* [CVE-2026-89677][MODERATE 7.0] nfsd: fix possible fh_compose of wrong dentry in nfsd4_create_file()
@ 2026-09-12 8:43 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 8:43 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89677
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: fix possible fh_compose of wrong dentry in nfsd4_create_file()
Commit: 033e783709ce99662cf81f6abcad746a8da9d35f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=033e783709ce99662cf81f6abcad746a8da9d35f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89677
Analysis date: Sat, 12 Sep 2026 04:43:27 -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.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89677 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89677
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89677 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89677 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:H/I:H/A:H';*CWE-416;CWE-825;*CWE-664;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'An NFS server use-after-free can occur in nfsd4_create_file because fh_compose may store a filehandle for a dentry before the final dentry is known. If dentry_create returns a different dentry than the original child, the previously stored child can be freed and later dereferenced through the response filehandle path. A remote NFS client with create or open access to an affected export can potentially trigger this through NFSv4 create/open handling, especially in unusual setups such as NFS re-export where dentry aliasing can occur. For the CVSS the PR:L is used because reliable triggering typically requires valid low privilege access to an exported filesystem with permission to create or open files. The issue is network reachable through nfsd, but AC:H is used because the described condition is hypothetical and depends on specific filesystem and dcache behavior. Impact is at least remote denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact due to the use-after-free class.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) SKIP CVE-2026-89677 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DISK UAF NETWORK KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
No floor or cap changed the conservative score. No floor or cap changed the paranoid score.
2. Signal breakdown
Conservative positive signals:
* +2 Real UAF / lifetime violation. The commit directly states that the dentry stored in resfhp can be freed and later dereferenced.
* +2 Remote reachable / directly network-triggerable. The vulnerable path is in nfsd NFSv4 create/open handling and can be reached by a remote NFS client that has access to the affected export.
* +1 Reliable kernel crash / strong DoS. A freed dentry later being dereferenced in kernel context supports a realistic nfsd kernel crash or host DoS.
Conservative negative signals:
* No -2 host-admin penalty. Host-admin-equivalent privilege is not required for the remote NFS client once the export exists and the client has create/open permission.
* No -1 race penalty. The issue is not described as a narrow timing race.
* No -1 rare subsystem penalty. NFS re-export or unusual dentry aliasing is a constraining condition, but NFS server exposure itself is a normal deployed service and the commit presents NFS re-export as an example rather than a proven exclusive trigger.
Paranoid additional signal:
* +1 Attacker-influenced active-object lifetime surface. The attacker can repeatedly drive NFSv4 create/open operations and influence target path/object selection, while the stale object is an active structured dentry later used through the filehandle response path. This is not controlled reclaim, but it is enough to keep the case above ordinary crash-only triage.
Material non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary kernel read/write not demonstrated.
* Callback or function-pointer control not demonstrated.
* Concrete repeated release/refcount mutation not demonstrated.
* Structured stale-object exploitation leverage was not separately awarded beyond the attacker-influenced lifetime surface.
* Weak LPE was not separately awarded merely because the bug is a UAF.
* Extra Availability was not awarded because the same crash is already counted once.
* Confidentiality and Integrity bonuses were not awarded in the arithmetic because no disclosure, overwrite, protected-resource access, or demonstrated LPE primitive is established.
Repeated release/refcount checklist:
* First release can end lifetime: not established as a repeated-release pattern.
* Exact same reference survives to a second lifetime mutation: not established.
* Second operation is a real lifetime/refcount mutation: not established.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no. The raw conservative score already reaches 5.
3. Reachability analysis
The evaluated attacker is a remote NFS client, not a local host administrator. The client must be able to reach nfsd and must have sufficient export permissions to exercise NFSv4 create/open behavior. This maps to below-host-admin reachability, so HostAdminRequired=NO.
The complete trigger is not shown to be namespace-scoped. NFS server export configuration and backing filesystem selection are host-side administrative setup, while the attacker-controlled part is the remote protocol operation against an already exposed export. NamespaceScopedPrivilege is therefore UNKNOWN.
Prior host setup exists in the practical sense that nfsd must be running and an affected export must exist. That does not make the attacker host-admin once the service is exposed. The described special case may require unusual backing filesystem behavior, with NFS re-export given as an example. No special hardware is involved. No hard race or narrow timing condition is established.
4. Primitive and severity interpretation
The exact primitive is a stale dentry lifetime violation in nfsd4_create_file. fh_compose could store a filehandle using a child dentry before the final dentry is known. If dentry_create returns a different dentry, the earlier child stored in resfhp may be freed and later dereferenced.
This is an active structured object UAF, not fixed stale storage. The stale object is a dentry participating in filesystem and filehandle processing. The demonstrated consequence is at least remote DoS via kernel crash. The speculative consequence is broader exploitation from stale dentry reuse, but controlled reclaim, arbitrary R/W, or LPE is not established.
PrimitiveClass=CLASS_B because a real kernel lifetime primitive exists and exploitation-leverage analysis is required, but no Class A controlled corruption primitive is demonstrated. ImportantCandidate=YES because this is a network-reachable active-object UAF in nfsd with unresolved post-lifetime behavior. AutoDowngradeAllowed=NO because automatically closing this as ordinary Moderate would lose a meaningful unresolved kernel lifetime primitive in a remote server path.
Likely operational severity is Actionable Moderate at minimum, with Strong Important review warranted if additional analysis finds controlled dentry reuse, sensitive filehandle confusion, or another exploitation primitive beyond crash.
5. One-sentence report phrase
A remote NFS client with create or open access to an affected export may trigger a stale dentry use in nfsd4_create_file, causing at least kernel DoS and requiring manual review because the bug is a network reachable active object UAF.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a network-reachable nfsd UAF candidate involving an active filesystem object, and the available patch context establishes a real stale post-lifetime dereference but does not fully resolve exploitation leverage beyond crash.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: nfsd: fix possible fh_compose of wrong dentry in nfsd4_create_file()
Commit: 033e783709ce99662cf81f6abcad746a8da9d35f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=033e783709ce99662cf81f6abcad746a8da9d35f
Commit description:
dentry_create() can hypothetically provide a different dentry than the
one passed in. This could happen, for example, if the exported
filesystem is NFS, and the server returned to OPEN a filehandle which
matched a directory that was already in the dcache. Clearly this would
not be expected!
If this were to happen the dentry (child) that was already stored in
resfhp could be freed and later dereferenced.
We shouldn't call fh_compose() until we are certain that we have the
final dentry, so this patch moved the fh_compose() call to two places:
one for the case where the target already exists, and one after
dentry_create() where it was created.
Fixes: 64a989d ("VFS/knfsd: Teach dentry_create() to use atomic_open()")
Cc: [email protected]
Signed-off-by: NeilBrown <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Chuck Lever <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
Reviewed-by: Benjamin Coddington <[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=033e783709ce99662cf81f6abcad746a8da9d35f
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89677 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89677
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:N/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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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