* [CVE-2026-89675][IMPORTANT] nfsd: fix UAF in async copy cancel and shutdown [ Upstream
@ 2026-09-12 10:27 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 10:27 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89675
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: nfsd: fix UAF in async copy cancel and shutdown [ Upstream
Commit: 9031493ef7369d5c59c4bacc96f0c85965f09a98
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9031493ef7369d5c59c4bacc96f0c85965f09a98
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89675
Analysis date: Sat, 12 Sep 2026 06:27:29 -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-2026-89675 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89675
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-2026-89675 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-89675 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-672;CWE-415;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 NFSD async COPY lifetime race can lead to use-after-free when teardown paths such as OFFLOAD_CANCEL, nfsd4_shutdown_copy, or nfsd4_cancel_copy_by_sb race the copy kthread and the async copy reaper. The vulnerable state can release nfsd_file references, clear nf_src or nf_dst, or free the embedded callback storage while the kthread or CB_OFFLOAD path can still use those objects. For the CVSS the PR:L is used because reliable triggering normally requires an NFS client that can create an async COPY operation and race cancellation or teardown on an accessible export. The issue is network reachable when the NFS server is exposed to such clients, although NFS servers are commonly deployed on trusted storage or data center networks rather than the public Internet. Impact is at least denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the bug class includes real use-after-free and lifetime corruption rather than only a warning or resource leak.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7) YES LOCK DISK INIT RACE ERRORPATH UAF NETWORK LINUS KPANIC NO NO 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. No floor or cap changed the score.
* Raw paranoid score: 7
* Paranoid score: 7. No floor or cap changed the score.
* Final recommended bucket: **Strong Important candidate**::
2. Signal breakdown
Conservative signals:
* +2 Real UAF / active lifetime violation. The commit explicitly describes async copy objects, nfsd_file references, task_struct, and embedded callback storage being used after lifetime protection is lost.
* +2 Remote reachable / network-triggerable. The vulnerable operations are in NFSD async COPY and OFFLOAD_CANCEL handling, reachable by an NFS client with access to the exported service.
* +1 Reliable kernel crash / strong DoS. The described UAF, NULL/UAF on nf_dst, freed task_struct access, and callback lifetime bugs realistically support kernel crash or fatal fault.
* +1 Concrete repeated release/refcount operation. release_copy_files() could run concurrently in the reaper and cancel/shutdown paths, double-putting nfsd_file references and potentially freeing them early.
Paranoid additional signal:
* +1 Distinct structured stale-object leverage. The embedded co_cb callback can remain in flight while the containing copy object is freed, and the s2s_cp_stateids entry can dangle for later laundromat or manage_cpntf_state traversal. This is separate from the double-put fact and leaves meaningful post-lifetime behavior unresolved.
Important non-awarded signals:
* +2 Strong LPE / strong exploitation leverage not awarded. Controlled reclaim, attacker-shaped replacement object, arbitrary write, callback pointer control, and demonstrated privilege escalation are not established.
* Confidentiality and integrity bonuses not awarded directly. They remain plausible only in the unresolved paranoid interpretation of a real UAF, not as proven read/write primitives.
* Extra Availability point not awarded. The crash/DoS consequence is already counted once.
* Race penalty not applied. The bug is a race, but the supplied evidence does not prove a narrow one-shot externally uncontrollable timing window.
* Host-admin penalty not applied. Server host-admin-equivalent privilege is not required for a remote NFS client to exercise protocol operations on an accessible export.
* Concrete repeated release/refcount check: first release can end lifetime of nfsd_file references. The same nf_src/nf_dst references can remain reachable through the racing copy object. The second release is a real put/lifetime mutation. +1 was awarded. Paranoid repeated-refcount +1 was not separately awarded because the paranoid point is assigned to distinct callback/stateid structured stale use. FLOOR 5 would apply, but raw conservative score is already above 5.
3. Reachability analysis
The evaluated attacker is a remote NFS client, not a server host administrator. The attacker must be able to reach the NFSD service and use an export that permits the relevant NFSv4 async COPY and cancellation behavior. This is not Internet-default exposure in many deployments, but NFS servers are routinely reachable on internal storage or data-center networks.
The complete trigger does not appear namespace-scoped. NamespaceScopedPrivilege is NO because the relevant security principal is a network NFS client interacting with the server, not a local namespace capability holder.
Prior privileged setup exists only in the normal sense that an administrator must have configured and exported NFS service. That does not make the vulnerability host-admin-only once the service is exposed to clients.
Race repeatability is plausible because the same client can create async COPY operations and attempt cancellation or teardown repeatedly. The race involves copy kthread, reaper, OFFLOAD_CANCEL, shutdown, and sb-cancel paths, but the evidence does not justify treating it as a rare one-shot timing condition for ActionableScore.
4. Primitive and severity interpretation
The exact primitive is an active structured lifetime bug in NFSD async COPY state. It includes UAF of struct nfsd4_copy-related state, double-put of nfsd_file references, possible NULL/UAF on copy->nf_dst while the kthread still performs nfsd_update_cmtime_attr(), unsafe kthread_stop() interaction with an unpinned task_struct, and an embedded callback object that can outlive the containing copy.
This is active structured object state, not fixed stale storage. It is not merely an ordinary cleanup stale dereference because at least one path performs a second concrete lifetime/refcount mutation after another racing path may already have ended the object lifetime. The repeated lifetime mutation is therefore category B from the prompt: second lifetime/refcount mutation through a potentially dangling object.
PrimitiveClass is CLASS_B. It is not CLASS_A because controlled reclaim, arbitrary kernel read/write, refcount takeover, function-pointer control, or demonstrated LPE is not established. The demonstrated consequence is kernel crash/DoS. The speculative consequence is stronger memory-corruption impact from unresolved post-lifetime state, but that requires manual review rather than automatic elevation to a proven exploit.
ImportantCandidate is YES because this is network-reachable server-side kernel code with real UAF, double lifetime mutation, and distinct in-flight callback/stateid lifetime hazards. AutoDowngradeAllowed is NO because automatically closing this as ordinary Moderate would lose a concrete unresolved lifetime/refcount corruption primitive.
Likely operational severity is at least Actionable Moderate and plausibly Strong Important pending manual review.
5. One-sentence report phrase
NFSD async COPY cancellation and shutdown can race the copy kthread and reaper, causing real use-after-free and double lifetime release of copy related objects reachable by an NFS client, with impact at least remote kernel crash and unresolved memory corruption leverage.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: real network-reachable NFSD UAF with concrete repeated release/refcount mutation and additional embedded callback/stateid lifetime hazards remains unresolved beyond simple crash-only behavior.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: nfsd: fix UAF in async copy cancel and shutdown [ Upstream
Commit: 9031493ef7369d5c59c4bacc96f0c85965f09a98
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9031493ef7369d5c59c4bacc96f0c85965f09a98
Changed files:
fs/nfsd/nfs4proc.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=9031493ef7369d5c59c4bacc96f0c85965f09a98
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89675 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89675
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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