[CVE-2026-90318][MODERATE REGULAR] fat: release buffer head after rebuilding parent [ Upstream

[CVE-2026-90318][MODERATE REGULAR] fat: release buffer head after rebuilding parent [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90318
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fat: release buffer head after rebuilding parent [ Upstream
Commit: 5e44b6944e11fd6647f028219ad0be4e85e9acee
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5e44b6944e11fd6647f028219ad0be4e85e9acee
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90318
Analysis date: Thu, 17 Sep 2026 17:44:12 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-90318 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90318

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 REGULAR
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-2026-90318	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90318 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:N/A:H';*CWE-772;*CWE-401;**CWE-400;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'A buffer_head reference leak can occur in the FAT NFS export path because fat_rebuild_parent uses the directory entry returned by fat_scan_logstart but does not release sinfo.bh after rebuilding the parent inode. A remote NFS client may be able to trigger this path when a FAT filesystem is exported and the nostale_ro rebuild logic is used, but the scenario depends on a specific storage and export configuration. For the CVSS the PR:L is used because reliable triggering typically requires authorized access to the NFS export or an equivalent client context rather than only sending arbitrary Internet traffic. The issue is network reachable only in deployments that expose FAT over NFS. Impact is denial of service through gradual kernel resource exhaustion. No concrete UAF, OOB access, information leak, or privilege escalation primitive is indicated by the code change.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	DISK SIMPLEFIX LOG LINUS 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3. No floor or cap applied.
* Raw paranoid score: 4
* Paranoid score: 4. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +1 Concrete persistent kernel resource/reference leak. `fat_scan_logstart()` leaves `sinfo.bh` for the caller, and `fat_rebuild_parent()` failed to call `brelse(sinfo.bh)` after successful use.
* +1 Sustained resource exhaustion concern. Repeated successful rebuilds in the NFS export path can accumulate unreleased `buffer_head` references.
* +2 Network reachable in affected deployments. The vulnerable path is tied to FAT exported through NFS, so an NFS client may trigger it if the export and `nostale_ro` path are in use.
* -1 Specialized configuration. The issue requires FAT used through the NFS export rebuild path, which is narrower than a generic local filesystem operation.

Paranoid signals:

* +1 Concrete persistent kernel resource/reference leak.
* +1 Sustained resource exhaustion concern.
* +2 Network reachable in affected deployments.
* The specialized-configuration reduction is withheld in the paranoid score to avoid auto-downgrading a remotely reachable NFS-export issue.

Non-awarded signals:

* RealUAF not awarded. The patch fixes a missing `brelse()`, not a stale post-lifetime dereference.
* Concrete repeated release/refcount not awarded. There is no first release followed by a second lifetime mutation on the same dangling reference.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity impact not awarded.
* Privilege escalation not supported by the patch.
* Reliable immediate kernel crash not established. The impact is gradual resource exhaustion rather than direct panic.

3. Reachability analysis

The practical attacker is an NFS client able to access an affected export. Host administrator privileges are not required for the client-side trigger once the FAT filesystem is exported, so `HostAdminRequired=NO`.

The complete setup does require prior host-side configuration: a FAT filesystem exported through NFS and the `nostale_ro` rebuild path. The attacker cannot usually create that export, but this is treated as deployment exposure rather than host-admin requirement for the actual trigger.

No namespace-scoped capability is involved. No special hardware is required. No race or timing window is involved. The issue is repeatable through repeated operations that force parent inode rebuilds in the affected export path.

4. Primitive and severity interpretation

The exact primitive is a persistent `buffer_head` reference leak. This is not memory corruption, not UAF, not double free, and not an authorization bypass.

The demonstrated consequence is resource leakage that may lead to denial of service after sustained triggering. The stronger consequence remains availability-only. There is no evidence of attacker-controlled memory corruption, information disclosure, arbitrary write, or privilege escalation.

Primitive class is `CLASS_C` because the behavior is DoS/resource-exhaustion oriented without an independently useful corruption primitive.

`ImportantCandidate=NO` because the primitive is a missing release/reference leak with no C/I or LPE leverage. `AutoDowngradeAllowed=YES` because no unresolved memory-corruption, ownership, authorization, or repeated lifetime-mutation primitive remains.

Likely operational severity is low-to-moderate, with manual review recommended mainly due to possible NFS network exposure.

5. One-sentence report phrase

A missing `brelse(sinfo.bh)` in the FAT NFS export parent rebuild path can leak `buffer_head` references on successful scans, allowing an authorized remote NFS client in affected deployments to cause gradual kernel resource exhaustion and denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the bug is resource-leak only and does not indicate memory corruption, but the affected path may be reachable over NFS in configured deployments.

7. Call-site confidence

Call-site confidence: medium

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: fat: release buffer head after rebuilding parent [ Upstream
Commit: 5e44b6944e11fd6647f028219ad0be4e85e9acee
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5e44b6944e11fd6647f028219ad0be4e85e9acee

Changed files:
  fs/fat/nfs.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=5e44b6944e11fd6647f028219ad0be4e85e9acee

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-90318 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90318

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:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.3

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: 3
  Paranoid ActionableScore: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).