[CVE-2026-89689][IMPORTANT] nfsd: don't free session slots that are still in use

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

CVE: CVE-2026-89689
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: nfsd: don't free session slots that are still in use
Commit: 72d40b103bb03f0124d3a3c8f71a30cbafaf6afa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=72d40b103bb03f0124d3a3c8f71a30cbafaf6afa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89689
Analysis date: Fri, 11 Sep 2026 20:19:00 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89689 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89689

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: IMPORTANT
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-2026-89689	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89689 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-787;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'An NFSv4.1 server side use after free can occur in nfsd4_sequence when the session slot shrink path frees slots that are still active. A remote client that already has an NFSv4.1 session can use a high numbered slot at or above se_target_maxslots and acknowledge the reduced maxslots value, causing free_session_slots to free the callers current slot or a slot used by another concurrent request. The request path then continues to update the freed slot metadata and later may copy a compound reply into the freed sl_data buffer, corrupting the slab object now occupying that memory. For the CVSS the PR:N is used for paranoid scoring because a network reachable NFSv4.1 service may allow a remote client to create and use a session without a separate local account on the server, although many deployments restrict access through export policy, host allow lists, or storage network isolation. Impact is at least remote denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel memory corruption and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)  SKIP CVE-2026-89689 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	DANGER DISK UAF NETWORK KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=7
ActionableScoreLower=6
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 6
* Conservative score: 6
* Raw paranoid score: 7
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate**::

No floor or cap changed the score. REAL UAF FLOOR 5 is satisfied but does not change the conservative score because the raw conservative score is already 6.

2. Signal breakdown

Conservative positive signals:

* +2 Real UAF / active lifetime violation: `free_session_slots()` can `kfree()` the current active NFSv4.1 session slot while `nfsd4_sequence()` continues processing it.
* +2 Remote reachable / directly network-triggerable: the vulnerable state is reached through NFSv4.1 client protocol traffic against an exposed nfsd service.
* +1 Reliable kernel crash / strong DoS: post-free use of the slot and later writes through the dangling `cstate->slot` can plausibly crash the kernel.
* +1 Structured stale-object exploitation leverage: after the slot is freed, the path continues to write slot metadata and later copies a compound reply into freed `sl_data[]`, corrupting whatever slab object reuses that memory.

Conservative negative signals:

* No negative privilege penalty: host-admin-equivalent privilege is not required by the supplied evidence.
* No race penalty: the primary current-slot scenario is an ordered protocol/state bug rather than a narrow timing race.
* No rare subsystem penalty: NFSv4.1 server exposure may be deployment-dependent, but the patch does not establish a genuinely rare or physical-only prerequisite.

Paranoid additional signal:

* +1 Attacker-influenced active-object lifetime/state: the remote client controls relevant protocol fields such as `slotid` and `sa_highest_slotid` and can participate in triggering the shrink acknowledgement condition that frees an active high-numbered slot. This supports stronger review sensitivity, but does not by itself prove controlled reclaim or arbitrary write.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary kernel read/write not demonstrated.
* CLASS_A not assigned because the destination is a freed slot address and controlled replacement of the slab object is not established.
* Confidentiality and integrity bonuses are not separately awarded as ActionableScore C/I signals merely because memory corruption exists.
* Extra Availability is not awarded because the same crash/DoS is already counted once.
* Concrete repeated release/refcount signal is not awarded. The bug is active-slot free followed by post-free writes/copy, not a second `put` or refcount mutation on the same dangling reference.
* Weak LPE is not separately awarded because the meaningful unresolved exploitation concern is already represented by structured stale-object leverage and the paranoid active-state signal.

Repeated-release/refcount checklist:

* First release can end lifetime: yes, `free_session_slots()` can `kfree()` the slot.
* Exact same reference survives: yes, the current `slot` and `cstate->slot` can continue to reference freed storage.
* Second operation is a real lifetime/refcount mutation: no, the later operations are metadata writes and reply-data copy, not another lifetime/refcount release.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: yes conceptually, because this is a below-host-admin remote active-object UAF with post-free writes into freed slab storage. It does not change the score.

3. Reachability analysis

The evaluated attacker is a remote NFSv4.1 client that can establish or use a session with the vulnerable nfsd server. The client can use a high-numbered slot in the range at or above `se_target_maxslots` and send a highest-slot acknowledgement that satisfies the shrink condition. This does not require host administrator privileges on the server.

The complete reachability may be restricted in real deployments by export policy, authentication, firewall rules, host allow lists, or isolated storage networks. Those are deployment restrictions, not proof of host-admin-equivalent privilege. Therefore `HostAdminRequired=NO`.

This is not a namespace-scoped capability issue. No evidence indicates that a namespace-local capability is needed or sufficient, so `NamespaceScopedPrivilege=NO`.

There is prior server-side state involved: `se_target_maxslots` must have been reduced below `se_fchannel.maxreqs`. The supplied commit states this is part of the session shrinker behavior. It is a state prerequisite, but not evidence of a separate host-admin setup requirement.

A concurrent-thread variant is also described, but the main current-slot UAF does not depend on a hard race. The client-controlled protocol sequence can trigger freeing of the slot currently being processed once the shrink state exists.

No special hardware, boot option, or unusual kernel configuration is established beyond running an affected NFSv4.1 server.

4. Primitive and severity interpretation

The exact primitive is a server-side NFSv4.1 session slot use-after-free. `nfsd4_sequence()` can call `free_session_slots()` on a range that includes the slot currently used by the same request. The code then continues to update fields of the freed slot and stores a dangling pointer in `cstate->slot`. Later `nfsd4_store_cache_entry()` may copy compound reply data into the freed `sl_data[]` buffer.

This is an active structured object UAF, not fixed stale storage. The stale object is an `nfsd4_slot` whose fields are used for NFS session state and reply caching. The stale use is broader than an ordinary cleanup-only dereference because the request path continues normal protocol processing and performs post-free writes.

This is not a concrete repeated release/refcount case. The important distinction is not a second `put`, but post-free structured writes and a later copy into freed slab memory.

Primitive class is CLASS_B. The patch establishes real kernel lifetime corruption and slab corruption, but does not establish controlled reclaim, arbitrary write, callback control, credential overwrite, or a demonstrated privilege escalation chain. Therefore CLASS_A would be too strong from the supplied evidence alone.

Demonstrated consequence is at least remote denial of service through kernel crash or memory corruption. Speculative consequence is stronger exploitation from post-free slab corruption if an attacker can influence reclaim, replacement object selection, or useful post-free contents. That stronger chain is not proven, but the primitive is meaningful enough that automatic downgrade would be unsafe.

ImportantCandidate=YES because this is a remotely reachable active-object UAF with post-free writes into freed slab storage. Manual Important-vs-Moderate review is required.

AutoDowngradeAllowed=NO because the unresolved kernel memory corruption primitive would be lost by treating the issue as ordinary crash-only DoS.

Likely operational severity is at least Actionable Moderate and more likely a Strong Important candidate for exposed NFSv4.1 servers, especially where untrusted or semi-trusted clients can reach nfsd.

5. One-sentence report phrase

A remote NFSv4.1 client can trigger a server side use after free in `nfsd4_sequence()` when session slot shrinking frees an active slot, leading to post-free writes into freed slab memory and at least kernel crash DoS with stronger exploitation requiring manual review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes a real remotely reachable active-object UAF with post-free metadata writes and later reply-data copy into freed `sl_data[]`, which is materially stronger than ordinary crash-only cleanup behavior.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: don't free session slots that are still in use
Commit: 72d40b103bb03f0124d3a3c8f71a30cbafaf6afa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=72d40b103bb03f0124d3a3c8f71a30cbafaf6afa

Commit description:

nfsd4_sequence() can free the very slot it is currently processing.
When the session shrinker has reduced se_target_maxslots below
se_fchannel.maxreqs, the shrink path checks three conditions before
calling free_session_slots():

  1. se_target_maxslots < maxreqs  (shrink was advertised)
  2. slot->sl_generation == se_slot_gen  (slot is up-to-date)
  3. seq->maxslots <= se_target_maxslots  (client acknowledges)

However, seq->slotid is never checked against se_target_maxslots.
A client using a slot in the range [se_target_maxslots, maxreqs) can
satisfy all three conditions: its slot has the current generation
(set by a prior SEQUENCE), and it sends sa_highest_slotid <=
se_target_maxslots to acknowledge the reduction.

free_session_slots() then kfrees every slot at index >=
se_target_maxslots, including the caller's own slot. The function
continues to write sl_seqid, sl_flags, sl_generation, and stores the
dangling pointer in cstate->slot. Later, nfsd4_store_cache_entry()
copies up to maxresp_cached bytes of the compound reply into the freed
sl_data[] array, corrupting whatever slab object now occupies that
address.

Additionally, a concurrent thread processing SEQUENCE on a different
high-numbered slot can have its slot freed out from under it.
NFSD4_SLOT_INUSE is set under nn->client_lock before the lock is
released, so any concurrent thread past SEQUENCE will have its slot
marked. However, free_session_slots() does not check NFSD4_SLOT_INUSE
before freeing.

Fix both problems by:
 1. Checking that the current request's slotid is below the shrink
    boundary.
 2. Scanning slots in the to-be-freed range for NFSD4_SLOT_INUSE and
    deferring the shrink if any are active.

Fixes: fc8738c ("nfsd: add support for freeing unused session-DRC slots")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260526-nfsd4_sequence_shrink_uaf_on_loaded_slot-v2-1-74a89db0639e@kernel.org
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=72d40b103bb03f0124d3a3c8f71a30cbafaf6afa

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

The original Linux kernel CVE announcement for CVE-2026-89689 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89689

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: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: 6
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).