* [CVE-2026-89706][MODERATE 7.0] nfsd: Reset write verifier when async COPY writeback fails
@ 2026-09-12 8:21 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 8:21 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89706
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: Reset write verifier when async COPY writeback fails
Commit: 31d4d0a62ec4bafbcdbad142f11ab1f90794b1d0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=31d4d0a62ec4bafbcdbad142f11ab1f90794b1d0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89706
Analysis date: Sat, 12 Sep 2026 04:21:32 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
Manual review required: NO
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89706 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89706
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: NO
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89706 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89706 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:H/A:L';CWE-703;CWE-754;CWE-404;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N';BEST CVSS score: '5.9';DESCR 'An NFSd async COPY durability flaw can occur when post copy writeback or fsync reports an error but the server write verifier is not rotated. A client receives NFS_UNSTABLE via CB_OFFLOAD and then sends COMMIT, but the unchanged verifier can make the client conclude that failed writeback data is durable. For the CVSS the PR:L is used because the attacker typically needs authenticated NFS client access and permissions on an exported filesystem rather than only unauthenticated network reachability. The issue is network reachable through NFSv4.2 async COPY but practical triggering also depends on a server side writeback failure such as storage or filesystem error conditions. Impact is primarily integrity loss through silent data loss or false durability confirmation. There is no evidence of memory corruption, information disclosure, kernel crash, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5) YES WRITE OOB DISK NETWORK KPANIC YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate**::
No floor or cap was applied.
2. Signal breakdown
Conservative signals:
* +2 Remote reachable / network-triggerable. The vulnerable behavior is in NFSd async COPY handling and is reachable through NFSv4.2 protocol operations by a client with access to the export.
* +1 Integrity plausible. The server can falsely confirm durability after writeback failure, causing silent data loss or incorrect durable-state reporting.
* +1 Important filesystem/storage path. The bug affects NFS server writeback, COMMIT, and durability semantics.
* +1 Practical consequence chain strongly implied. The commit describes CB_OFFLOAD returning NFS_UNSTABLE, the client issuing COMMIT, and unchanged verifier causing the client to treat failed writeback data as durable.
* -1 Special triggering condition. The harmful behavior depends on post-copy fsync or writeback error conditions on the server side, not merely on a normal COPY request.
Paranoid difference:
* The paranoid score does not subtract the writeback-failure condition because such failures may be attacker-influenced in some deployments, for example through quota, space exhaustion, backend errors, or storage-failure timing. This is still grounded in the described mechanics and does not assume memory corruption.
Important non-awarded signals:
* No RealUAF. The patch does not establish use-after-free, stale object use, refcount corruption, or double release.
* No controlled reclaim, arbitrary write, OOB access, or kernel memory corruption is demonstrated.
* No confidentiality impact is awarded because no disclosure path is shown.
* No privilege escalation is awarded because the bug affects durability semantics, not credentials, authorization, or kernel control flow.
* No extra availability point is awarded conservatively because the primary effect is data integrity loss, not a demonstrated kernel crash or service outage.
3. Reachability analysis
The relevant attacker is an NFS client that can access an exported filesystem and issue NFSv4.2 async COPY operations. This is below host-admin privilege on the NFS server, so HostAdminRequired=NO. It is not an unauthenticated Internet-style trigger in typical deployments because NFS export access and file permissions are normally required, but the vulnerable operation is network reachable once that access exists.
Namespace-scoped privilege is not relevant here. There is no indication that the trigger depends on Linux namespaces, CAP_NET_ADMIN, local syscalls, local device access, or host administrative control. A prior server configuration exposing NFS exports is required as normal service setup, but that does not make the attacker host-admin.
The main practical constraint is the need for a writeback or fsync failure after async COPY. This makes exploitation more conditional than a simple malformed-packet crash, but the commit explicitly describes the durability failure sequence.
4. Primitive and severity interpretation
The exact primitive is a protocol/storage semantic integrity flaw: after async COPY writeback fails, the server fails to rotate the write verifier, so a later COMMIT can incorrectly validate data that was not durably written.
This is not active structured UAF, fixed stale storage, memory corruption, or cleanup/destructor lifetime misuse. There is no concrete repeated release or refcount operation. The demonstrated consequence is silent data loss or false durability confirmation. The speculative consequence is broader application-level data integrity impact if clients rely on NFS COMMIT semantics for correctness.
PrimitiveClass=NONE because no kernel corruption, lifetime, ownership, authorization, or memory-safety primitive is established. ImportantCandidate=NO because there is no LPE, memory corruption, authorization bypass, or cross-owner COW violation. AutoDowngradeAllowed=NO because the bug is network reachable and can cause high-value storage integrity loss, so it should not be automatically closed as an ordinary low-impact issue without product/context review.
Likely operational severity is actionable Moderate: important for NFS server deployments, especially storage systems and data-center environments, but not an Important-class kernel exploitation primitive based on the supplied evidence.
5. One-sentence report phrase
An NFSd async COPY durability flaw can let an authenticated NFS client receive false COMMIT confirmation after server-side writeback failure, causing silent data loss or incorrect durable-state reporting without kernel memory corruption or privilege escalation.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Reason: the issue is network reachable and affects storage integrity semantics, but it does not establish memory corruption, authorization bypass, kernel crash, or privilege escalation.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: nfsd: Reset write verifier when async COPY writeback fails
Commit: 31d4d0a62ec4bafbcdbad142f11ab1f90794b1d0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=31d4d0a62ec4bafbcdbad142f11ab1f90794b1d0
Commit description:
Async COPY captures nn->writeverf at request time and reports it to
the client via CB_OFFLOAD after the worker kthread completes. When
the post-copy vfs_fsync_range() or filemap_check_wb_err() in
_nfsd_copy_file_range() reports an error, the worker correctly
leaves NFSD4_COPY_F_COMMITTED clear so that CB_OFFLOAD encodes
wr_stable_how as NFS_UNSTABLE, but the server's write verifier is
not rotated.
A client that receives NFS_UNSTABLE in CB_OFFLOAD follows up with
COMMIT to make the copied data durable. With the verifier
unchanged, COMMIT returns the same value the client just received
via CB_OFFLOAD, and the client concludes the copy is durable --
silently dropping the data whose writeback in fact failed. This
violates the UNSTABLE+COMMIT durability contract (RFC 7862 section
15.1, RFC 8881 section 18.32) and matches the bug just fixed in
nfsd_vfs_write() and nfsd_commit().
Rotate nn->writeverf at the writeback-failure site. The async COPY
worker has no svc_rqst, so commit_reset_write_verifier() is not
available here; calling nfsd_reset_write_verifier() directly
mirrors the trace-less reset already used by
nfsd_file_check_write_error() for the same purpose. Filter out
-EAGAIN and -ESTALE, matching commit_reset_write_verifier(), since
neither indicates a durable-storage failure.
Fixes: eac0b17 ("NFSD add vfs_fsync after async copy is done")
Cc: [email protected]
Assisted-by: kres:claude-opus-4-7
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/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=31d4d0a62ec4bafbcdbad142f11ab1f90794b1d0
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89706 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89706
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:H/A:N
The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L
The Best / paranoid CVSS score: 5.9
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: 4
Paranoid ActionableScore: 5
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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