[CVE-2026-89641][MODERATE 7.0] cifs: clear tcon after cifsFileInfo_put() in cifs_file_set_size()

[CVE-2026-89641][MODERATE 7.0] cifs: clear tcon after cifsFileInfo_put() in cifs_file_set_size()

[AL-KERNEL]

CVE: CVE-2026-89641
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: cifs: clear tcon after cifsFileInfo_put() in cifs_file_set_size()
Commit: 4f18c9e7ee464aaae5cd9fccdb943b3fcb4655d4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4f18c9e7ee464aaae5cd9fccdb943b3fcb4655d4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89641
Analysis date: Sat, 12 Sep 2026 01:07:01 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89641 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89641

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89641	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89641 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in cifs_file_set_size() because tcon and server are borrowed from an open file tlink and then reused after cifsFileInfo_put() may drop the last reference. If the handle based set_file_size() RPC fails and the code falls back to set_path_size(), the stale tcon can be used after the tlink was removed during reconnect or session teardown. For the CVSS the PR:L is used because a local process with access to a mounted CIFS path is normally needed to trigger the size change, while a malicious or unstable SMB server can make the race more reachable but does not by itself provide a pure packet only trigger. The issue is not directly network reachable in the selected scoring, but it is network assisted in SMB/CIFS deployments. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel use-after-free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	DISK UAF SERVERTOCLIENT LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap changed the score.
* Raw paranoid score: 6
* Paranoid score: 6. No floor or cap changed the score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive conservative signals:

* +2 Real UAF / lifetime violation: `tcon` and `server` are borrowed from the open file tlink, then `cifsFileInfo_put(open_file)` may drop the final reference and free `tcon`, while the same borrowed pointer remains reachable.
* +1 Reliable kernel crash / strong DoS: subsequent `set_path_size()` may use freed `tcon` / `server`, supporting kernel fault or crash.
* +1 Local below-host-admin trigger: a local process with access to an already mounted CIFS path can normally exercise file-size changing paths such as truncate or O_TRUNC behavior.
* +1 Important filesystem/storage path: the stale object is used in SMB/CIFS client file size update logic, not in a purely diagnostic path.

Positive paranoid-only signal:

* +1 Weak exploitation concern: the stale object is an active structured CIFS connection/tree object consumed by a path-based RPC fallback after lifetime loss. This is more meaningful than a shallow cleanup-only stale dereference, although controlled reclaim or arbitrary write is not demonstrated.

Non-awarded / withheld signals:

* +0 Controlled reclaim not demonstrated.
* +0 Arbitrary or directed kernel write not demonstrated.
* +0 Confidentiality and integrity impact not awarded as direct signals. They remain worst-case concerns from UAF semantics, not established consequences.
* +0 Remote reachability not awarded. A malicious or unstable SMB server may assist by causing failure/reconnect/session teardown, but the selected attacker path still needs local CIFS filesystem access.
* +0 Concrete repeated release/refcount signal not awarded. `cifsFileInfo_put()` can end the lifetime, and the exact borrowed `tcon` survives, but the later operation is `set_path_size()` using the stale object, not a second concrete `put/release/refcount` mutation on the same reference.
* +0 Structured stale-object leverage was not awarded conservatively because controlled replacement or attacker-shaped post-free contents are not established.
* +0 Extra availability point not awarded because the same crash is already counted.
* +0 Race/timing penalty not applied. The bug depends on reconnect/session teardown and RPC failure conditions, but the supplied evidence does not prove a one-shot or externally uncontrollable timing window.

Repeated-release/refcount checklist:

* First release can end lifetime: yes, `cifsFileInfo_put()` may lead to `cifs_put_tlink()` freeing `tcon`.
* Exact same reference survives: yes, borrowed `tcon` remains non-NULL and bypasses the `if (tcon == NULL)` reacquire path.
* Second operation is a real lifetime/refcount mutation: no, the second operation is stale use by `set_path_size()`.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no explicit floor needed because raw conservative score already reaches 5.

3. Reachability analysis

The practical trigger is local code operating on an existing CIFS mount and causing the file-size update path to use a writable handle first, then fall back to the path-based operation after `set_file_size()` fails. A malicious or unstable SMB server, reconnect event, or session teardown can make the lifetime condition more reachable, because the tlink may already be removed from the tlink tree when `cifsFileInfo_put()` drops the final reference.

Host administrator privilege is not required for the file operation itself once the CIFS mount is available to the user, so `HostAdminRequired=NO` for the evaluated vulnerable operation. There may be prior privileged setup for creating the CIFS mount, but that is deployment setup rather than proof that the attacker must be host admin to trigger the bug on an already configured system.

Namespace-scoped privilege is not the relevant model here, so `NamespaceScopedPrivilege=NO`. This is not a pure network packet-triggered vulnerability from an unauthenticated remote host. It is best described as local filesystem access with possible SMB-server-assisted timing/failure conditions.

Race repeatability is unresolved but not proven to be prohibitively narrow. Repeated local file-size operations plus reconnect or teardown conditions may make attempts repeatable in practice.

4. Primitive and severity interpretation

The exact primitive is a kernel use-after-free of a CIFS `tcon` / `server`-related object reference. The stale pointer is an active structured object used by SMB/CIFS client path-based file size update logic. This is not fixed stale storage and not merely passive metadata.

The stale use is broader than ordinary cleanup-only dereference because the freed connection/tree context can be passed into `set_path_size()` after the handle-based RPC failed. However, the patch does not establish controlled reclaim, attacker-shaped replacement contents, callback control, arbitrary read/write, credential modification, or a demonstrated LPE chain.

This is `CLASS_B`: a real active-object lifetime violation requiring exploitation-leverage review. The demonstrated consequence is local DoS via kernel crash. The speculative consequence is memory corruption with possible confidentiality/integrity impact or privilege escalation, but that is not proven by the supplied patch. Because the stale object is active and the bug is a real UAF in filesystem client code reachable below host admin on configured systems, it should not be automatically downgraded to ordinary Moderate.

ImportantCandidate is YES because meaningful Important-vs-Moderate ambiguity remains for an active structured UAF. AutoDowngradeAllowed is NO because automatic closure would risk discarding an unresolved kernel lifetime-corruption primitive.

5. One-sentence report phrase

A use-after-free in `cifs_file_set_size()` can reuse a borrowed CIFS `tcon` after `cifsFileInfo_put()` drops the last tlink reference, allowing a local user with access to a mounted CIFS path and suitable reconnect or RPC failure conditions to trigger a kernel crash, with unresolved memory-corruption impact requiring manual review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real active structured kernel UAF in CIFS client file-size handling. DoS is the demonstrated impact, but the stale object is used in nontrivial filesystem/network-client operation and the exploitation leverage beyond crash is unresolved.

7. Call-site confidence

Call-site confidence: medium

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

Patch: cifs: clear tcon after cifsFileInfo_put() in cifs_file_set_size()
Commit: 4f18c9e7ee464aaae5cd9fccdb943b3fcb4655d4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4f18c9e7ee464aaae5cd9fccdb943b3fcb4655d4

Commit description:

When the else branch of cifs_file_set_size() finds a writable file handle
via find_writable_file(), it borrows tcon and server from the handle's
tlink, attempts the handle-based set_file_size() RPC, and then releases
the handle with cifsFileInfo_put().

If set_file_size() fails, execution falls through to the path-based
fallback, which reuses the borrowed tcon and server under the
"if (tcon == NULL)" guard.  Since tcon is not NULL at that point, the
guard is skipped.  If cifsFileInfo_put() dropped the last reference on a
tlink that was already removed from the tlink tree (TCON_LINK_IN_TREE
cleared, as happens during reconnection or session teardown),
cifs_put_tlink() will have freed tcon; the subsequent set_path_size()
call is then a use-after-free.

Setting tcon = NULL after cifsFileInfo_put() causes the existing guard
to take the cifs_sb_tlink() path, which acquires a fresh reference for
the path-based operation or fails cleanly if the session is gone.

Fixes: 110fee6 ("smb: client: fix missing timestamp updates with O_TRUNC")
Cc: stable@vger.kernel.org
Cc: Paulo Alcantara <pc@manguebit.com>
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/client/inode.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=4f18c9e7ee464aaae5cd9fccdb943b3fcb4655d4

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

The original Linux kernel CVE announcement for CVE-2026-89641 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89641

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:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 6

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).