper with SMB traffic. No privileges are required on the vulnerable client in that server-to-client model, but the victim must connect to or already use the affected SMB share. This is network reachable, but not Internet-wide by default for most clients because SMB mounts are usually configured for enterprise, domain, VPN, or internal file-sharing environments. Containers and namespaces do not materially reduce the server-side trigger requirement, although any workload that can cause the host kernel SMB client to access a malicious share may expose the bug.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup bug. The realistic impact is likely kernel crash DoS because the trigger path is conditional and depends on replay/error sequencing. However, the underlying primitive is a real kernel double-free in a network-facing filesystem client path, so paranoid triage should treat it as an Important candidate requiring manual review. Theoretical privilege escalation is not proven, but memory corruption is explicit enough to avoid auto-closing.

## 5. One-sentence report phrase

A malicious or compromised SMB server may trigger a replay-path double-free in SMB2_query_directory() by causing stale response buffer bookkeeping to be reused after a replayable error, leading at minimum to kernel crash DoS and requiring manual review for possible memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: explicit network-influenced kernel double-free in SMB client code. Even though practical exploitation beyond DoS is not demonstrated, the memory lifetime primitive is strong enough that this should not be auto-closed.

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

Patch: smb: client: fix query directory replay double-free
Commit: 3409aedf3c81a810243da94164f6621c9d205c98
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3409aedf3c81a810243da94164f6621c9d205c98

Commit description:

A response-bearing attempt can return a replayable error and free its
response buffer. If SMB2_query_directory_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=3409aedf3c81a810243da94164f6621c9d205c98

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

The original Linux kernel CVE announcement for CVE-2026-64387 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64387

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: 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).