* [CVE-2026-89661][MODERATE REGULAR] NFSD: Prevent post-shutdown use-after-free in unlock_filesystem
@ 2026-09-12 1:13 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-12 1:13 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89661
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: NFSD: Prevent post-shutdown use-after-free in unlock_filesystem
Commit: 0c1a755b7212e0835398d4df5e782ea85ccd7476
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0c1a755b7212e0835398d4df5e782ea85ccd7476
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89661
Analysis date: Fri, 11 Sep 2026 21:13:28 -0400
ActionableScore: 4
ActionableScore lower bound: 1
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
A post-shutdown UAF in the NFSD unlock_filesystem control path lets a local CAP_SYS_ADMIN capable actor trigger stale NFSD state-table traversal after server shutdown, causing a kernel crash, with no direct network trigger or demonstrated privilege-escalation primitive.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89661 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89661
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-2026-89661 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89661 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-664;CWE-667;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.8';DESCR 'A post-shutdown use-after-free exists in the NFSD unlock_filesystem control path because nfsd4_cancel_copy_by_sb() could run before nfsd_mutex is held and before the handler confirms that the nfsd service is still running. After nfsd shutdown, conf_id_hashtbl may already be freed while the stale pointer remains, so the cancel helper can traverse freed slab memory and dereference a bogus nfs4_client state object. For the CVSS the PR:L is selected for the paranoid score because CAP_SYS_ADMIN may be delegated to a reduced capability administrator, container root, or service account rather than a fully trusted host root. The issue is not network reachable directly and requires local access to the NFSD procfs control interface. Impact is at least 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, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) SKIP CVE-2026-89661 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DANGER DISK UAF NETWORK LINUS INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
ActionableScoreLower=1
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
## 1. ActionableScore
* Raw conservative score: 1
* +2 Real UAF / lifetime violation.
* +1 Reliable kernel crash / strong DoS.
* -2 Host-admin-equivalent CAP_SYS_ADMIN interpretation from the supplied text.
* Conservative score: 1
* No floor applied because the lower interpretation treats host-admin-equivalent privilege as established.
* Raw paranoid score: 3
* +2 Real UAF / lifetime violation.
* +1 Reliable kernel crash / strong DoS.
* No -2 host-admin penalty in the final actionable interpretation because the supplied evidence does not fully resolve whether the complete CAP_SYS_ADMIN requirement is strictly host-admin-equivalent or may be namespace/delegation scoped in all evaluated environments.
* Paranoid score: 4
* Applied Real UAF floor 4: 3 -> 4.
* Reason: this is a real stale structured NFSD state-table use, not a mere warning or bookkeeping bug, and final host-admin equivalence is unresolved.
* Final recommended bucket: **Borderline / manual review recommended**::
## 2. Signal breakdown
Positive signals:
* +2 Real UAF / lifetime violation: after NFSD shutdown, `nn->conf_id_hashtbl` can already be freed while the pointer remains usable by `nfsd4_cancel_copy_by_sb()`.
* +1 Reliable kernel crash / strong DoS: KASAN reports slab-use-after-free, and the stale traversal can dereference a bogus `nfs4_client` and take `clp->async_lock`.
Negative signals:
* -2 in the lower score only: supplied text says the reachable actor is a local administrator holding `CAP_SYS_ADMIN`, which can be host-admin-equivalent.
* No race/timing penalty: this is mainly an ordered post-shutdown lifetime bug, not a narrow externally uncontrollable race.
* No rare hardware or niche hardware penalty: no special hardware is required.
Important non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Information disclosure not demonstrated.
* Privilege escalation not demonstrated.
* Remote reachability not established.
* Structured stale-object exploitation leverage not awarded because the demonstrated stale path is a post-shutdown control/cleanup path and no attacker-controlled replacement or post-free contents are shown.
* Weak LPE not awarded merely because this is a UAF.
* Extra Availability not awarded because the same kernel crash is already counted once.
* C/I impact not awarded merely because stale slab memory is traversed.
* Concrete repeated release/refcount signal not awarded.
Repeated release/refcount checklist:
* First release can end lifetime: yes for `conf_id_hashtbl` cleanup during `nfs4_state_destroy_net()`.
* Exact same reference survives: the stale pointer to `conf_id_hashtbl` remains intact.
* Second operation is a real lifetime/refcount mutation: no. The second operation is traversal/dereference/lock through stale state, not a concrete second `put`, `release`, `free`, or refcount decrement on the same object.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no. The UAF is real, but no concrete second lifetime mutation, controlled reclaim, or meaningful attacker-shaped stale reuse is established.
## 3. Reachability analysis
The supplied context states that a local administrator holding `CAP_SYS_ADMIN` can trigger the bug by stopping NFSD and then writing a filesystem path to `/proc/fs/nfsd/unlock_filesystem`.
The exact complete privilege boundary is not fully resolved from the supplied patch alone. The conservative lower score treats this as host-admin-equivalent CAP_SYS_ADMIN. The final actionable score keeps `HostAdminRequired=UNKNOWN` because the prompt requires positive proof before asserting strict host-admin equivalence across all relevant namespace/delegation cases.
`NamespaceScopedPrivilege=UNKNOWN`: the code uses `netns(file)` and NFSD has per-net state, but the supplied evidence does not prove that all required stages, including stopping the server and writing the relevant procfs control file, are fully attacker-obtainable inside an unprivileged-created namespace.
No direct network trigger is established. NFS clients over the network are not shown to reach this path directly. The trigger is a local NFSD control-plane operation.
Prior setup exists in practice because NFSD must have been running and then shut down, but this is part of the described administrative lifecycle rather than special hardware or a rare boot mode.
Race repeatability is not the main issue. The vulnerable state follows from post-shutdown ordering: the helper can run after the state table has been freed.
## 4. Primitive and severity interpretation
Exact primitive: post-shutdown use-after-free / stale pointer use in the NFSD `unlock_filesystem` control path.
The stale object is active structured kernel state: the helper treats freed `conf_id_hashtbl` slab memory as an array of `struct list_head` and may reach a bogus `nfs4_client` before taking `async_lock`.
This is broader than a passive stale value, but the demonstrated path is still primarily a cleanup/control-plane stale traversal. There is no evidence of controlled reclaim, attacker-shaped replacement object, arbitrary read/write, callback hijack, sensitive disclosure, or demonstrated LPE.
Primitive class: CLASS_B. It is a real kernel lifetime violation, but not a strong Class A primitive.
Demonstrated consequence: local kernel crash / DoS.
Speculative consequence: stronger exploitation might be investigated because stale structured slab memory is traversed, but the supplied evidence does not establish meaningful control over the reclaimed contents or a non-crash exploitation primitive.
ImportantCandidate=NO because this looks like a constrained post-shutdown NFSD control-plane UAF with crash as the demonstrated outcome and no concrete repeated refcount/lifetime mutation or controlled reuse.
AutoDowngradeAllowed=YES because the issue does not currently show unresolved Important-grade leverage beyond a real but constrained UAF. Manual review is still recommended due to real kernel UAF and ambiguity around CAP_SYS_ADMIN scoping.
Likely operational severity: borderline Moderate. It should not be ignored as a simple warning, but it is not a strong Important candidate on the supplied evidence.
## 5. One-sentence report phrase
A post-shutdown UAF in the NFSD unlock_filesystem control path lets a local CAP_SYS_ADMIN capable actor trigger stale NFSD state-table traversal after server shutdown, causing a kernel crash, with no direct network trigger or demonstrated privilege-escalation primitive.
## 6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Reason: real kernel UAF in NFSD state-table traversal is established, but the available evidence points mainly to local admin/control-plane crash-only impact and does not establish controlled reclaim, repeated lifetime mutation, or LPE.
## 7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: NFSD: Prevent post-shutdown use-after-free in unlock_filesystem
Commit: 0c1a755b7212e0835398d4df5e782ea85ccd7476
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0c1a755b7212e0835398d4df5e782ea85ccd7476
Commit description:
Writing a filesystem path to /proc/fs/nfsd/unlock_filesystem runs
nfsd4_cancel_copy_by_sb() before nfsd_mutex is held and before the
handler confirms that nn->nfsd_serv is set. Once nfsd has shut down,
nfs4_state_destroy_net() has freed nn->conf_id_hashtbl but left the
pointer intact, so the cancel helper iterates freed slab memory as an
array of struct list_head and then dereferences a bogus nfs4_client
when it takes clp->async_lock. A local administrator holding
CAP_SYS_ADMIN can reach this use-after-free by stopping the server and
then writing to unlock_filesystem; KASAN reports a slab-use-after-free
read in nfsd4_cancel_copy_by_sb().
nfsd4_revoke_states() walks the same state tables and for that reason
already runs only under nfsd_mutex with nn->nfsd_serv confirmed
present. Move the async COPY cancel into that protected section so
every NFSv4 state-table walker on this path observes a running server.
Async copies exist only while the server runs, so gating the cancel on
nn->nfsd_serv loses nothing.
Reported-by: Musaab Khan <[email protected]>
Fixes: 3daab31 ("nfsd: cancel async COPY operations when admin revokes filesystem state")
Cc: [email protected]
Reviewed-by: Jeff Layton <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Chuck Lever <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
fs/nfsd/nfsctl.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=0c1a755b7212e0835398d4df5e782ea85ccd7476
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89661 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89661
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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7.8
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: 1
Paranoid ActionableScore: 4
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
^ permalink raw reply [relevance 21%]
Results 1-1 of 1 | reverse | sort options + mbox downloads above
-- links below jump to the message on this page --
2026-09-12 1:13 21% [CVE-2026-89661][MODERATE REGULAR] NFSD: Prevent post-shutdown use-after-free in unlock_filesystem AL-KERNEL
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox