* [CVE-2026-90151][MODERATE 7.0] NFSv4: remove callback IDR entry on client allocation failure [ Upstream
@ 2026-09-18 7:06 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-18 7:06 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90151
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSv4: remove callback IDR entry on client allocation failure [ Upstream
Commit: 68c721391b761dbe38d5b0094d2bb6e8489ad92b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=68c721391b761dbe38d5b0094d2bb6e8489ad92b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90151
Analysis date: Fri, 18 Sep 2026 03:06:37 -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-2026-90151 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90151
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-2026-90151 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90151 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:N/UI:R/S:U/C:H/I:H/A:H';*CWE-416;CWE-825;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'An NFSv4.0 callback IDR entry can remain published after nfs4_alloc_client() fails and frees the associated nfs_client through nfs_free_client(). A later callback lookup by cb_ident may retrieve the stale pointer and take a reference to freed memory, creating a use-after-free risk in the NFS client callback path. For the CVSS the PR:N is used for the paranoid score because a malicious or compromised NFSv4.0 server does not need an account on the victim, although the victim must mount or automount the server for the vulnerable client path to be reached. The issue is network reachable in deployments where NFSv4.0 callbacks from the server to the client are possible. Impact is at least denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel use-after-free.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) YES LOCK DISK ERRORPATH UAF NETWORK LINUS KPANIC DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
Applied conservative floor: 4 -> 5. The floor is justified because this is a real stale published kernel object pointer in an active callback lookup structure, not fixed stale storage or a cleanup-only dereference.
2. Signal breakdown
Conservative positive signals:
* +2 Real UAF / lifetime violation: nfs4_alloc_client() can publish an NFSv4.0 callback identifier in cb_ident_idr, then fail later and free the nfs_client via nfs_free_client() without removing the IDR entry.
* +1 Secondary index / registration table staleness: cb_ident_idr remains synchronized with a freed nfs_client, and later callback lookup can discover the obsolete association.
* +1 Reliable kernel crash / strong DoS concern: a later callback lookup can retrieve the stale pointer and take a reference to freed memory, which is a realistic kernel fault or memory-lifetime corruption path.
Paranoid additional signals:
* +2 Remote callback exposure in the stronger attacker model: a malicious or compromised NFSv4.0 server can participate in the callback path after the victim mounts or automounts the server. This is not treated as arbitrary Internet exposure, but it is a meaningful network-involved trigger surface.
Important non-awarded signals:
* Controlled reclaim is not demonstrated.
* Arbitrary kernel read/write is not demonstrated.
* CLASS_A is not assigned because no controlled replacement, callback hijack, refcount takeover, or demonstrated LPE is established.
* Confidentiality and integrity bonuses are not awarded separately. The UAF could theoretically affect them, but no concrete disclosure or write primitive is shown.
* Concrete repeated release/refcount signal is not awarded under the dedicated repeated-release rule, because the patch establishes stale lookup and reference acquisition, not a visible first put followed by a second put/release on the exact same reference.
* Structured stale-object leverage is not separately stacked beyond the IDR staleness and UAF facts, to avoid double-counting the same stale lookup mechanism.
* Extra Availability is not awarded because the crash/lifetime fault is already counted once.
* No race penalty is applied. The described issue is an ordered publish-before-free lifetime bug, not a proven narrow timing race.
* No host-admin penalty is applied because the supplied scenario includes a malicious or compromised NFS server that does not need an account on the victim, although victim-side mount or automount is still required.
Repeated-release/refcount checklist:
* First release can end lifetime: yes, nfs_free_client() frees the failed nfs_client.
* Exact same reference survives: yes, the stale pointer remains reachable through cb_ident_idr.
* Second operation is a real lifetime/refcount mutation: partially, later lookup may take a reference to freed memory, but this is a stale get/reference acquisition rather than the dedicated repeated put/release pattern.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: yes, because the stale object remains published in an active IDR lookup path and is later used by NFSv4.0 callback handling.
3. Reachability analysis
The vulnerable object is created during NFSv4 client allocation. The bug is triggered when a callback identifier is allocated and published, then a later initialization step fails, causing direct free through nfs_free_client() instead of the normal nfs_put_client() teardown path.
The strongest supplied attacker model is a malicious or compromised NFSv4.0 server. That attacker does not need host-admin privileges or a local account on the victim, but the victim must mount or automount the server for the vulnerable client path to be reached. This is why HostAdminRequired=NO is selected for the evaluated attacker role, while the report should still mention the mount or automount prerequisite.
The issue is not namespace-scoped. No evidence is supplied that an attacker-created user or network namespace is relevant to the complete trigger chain.
Prior victim-side setup exists in practice: an NFSv4.0 mount or automount must be attempted. In common systems, creating arbitrary NFS mounts is often restricted, but the malicious-server scenario can also arise from existing administrator-configured mounts or automounts. Therefore the conservative score does not assume ordinary local unprivileged mount control, and the paranoid score considers the network callback role after the mount relationship exists.
No special hardware is required. No narrow race is required. The key condition is an allocation or initialization failure after cb_ident publication and before successful client setup.
4. Primitive and severity interpretation
The exact primitive is a publish-before-free lifetime bug. A callback IDR entry can remain visible after the associated struct nfs_client has been freed. A later NFSv4.0 callback lookup by cb_ident can retrieve that stale pointer and take a reference to it.
This is an active structured object case. struct nfs_client is a nontrivial kernel object used by NFS client state and callback handling. The stale pointer is not merely a fixed stale buffer and not just a destructor-local cleanup pointer.
The demonstrated consequence is at least kernel memory-lifetime misuse and likely DoS through kernel crash. The speculative consequence is broader exploitation from use-after-free, including possible confidentiality or integrity impact, but the supplied patch does not establish controlled reclaim, arbitrary read/write, callback control, or privilege escalation.
PrimitiveClass=CLASS_B because a real UAF/lifetime primitive is established, but no strong controlled exploitation primitive is shown.
ImportantCandidate=YES because a stale active kernel object remains published in a lookup table and can be used later by a network-involved callback path. That is meaningful unresolved exploitation leverage beyond an ordinary crash-only warning.
AutoDowngradeAllowed=NO because automatic closure as ordinary Moderate would discard a real published stale-pointer UAF in an active callback lookup path. This needs manual review even though CLASS_A exploitation is not proven.
Likely operational severity is Actionable Moderate at minimum, with Strong Important candidacy depending on whether practical callback-trigger control, allocation-failure inducement, and post-free reuse can be demonstrated.
5. One-sentence report phrase
An NFSv4.0 callback IDR entry may remain published after nfs4_alloc_client() frees the associated nfs_client on an allocation failure, allowing a later callback lookup to take a reference to freed memory and creating a network-involved use-after-free risk when a malicious or compromised NFS server is mounted.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a real stale-pointer UAF involving an active NFSv4.0 callback lookup table, with possible malicious-server reachability and unresolved exploitation leverage beyond a simple local crash.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: NFSv4: remove callback IDR entry on client allocation failure [ Upstream
Commit: 68c721391b761dbe38d5b0094d2bb6e8489ad92b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=68c721391b761dbe38d5b0094d2bb6e8489ad92b
Changed files:
fs/nfs/client.c
fs/nfs/internal.h
fs/nfs/nfs4client.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=68c721391b761dbe38d5b0094d2bb6e8489ad92b
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90151 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90151
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:L/I:L/A:H
The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:R/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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