* [CVE-2026-89668][MODERATE 7.0] nfsd: move nfsd_debugfs_init() after nfsd4_init_slabs() in init_nfsd()
@ 2026-09-12 10:59 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-12 10:59 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89668
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: move nfsd_debugfs_init() after nfsd4_init_slabs() in init_nfsd()
Commit: 4ae5d7490ae6a7b83b3d607f88c499f4f85e753a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4ae5d7490ae6a7b83b3d607f88c499f4f85e753a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89668
Analysis date: Sat, 12 Sep 2026 06:59:29 -0400
ActionableScore: 5
ActionableScore lower bound: 4
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-2026-89668 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89668
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-2026-89668 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89668 CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;CWE-404;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'An NFSD module initialization failure can leave debugfs files behind with stale fops pointers into module text that is no longer valid. This happens when nfsd_debugfs_init() succeeds but nfsd4_init_slabs() fails and the init path returns without removing the debugfs entries. A later access to the orphan /sys/kernel/debug/nfsd files can dereference freed module code pointers, causing a kernel crash and creating a UAF-style control flow risk. For the CVSS the PR:L reflects the paranoid case where a local user or service account can access the orphan debugfs interface after the failed module initialization, while reliable creation of that state is harder and normally administrative. The issue is not network reachable through NFS traffic. Impact is at least local denial of service and in the worst case may include confidentiality or integrity impact due to stale function pointers into freed module text.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) SKIP CVE-2026-89668 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DISK INIT ERRORPATH UAF NETWORK DEBUGFS LINUS KPANIC KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 5
* Applied paranoid floor/cap: Real UAF floor 5 applied because stale externally reachable debugfs state contains fops pointers into freed module text and later VFS/debugfs operations may consume those stale function pointers.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Positive signals:
* +2 Real UAF / lifetime violation: debugfs files can remain after failed nfsd module initialization with stale fops pointers into module text that may have been freed after init failure.
* +1 Reliable kernel crash / strong DoS: later access to orphan debugfs entries can dereference or call through stale fops pointers and plausibly crash the kernel.
* +1 Weak exploitation concern: the stale object is not just passive metadata. It is an externally reachable debugfs/VFS object whose fops are active callback/control-flow data.
Negative signals:
* No -2 host-admin penalty applied in the score because the supplied evidence strongly suggests module initialization is privileged, but the complete security-relevant chain after orphan debugfs creation is not fully resolved.
* No race/timing penalty: this is an ordered init/unwind bug, not a narrow race.
Important non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity bonuses not awarded directly.
* Strong LPE +2 not awarded because attacker control over replacement module text or fops target is not established.
* Concrete repeated release/refcount +1 not awarded. This is stale callback/lifetime state, not a proven second put/release/refcount mutation on the exact same object.
* Extra availability point not awarded because the same kernel crash is already counted once.
* Remote reachability not awarded. The issue is not triggered by NFS protocol traffic.
3. Reachability analysis
The vulnerable state is created during nfsd module initialization. In normal deployments, loading or initializing nfsd module state is host-administrator controlled. The immediate failure condition is nfsd4_init_slabs() failing after nfsd_debugfs_init() has already created debugfs files.
A later attacker would need access to the orphan `/sys/kernel/debug/nfsd` entries. Debugfs is commonly restricted to root or privileged diagnostic contexts, but the supplied input does not fully establish the permissions or the complete actor model for the post-failure access path. Therefore HostAdminRequired is set to UNKNOWN rather than YES or NO.
No namespace-scoped privilege path is shown. No network path through NFS traffic is shown. No special hardware requirement is present. No race repeatability issue is central to the bug.
4. Primitive and severity interpretation
The exact primitive is a real stale lifetime use: orphan debugfs files keep fops pointers into module text after failed nfsd initialization can free that module text.
This is an active structured stale-object case, not fixed stale storage. The stale state is consumed by debugfs/VFS operations through file operation callbacks. It is not merely cleanup metadata.
This is not a concrete repeated release/refcount case. There is no visible first put followed by a second put on the exact same dangling reference. The stronger concern is stale callback/control-flow data surviving after module lifetime ends.
PrimitiveClass is CLASS_B. The demonstrated consequence is local kernel crash or host DoS. The speculative consequence is stronger exploitation if freed module text or related callback targets can be influenced, but controlled reclaim or callback hijack is not established by the supplied evidence.
ImportantCandidate is YES because stale fops pointers into freed module text are a meaningful unresolved kernel lifetime/control-flow primitive. AutoDowngradeAllowed is NO because automatic closure would discard a real stale callback primitive requiring manual exploitation-leverage review.
Likely operational severity: actionable Moderate at minimum, with manual review needed to decide whether this reaches Important in a given product configuration.
5. One-sentence report phrase
An nfsd initialization failure can leave orphan debugfs entries with stale file operation pointers into freed module text, allowing later local debugfs access to trigger a kernel crash and requiring manual review for possible stale callback exploitation leverage.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: real stale fops pointers into freed module text are established, and the post-lifetime callback/control-flow implications are not safely reducible to ordinary crash-only cleanup behavior.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: nfsd: move nfsd_debugfs_init() after nfsd4_init_slabs() in init_nfsd()
Commit: 4ae5d7490ae6a7b83b3d607f88c499f4f85e753a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4ae5d7490ae6a7b83b3d607f88c499f4f85e753a
Commit description:
nfsd_debugfs_init() runs before nfsd4_init_slabs() in init_nfsd().
If the slab allocation fails, the bare "return retval" bypasses
nfsd_debugfs_exit(), leaving orphan debugfs files with stale fops
pointers into the freed module text.
Move nfsd_debugfs_init() to after the slab init succeeds, so the
early return has no debugfs state to clean up.
Since debugfs is now the more recently initialized of the two, also
update the unwind paths to match reverse-initialization (LIFO) order:
run nfsd_debugfs_exit() before nfsd4_free_slabs() in both the
init_nfsd() error path and exit_nfsd(). The nfsd debugfs files only
reference module-global state and have no dependency on the slab
caches, so that reordering is a cleanup with no functional change.
Fixes: 9fe5ea7 ("NFSD: Add /sys/kernel/debug/nfsd")
Cc: [email protected]
Assisted-by: Claude:claude-opus-4-8
Signed-off-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=4ae5d7490ae6a7b83b3d607f88c499f4f85e753a
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89668 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89668
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:H/PR:H/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7
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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