[CVE-2026-89712][MODERATE 7.0] NFSD: restart ssc_expire_umount walk after dropping nfsd_ssc_lock

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

CVE: CVE-2026-89712
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSD: restart ssc_expire_umount walk after dropping nfsd_ssc_lock
Commit: 77de363d9a1c8cd35f20482782c612cda085791a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=77de363d9a1c8cd35f20482782c612cda085791a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89712
Analysis date: Fri, 11 Sep 2026 17:05:11 -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-2026-89712 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89712

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
======================================================================

CVE-2026-89712	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89712 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'An NFS server side copy expiry race can dereference a freed nfsd4_ssc_umount_item because nfsd4_ssc_expire_umount drops nfsd_ssc_lock around mntput while the saved next list entry is not pinned. A concurrent nfsd RPC thread can delete and free that saved next entry via nfsd4_ssc_cancel_dul, so the expire walk can continue with a stale pointer. This is a race based use after free in the NFSD control path for inter server copy cleanup. For the CVSS the PR:L is selected because the attacker normally needs network access as an NFS client able to exercise server side copy or related source mount failure paths, but does not need local shell access on the server. The issue is network reachable through NFS service behavior rather than generic packet processing. Impact is at least denial of service via kernel crash, and the freed list object plus later list traversal makes high confidentiality and integrity impact plausible enough for manual review in the paranoid score.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	LOCK DISK UAF NETWORK 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

Conservative positive signals:

* +2 Real UAF / lifetime violation. `nfsd4_ssc_expire_umount()` saves the next list item in `tmp`, drops `nfsd_ssc_lock`, and another NFSD RPC thread can delete and free that saved item through `nfsd4_ssc_cancel_dul()`. The next iteration can dereference the freed `nfsd4_ssc_umount_item`.
* +2 Network reachable service path. The bug is in NFSD server-side copy / inter-server copy cleanup behavior and can be influenced through NFS service operations by a client able to exercise that feature and related failure paths.
* +1 Reliable kernel crash / strong DoS. Dereferencing a freed list object in kernel list traversal can realistically cause an Oops, panic, or corrupted list traversal leading to host DoS.

Paranoid additional signal:

* +1 Structured stale-object exploitation leverage. The stale object is an active list item, and later stale use may consume list pointers, expiry state, refcount state, and may continue into list traversal / deletion logic. Controlled reclaim or arbitrary write is not established, so this remains Class B rather than Class A.

Non-awarded material signals:

* Controlled reclaim is not demonstrated.
* Arbitrary read/write is not demonstrated.
* Confidentiality and integrity are not awarded as direct signals. They remain unresolved paranoid consequences of a kernel UAF, not established primitives.
* Extra availability is not awarded because the crash/DoS is already counted once.
* Race penalty is not applied. The bug is a race, but supplied evidence does not prove that the timing is one-shot, externally uncontrollable, or too unreliable for repeated attempts.
* Concrete repeated release/refcount +1 is not awarded. A prior free of the saved `tmp` object is established, and the stale pointer survives in the iterator, but the supplied evidence directly establishes stale dereference rather than a deterministic second lifetime/refcount mutation on the exact same object.
* Paranoid repeated-refcount +1 is withheld for the same reason.
* FLOOR 5 is not separately applied because the conservative arithmetic already reaches 5.

Repeated-release checklist:

* First release can end lifetime: yes, `nfsd4_ssc_cancel_dul()` can `list_del` and `kfree` its item.
* Exact same reference survives: yes, as saved `tmp` in the expire walk.
* Second operation is a concrete lifetime/refcount mutation: not sufficiently established as deterministic from the supplied text.
* Dedicated +1 Concrete repeated release/refcount: not awarded.
* Paranoid repeated-refcount leverage +1: not awarded.
* FLOOR 5: not needed, conservative score is already 5.

3. Reachability analysis

The evaluated attacker is a remote NFS client or client-controlled workflow able to exercise NFSv4 server-side copy / inter-server copy paths and cause or race source-server mount failure cleanup. This does not require local shell access or host administrator privilege on the NFSD server, so `HostAdminRequired=NO`.

The operation is not evaluated as namespace-scoped privilege. It is a network service path in NFSD, not a local namespace capability path, so `NamespaceScopedPrivilege=NO`.

There is prior server setup: NFSD must be running, relevant exports and server-side copy behavior must be available, and the attacker must be able to reach the NFS service. This is a normal service-exposure condition, not host-admin-equivalent privilege for the attacker.

The race involves the expire delayed work dropping `nfsd_ssc_lock` around `mntput()` while another RPC thread deletes an item from the same list. It is timing-dependent, but it is tied to concurrent service operations and cleanup rather than a single uncontrollable hardware event.

4. Primitive and severity interpretation

The exact primitive is a race-based use-after-free of `struct nfsd4_ssc_umount_item`. The stale object is an active structured kernel list object, not fixed stale storage. The demonstrated stale use is list traversal after another thread has deleted and freed the saved next element.

This is not merely a passive stale metadata issue. The freed object participates in active list traversal and cleanup state. However, no controlled replacement, attacker-shaped reclaimed object, arbitrary kernel read/write, callback control, or demonstrated LPE is supplied.

Primitive class is `CLASS_B`: real kernel lifetime corruption with unresolved exploitation leverage. Demonstrated consequence is at least kernel crash / DoS. Speculative consequence is broader memory corruption if stale list fields are consumed after reuse, but that is not established strongly enough for Class A.

ImportantCandidate is `YES` because this is a network-reachable active-object UAF in NFSD with meaningful unresolved post-lifetime behavior. AutoDowngradeAllowed is `NO` because automatically closing it as ordinary Moderate would discard a real network-reachable kernel lifetime primitive requiring manual exploitation-leverage review.

Likely operational severity is Actionable Moderate at minimum, with Strong Important consideration depending on whether further analysis shows practical stale-object reuse, list corruption leverage, or attacker-controlled replacement.

5. One-sentence report phrase

An NFSD server-side copy cleanup race can leave the expire worker using a freed `nfsd4_ssc_umount_item` after dropping `nfsd_ssc_lock`, allowing a network client able to exercise the relevant NFS server-side copy paths to trigger a kernel UAF and likely DoS, with broader memory-corruption impact requiring manual review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real network-reachable NFSD use-after-free of an active structured list object, and the post-free list traversal behavior leaves meaningful exploitation leverage unresolved even though controlled reclaim or LPE is not demonstrated.

7. Call-site confidence

Call-site confidence: high

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

Patch: NFSD: restart ssc_expire_umount walk after dropping nfsd_ssc_lock
Commit: 77de363d9a1c8cd35f20482782c612cda085791a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=77de363d9a1c8cd35f20482782c612cda085791a

Commit description:

nfsd4_ssc_expire_umount() walks nn->nfsd_ssc_mount_list with
list_for_each_entry_safe(ni, tmp, ...).  For each expired entry it
sets nsui_busy = true, drops nfsd_ssc_lock to run mntput() on the
source vfsmount, then reacquires the lock to list_del + kfree the
entry and continue iterating via the macro's saved tmp pointer.

The nsui_busy flag protects the current ni from concurrent
nfsd4_ssc_setup_dul() finders during the lock-drop window, but it
does not pin tmp.  Another nfsd RPC thread that fails its source-
server mount and reaches nfsd4_ssc_cancel_dul() will, during that
same window, take nfsd_ssc_lock, list_del + kfree its own ssc_umount
item, and release the lock.  If that item is the saved tmp of the
expire walk, the next iteration dereferences a freed
nfsd4_ssc_umount_item.

Restart the walk from the head after the mntput() unlock window so
no saved next pointer survives the lock-drop.  The list is bounded
by the number of active inter-server source mounts (typically small)
and the expire delayed-work runs periodically rather than per-IO,
so the restart is cheap.

Fixes: f4e44b3 ("NFSD: delay unmount source's export after inter-server copy completed.")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Link: https://patch.msgid.link/20260524130654.1924556-1-michael.bommarito@gmail.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  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=77de363d9a1c8cd35f20482782c612cda085791a

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

The original Linux kernel CVE announcement for CVE-2026-89712 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89712

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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).