[CVE-2026-89692][MODERATE REGULAR] nfsd: clear CALLBACK_RUNNING on failed delegation recall queue

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

CVE: CVE-2026-89692
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nfsd: clear CALLBACK_RUNNING on failed delegation recall queue
Commit: b137930ee52e3ef38915a3511d4c070463b63a32
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b137930ee52e3ef38915a3511d4c070463b63a32
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89692
Analysis date: Fri, 11 Sep 2026 21:51:46 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89692 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89692

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-2026-89692	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89692 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:L/UI:N/S:U/C:N/I:N/A:H';CWE-459;CWE-703;**CWE-400;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'An NFS server delegation recall can become permanently stuck when nfsd4_run_cb() fails to queue the recall work after NFSD4_CALLBACK_RUNNING was already set. Later break_lease() calls for the same delegation can hit the running guard and skip the recall, so the delegation is not broken and conflicting open or lock operations may stall. For the CVSS the PR:L is used because a realistic trigger requires an authenticated or otherwise allowed NFS client that can create or conflict with delegations on an exported filesystem. The issue is network reachable through NFSv4 server behavior, but exploitation depends on the failed callback queue path and is therefore AC:H. Impact is denial of service against NFS operations. No confidentiality or integrity impact is assigned because the patch does not indicate memory corruption, stale pointer traversal, information disclosure, or an arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)  SKIP CVE-2026-89692 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	DISK SIMPLEFIX NETWORK  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap was applied. The paranoid score is emitted as ActionableScore because it is higher than the conservative score.

2. Signal breakdown

Conservative positive signals:

* +2 Remote reachable / network-triggerable path: the affected code is in the NFSv4 server delegation recall path and can be reached through NFS client activity that creates or conflicts with delegations.
* +1 Availability impact: a stuck NFSD4_CALLBACK_RUNNING bit can cause later break_lease() calls to skip delegation recall, leaving conflicting open or lock operations stalled.

Conservative negative signals:

* -1 Hard or unreliable trigger condition: the harmful state requires nfsd4_run_cb() to fail after NFSD4_CALLBACK_RUNNING has already been set. The supplied evidence does not show that a remote client can directly and reliably force that queue failure.

Paranoid difference:

* The paranoid score withholds the -1 timing or reliability reduction because the permanent latch behavior is concrete once the failed queue branch is reached, and the NFS server path is network-facing.

Important non-awarded signals:

* Real UAF not awarded: the patch does not show post-lifetime dereference, stale traversal, double free, or use of freed memory.
* Concrete repeated release/refcount not awarded: refcount_inc() is followed by refcount_dec() only on the failed queue branch. There is no earlier release that can end the object lifetime followed by a second lifetime mutation through the same dangling reference.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity impact not awarded.
* Strong LPE and weak LPE not awarded.
* Reliable kernel crash not awarded: the demonstrated consequence is stalled NFS operations, not host panic.
* Extra Availability not double-counted.

Repeated release/refcount checklist:

* First release can end lifetime: NO, not established.
* Exact same dangling reference survives: NO, not established.
* Second lifetime/refcount mutation exists: NO, only a normal rollback refcount_dec() is visible.
* Concrete repeated release/refcount +1 awarded: NO.
* Paranoid repeated-refcount leverage +1 awarded: NO.
* FLOOR 5 applied: NO.

3. Reachability analysis

The evaluated attacker is a remote NFSv4 client that can access an exported filesystem and perform operations capable of creating or conflicting with delegations. Host administrator privileges are not required for the client-side NFS operations, so HostAdminRequired=NO.

The complete trigger is not namespace-scoped. It is an NFS server network path, not a local namespace capability path, so NamespaceScopedPrivilege=NO.

There is a prior deployment prerequisite: the target must be running nfsd with NFSv4 delegation behavior relevant to the workload. That is normal service exposure rather than a privilege required by the attacker.

The difficult part is the failed callback queue branch. The patch establishes what happens if nfsd4_run_cb() fails, but it does not establish that an attacker can reliably force that failure on demand. This supports conservative caution and keeps the issue below Important-class scoring.

No special hardware requirement is present.

4. Primitive and severity interpretation

The primitive is an invalid persistent callback state latch. nfsd_break_one_deleg() sets NFSD4_CALLBACK_RUNNING before attempting to queue delegation recall work. If queuing fails, the old code decremented the reference count but left NFSD4_CALLBACK_RUNNING set. Since the normal clearing site is the callback workqueue completion path, and no work was queued, the bit can remain set permanently.

This is not an active structured UAF and not fixed stale storage. It is a logic and cleanup error in callback state handling.

The demonstrated consequence is denial of service against NFS operations: future delegation breaks for the same delegation can be skipped, and conflicting opens or locks may stall.

Speculative consequences beyond DoS are not supported by the patch. There is no memory corruption primitive, no stale pointer traversal, no arbitrary write, no protected object access, and no privilege escalation chain.

PrimitiveClass=CLASS_C because the established behavior is invalid persistent state causing DoS, without an independently useful corruption or authorization primitive.

ImportantCandidate=NO because there is no Class A or meaningful Class B corruption/lifetime primitive requiring Important-vs-Moderate exploitation review.

AutoDowngradeAllowed=YES because this is a network-facing but DoS-only logic bug with no unresolved memory corruption, ownership violation, or security-boundary bypass. Manual review is still recommended due to NFS server exposure and persistent operation stalls.

Likely operational severity: low-to-moderate network DoS against affected NFSv4 server workloads.

5. One-sentence report phrase

An NFSv4 server delegation recall can become permanently stuck if callback work queueing fails after NFSD4_CALLBACK_RUNNING is set, allowing an allowed NFS client workload to cause later conflicting open or lock operations to stall.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is network-facing in the NFS server and can cause persistent NFS operation stalls, but the patch does not establish memory corruption, privilege escalation, or a reliably attacker-controlled queue-failure trigger.

7. Call-site confidence

Call-site confidence: medium

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

Patch: nfsd: clear CALLBACK_RUNNING on failed delegation recall queue
Commit: b137930ee52e3ef38915a3511d4c070463b63a32
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b137930ee52e3ef38915a3511d4c070463b63a32

Commit description:

nfsd_break_one_deleg() sets NFSD4_CALLBACK_RUNNING via test_and_set_bit
at entry to serialize recall work, then calls nfsd4_run_cb() to queue
the recall.  When the queue attempt fails the refcount bump is undone,
but the RUNNING bit is left set.  The only site that clears the bit is
nfsd41_destroy_cb() (fs/nfsd/nfs4callback.c), which runs from the
workqueue and is therefore unreachable when nothing was queued.

The bit becomes a permanent latch on dp->dl_recall.cb_flags: every
subsequent break_lease() on the same delegation hits the early-return
guard in nfsd_break_one_deleg() and silently skips the recall, so the
delegation is never broken and the conflicting open or lock stalls.

Fix by clearing NFSD4_CALLBACK_RUNNING on the !queued branch alongside
the refcount_dec.

Fixes: 1054e8f ("nfsd: prevent callback tasks running concurrently")
Cc: stable@vger.kernel.org
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260526-cb_recall_any_callback_running_stuck-v1-2-310011a028f3@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfs4callback.c
  fs/nfsd/nfs4state.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=b137930ee52e3ef38915a3511d4c070463b63a32

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

The original Linux kernel CVE announcement for CVE-2026-89692 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89692

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:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.3

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: 2
  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).