[CVE-2026-97562][MODERATE REGULAR] smb: client: pin DFS superblock in iterator callback

[CVE-2026-97562][MODERATE REGULAR] smb: client: pin DFS superblock in iterator callback

[AL-KERNEL]

CVE: CVE-2026-97562
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: smb: client: pin DFS superblock in iterator callback
Commit: ea43a15cea36dc5ddd832be5bddeac7dd804cbcf
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea43a15cea36dc5ddd832be5bddeac7dd804cbcf
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97562
Analysis date: Fri, 25 Sep 2026 12:22:56 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
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-2026-97562 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97562

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 REGULAR
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-2026-97562	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97562 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-664;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 DFS failover path occurs because tcon_super_cb stores a raw superblock pointer and cifs_sb_active later acquires its active reference after iterate_supers_type has released its protection. Concurrent DFS automount expiry can free the superblock in this interval, causing cifs_sb_active to dereference freed slab memory. The race was reproduced deterministically with KASAN. A remote attacker may be able to influence the vulnerable path through a malicious or compromised SMB DFS server that is already trusted and mounted by the victim. For the CVSS the PR:N value used for the paranoid score assumes the attacker controls such a network endpoint and does not require a local account on the victim. Exploitation requires a difficult timing condition involving DFS expiry and failover, therefore AC:H is appropriate. The base impact is local confidentiality and integrity impact plus kernel denial of service. The paranoid impact is C:H I:H A:H because the bug is a confirmed kernel slab use after free, although a controlled memory corruption or privilege escalation primitive is not demonstrated by this patch. SMB DFS mounts are commonly used in enterprise or internal network environments. This issue should be reviewed manually where untrusted or externally managed SMB DFS servers can be mounted.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	DANGER DISK RACE UAF SERVERTOCLIENT LINUS  INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=3
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Real UAF or lifetime violation. `tcon_super_cb()` stores `sb` without an active reference. After `iterate_supers_type()` releases `s_umount` and its passive reference, concurrent DFS automount expiry can free the same `struct super_block` before `cifs_sb_active()` consumes it. The commit confirms a deterministic KASAN slab-use-after-free.
* +1 Realistic kernel DoS. Dereferencing a freed superblock in `cifs_sb_active()` can produce a kernel fault or KASAN failure.

Important non-awarded signals:

* +0 Direct remote reachability. An SMB DFS server may influence failover and automount behavior, but the supplied patch does not establish that unauthenticated or arbitrary remote traffic can reliably drive the complete free-versus-use sequence without a pre-existing client mount to an attacker-controlled or compromised server.
* +0 Controlled reclaim or replacement. No evidence shows that an attacker can place controlled data at the freed superblock address.
* +0 Arbitrary or directed write. The patch establishes a stale dereference during active-reference acquisition, not an attacker-directed write primitive.
* +0 Structured stale-object leverage. `struct super_block` is an active structured object, but the supplied evidence does not show attacker-controlled post-free fields, callback dispatch, replacement-object consumption, or meaningful state control after reuse.
* +0 Concrete repeated release or refcount mutation. The passive iterator reference is dropped before the stale use, but the supplied control flow does not establish a second `put`, `release`, or equivalent lifetime-ending operation on the exact same dangling reference.
* +0 Confidentiality and integrity impact. A UAF exists, but no disclosure, protected-object modification, privilege escalation, or controlled corruption mechanism is demonstrated.
* +0 Remote trigger bonus. A malicious trusted DFS server is a plausible deployment concern, but direct network reachability is not proven by the supplied patch.
* No hard-race penalty. The KASAN test is described as deterministic, and the supplied evidence does not establish a narrow or externally uncontrollable one-shot timing condition.

3. Reachability analysis

The vulnerable path is reached by an SMB client using DFS automounts and DFS failover or reconnect processing. A local process accessing an already mounted DFS path may participate in triggering automount activity, while SMB server behavior may influence disconnect, referral, reconnect, or failover conditions.

The complete attacker model is unresolved. The patch does not show whether an ordinary local user can create the affected DFS mount, configure referral behavior, force expiry, and cause failover without host-admin-equivalent mount or configuration privileges. It also does not establish that a remote attacker can trigger the complete sequence unless the victim already mounts an attacker-controlled or compromised SMB DFS server.

No namespace-specific capability check or complete namespace-scoped trigger chain is visible. No special hardware requirement is involved. DFS mounts and automount expiry are configuration-dependent, but that fact alone is not used as a negative score penalty.

4. Primitive and severity interpretation

The exact primitive is a real slab use-after-free of an SMB DFS `struct super_block`. It is an active structured kernel object, but the observed stale operation is limited to obtaining an active reference through `cifs_sb_active()` after object lifetime protection was lost.

This is not fixed asynchronous stale storage, but the supplied evidence still shows a constrained stale-object dereference rather than controlled post-free reuse. The patch does not establish controlled reclaim, attacker-shaped replacement data, callback execution, arbitrary read or write, repeated stale release, refcount takeover, information disclosure, or a security-boundary bypass.

The issue is therefore CLASS_B, not CLASS_A. The demonstrated consequence is kernel fault or denial of service. Privilege escalation and confidentiality or integrity impact remain theoretical possibilities of the general UAF class and are not scored as established consequences here.

ImportantCandidate is NO because the available evidence supports a real UAF and crash risk but does not establish meaningful exploitation leverage beyond the stale dereference. AutoDowngradeAllowed is YES because this appears to be a constrained lifetime bug with no demonstrated reclaim, write-like stale refcount mutation, disclosure, or boundary primitive. Manual review remains recommended for deployments that mount DFS shares from externally managed or insufficiently trusted SMB infrastructure.

5. One-sentence report phrase

A race between SMB DFS superblock lookup and automount expiry can cause cifs_sb_active() to dereference a freed superblock, resulting in a kernel use-after-free and potential denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The patch confirms a deterministic kernel slab use-after-free, but the supplied evidence does not establish controlled reuse, an attacker-controlled SMB endpoint path, or a practical privilege-escalation primitive.

7. Call-site confidence

Call-site confidence: medium

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

Patch: smb: client: pin DFS superblock in iterator callback
Commit: ea43a15cea36dc5ddd832be5bddeac7dd804cbcf
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea43a15cea36dc5ddd832be5bddeac7dd804cbcf

Commit description:

tcon_super_cb() stores a raw superblock pointer, but __cifs_get_super()
takes its active reference only after iterate_supers_type() has dropped
s_umount and its passive reference. Concurrent DFS automount expiry can
therefore free the superblock before cifs_sb_active() uses it.

A deterministic KASAN test reproduces the race as:

  BUG: KASAN: slab-use-after-free in cifs_sb_active+0x77/0x80

The same test passes with this change applied.

Take the active reference in the callback while iterate_supers_type()
still holds s_umount shared. cifs_put_tcp_super() remains the matching
release.

Fixes: bacd704 ("cifs: handle prefix paths in reconnect")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/client/misc.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=ea43a15cea36dc5ddd832be5bddeac7dd804cbcf

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

The original Linux kernel CVE announcement for CVE-2026-97562 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97562

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: unknown
  Paranoid ActionableScore: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).