[CVE-2026-74608][MODERATE 7.0] smb: client: Fix use-after-free in cifs_try_adding_channels()

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

CVE: CVE-2026-74608
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: Fix use-after-free in cifs_try_adding_channels()
Commit: 64d7584e62ac8cdc750455c5fdc6008fc2de4f06
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=64d7584e62ac8cdc750455c5fdc6008fc2de4f06
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74608
Analysis date: Sat, 22 Aug 2026 16:56:30 -0400
ActionableScore: 8
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in the SMB client multichannel failure path can free an iface object via kref_put and then write to iface weight_fulfilled, creating a network-assisted kernel use-after-free that can cause DoS and may warrant manual review for broader memory corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74608 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74608

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: MODERATE 7.0
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-2026-74608	MODERATE	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-416;CWE-362;CWE-667;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A use after free in the SMB client multichannel path can occur in cifs_try_adding_channels when cifs_ses_add_channel fails while a concurrent interface list refresh removes the remaining list reference to the same iface object. The failure path dropped the temporary kref before updating iface weight_fulfilled, so kref_put could free iface and the later increment could write to freed kernel memory. For the CVSS the PR:N is used because a malicious or compromised SMB server does not need local privileges on the client once the client is connected to the SMB session. The issue is network reachable in deployments using SMB multichannel, but reliable triggering depends on a race with interface list refresh and channel creation failure, so AC:H is appropriate. Impact is at least denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the bug is a kernel UAF write rather than a pure NULL pointer dereference.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	DISK UAF SERVERTOCLIENT KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: **5**
* Paranoid score: **8**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* **Remote reachable / network-triggerable: +2**. This is in the SMB client multichannel path. A malicious or compromised SMB server can plausibly influence channel creation failure after the client connects.
* **Memory corruption, strong corruption primitive: +2**. The bug is a real UAF with a write after free: `iface->weight_fulfilled++` can execute after `kref_put()` frees `iface`.
* **Real lifetime corruption: +1**. The issue is caused by incorrect reference lifetime ordering around `kref_put()` and concurrent interface list refresh.
* **Reliable kernel crash / strong DoS: +1**. A write to freed kernel memory can realistically crash the kernel.
* **Broad/default/common subsystem exposure: +1**. SMB/CIFS client is a broadly deployed kernel filesystem client, although the exact path depends on multichannel.
* **Hard or unreliable race / special timing required: -1**. Triggering requires a race between channel creation failure and interface list refresh.
* **Configuration-dependent multichannel path: -1**. The affected code is tied to SMB multichannel behavior and is not necessarily exercised by all SMB mounts.

Paranoid additional signals:

* **Weak LPE concern: +1**. The primitive is a stale write into freed kernel memory, but no attacker-controlled reclaim or object replacement is shown.
* **Confidentiality impact plausible: +1**. In the paranoid interpretation, UAF corruption of a reused object could theoretically affect kernel memory state beyond a crash.
* **Integrity impact plausible: +1**. The stale increment is a write after free, so integrity impact is plausible in worst case, although not demonstrated.

No arbitrary write, callback control, type confusion, or controlled reclaim is demonstrated by the patch.

## 3. Reachability analysis

The likely attacker is a malicious or compromised SMB server, or an attacker who can influence the SMB server endpoint used by the client. The client must establish an SMB session where multichannel channel creation is attempted. No local privileges on the client are needed for the remote server side of the trigger, so PR:N is reasonable for CVSS-style remote-client exposure.

Namespaces and containers do not materially lower the requirement for the remote-server scenario. If the only attacker is local, then the attacker would need the ability to cause SMB mounts or influence SMB session setup, which is more privileged. The realistic network condition is not generic Internet exposure of the kernel, but exposure through a client connecting to an untrusted or compromised SMB server.

Call-site confidence: **high for the UAF and stale write**, because the patch directly moves the write before `kref_put()`. Confidence is **medium for reliable remote triggering**, because it depends on SMB multichannel behavior and a race with interface refresh.

## 4. Severity interpretation

This is not an ordinary Moderate resource leak or validation-only fix. It is a real kernel lifetime bug with a write-after-free in a network filesystem client path.

Conservatively, it is an **Actionable Moderate** because exploitation requires a timing race and specific SMB multichannel conditions. Paranoid handling reaches **Strong Important candidate** because the primitive is not merely a NULL dereference. It is a stale write to freed kernel memory, and network-assisted triggering by a malicious SMB server is plausible.

Theoretical privilege escalation is not proven. However, the bug class and write-after-free behavior justify manual review rather than auto-closing as DoS-only.

## 5. One-sentence report phrase

A race in the SMB client multichannel failure path can free an iface object via kref_put and then write to iface weight_fulfilled, creating a network-assisted kernel use-after-free that can cause DoS and may warrant manual review for broader memory corruption impact.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.**

Reason: network-assisted SMB client UAF with a stale write after free. Even though the race and multichannel conditions reduce reliability, this should not be auto-closed because the primitive is real kernel memory corruption rather than a pure crash-only validation bug.

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UPSTREAM PATCH SUMMARY
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Patch: smb: client: Fix use-after-free in cifs_try_adding_channels()
Commit: 64d7584e62ac8cdc750455c5fdc6008fc2de4f06
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=64d7584e62ac8cdc750455c5fdc6008fc2de4f06

Commit description:

cifs_try_adding_channels() takes a temporary reference to an interface
before dropping iface_lock. If cifs_ses_add_channel() fails, it drops
that reference and then increments iface->weight_fulfilled.

A concurrent interface list refresh can remove the list reference while
channel creation is in progress. In that case, the failure-path
kref_put() releases the last reference and frees iface. Updating
weight_fulfilled afterward then accesses freed memory.

Increment weight_fulfilled before dropping the temporary reference,
keeping iface alive for the final access.

Fixes: 6aac002 ("cifs: failure to add channel on iface should bump up weight")
Cc: stable@vger.kernel.org
Signed-off-by: Shuangpeng Bai <shuangpeng.kernel@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/client/sess.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=64d7584e62ac8cdc750455c5fdc6008fc2de4f06

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

The original Linux kernel CVE announcement for CVE-2026-74608 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74608

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:N/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: 5
  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: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).