[CVE-2026-64383][IMPORTANT] smb: client: fix double-free in SMB2_flush() replay

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

CVE: CVE-2026-64383
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: smb: client: fix double-free in SMB2_flush() replay
Commit: 6e27f40b682a5e42a2daae3ce6d96f0e0e16dedb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e27f40b682a5e42a2daae3ce6d96f0e0e16dedb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64383
Analysis date: Sat, 25 Jul 2026 15:44:34 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A double-free in the Linux SMB client SMB2_flush replay path can be triggered by a malicious or compromised SMB server through stale response buffer state across replay attempts, causing at least kernel 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-2026-64383 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64383

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-2026-64383	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;CWE-703;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 in the SMB client SMB2_flush replay path can occur because resp_buftype and rsp_iov keep stale response buffer state across replay attempts. A malicious or compromised SMB server can return a replayable flush response and then cause a later retry to fail before cifs_send_recv stores a replacement response, so flush_exit may free the old response pointer again. For the CVSS the PR:N is used because the attacker does not need a local account or privileges on the client when acting as the SMB server or a network attacker able to tamper with the SMB session. Base impact is at least denial of service via kernel crash, while the paranoid score treats the kernel double-free as memory corruption with possible confidentiality, integrity, and availability impact. The issue is network reachable in deployments where the vulnerable client mounts or accesses an attacker controlled or compromised SMB share, though practical exploitation likely requires precise replay and failure conditions.';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 SMB2 flush replay responses over the network.
* Generic memory corruption, strong corruption primitive: +2. The commit explicitly fixes a double-free.
* Real lifetime corruption: +1. Stale response buffer bookkeeping survives across replay attempts and can be freed again.
* Reliable kernel crash / strong DoS: +1. Double-free in kernel SMB client response handling is a credible crash path.
* Important filesystem/storage path: +1. This is in the SMB client filesystem path.
* Special replay and retry failure conditions: -1. Triggering requires a replayable flush response followed by a retry failure before replacement response state is stored.

Conservative total: 6

Paranoid additions:

* Weak LPE concern: +1. Kernel double-free can sometimes become exploitable beyond DoS, although no reclaim or controlled replacement primitive is shown here.
* Confidentiality impact plausible: +1. Only for paranoid handling due to memory corruption class.
* Integrity impact plausible: +1. Only for paranoid handling due to memory corruption class.
* Practical cap for high-complexity network client double-free: final paranoid score kept at 8 rather than 9 because exploitation depends on replay/error sequencing and no controlled reclaim primitive is demonstrated.

Paranoid total: 8

## 3. Reachability analysis

The likely attacker is a malicious SMB server, compromised SMB server, or network attacker able to tamper with an SMB session. The victim side is the Linux SMB client. No local account on the victim is needed if the client already mounts or accesses the attacker-controlled share, so attacker-side PR is effectively none.

The path is not an Internet-listening server path on the victim. It is network reachable in the client-to-server trust direction. Realistic exploitation requires the victim to connect to a malicious or compromised SMB endpoint and requires specific replay behavior plus a retry failure before `cifs_send_recv()` replaces the response pointer.

Namespaces and containers do not materially reduce the remote-server attack model, but creating the SMB mount itself is usually privileged unless an existing mount, automount, desktop file manager, or managed environment causes access to the share.

Call-site confidence: high. The patch and commit directly identify `SMB2_flush()`, the replay loop, stale `resp_buftype` and `rsp_iov`, and the double-free cleanup path.

## 4. Severity interpretation

This is at least an Actionable Moderate issue because it is a network-influenced kernel double-free in a filesystem client path. It should not be auto-closed as an ordinary Moderate.

Realistically, the clearest impact is kernel crash or DoS. Theoretical exploitation beyond DoS is plausible because double-free is a memory corruption class, but the patch does not demonstrate attacker-controlled reclaim, object replacement, function pointer control, arbitrary write, or a reliable LPE chain. That keeps the conservative score below high-end Important, while the paranoid score is high enough for manual review.

## 5. One-sentence report phrase

A double-free in the Linux SMB client SMB2_flush replay path can be triggered by a malicious or compromised SMB server through stale response buffer state across replay attempts, causing at least kernel DoS and requiring manual review for possible memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a network-influenced kernel double-free in the SMB client. Even though practical exploitation likely requires specific replay and failure conditions, the memory-corruption class and remote server influence make it unsafe to auto-close.

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UPSTREAM PATCH SUMMARY
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Patch: smb: client: fix double-free in SMB2_flush() replay
Commit: 6e27f40b682a5e42a2daae3ce6d96f0e0e16dedb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e27f40b682a5e42a2daae3ce6d96f0e0e16dedb

Commit description:

SMB2_flush() keeps its response buffer bookkeeping across replay
attempts. If a replayable flush response is received and the retry then
fails before cifs_send_recv() stores a replacement response, flush_exit
will free the stale response pointer a second time.

Reinitialize resp_buftype and rsp_iov at the top of the replay loop so
cleanup only acts on response state produced by the current attempt.
This fixes a double-free without changing replay handling for successful
requests.

Fixes: 4f1fffa ("cifs: commands that are retried should have replay flag set")
Cc: stable@vger.kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Zhengchuan Liang <zcliangcn@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Assisted-by: Codex:GPT-5.4
Acked-by: Henrique Carvalho <henrique.carvalho@suse.com>
Signed-off-by: Zhao Zhang <zzhan461@ucr.edu>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
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=6e27f40b682a5e42a2daae3ce6d96f0e0e16dedb

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

The original Linux kernel CVE announcement for CVE-2026-64383 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64383

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).