[CVE-2026-45915][LOW] fat: avoid parent link count underflow in rmdir [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-45915
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: fat: avoid parent link count underflow in rmdir [ Upstream
Commit: 7fe0de287e931e07cb96ecf1f449b2ebdb0e1115
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7fe0de287e931e07cb96ecf1f449b2ebdb0e1115
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45915
Analysis date: Wed, 27 May 2026 21:37:13 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A corrupted FAT/VFAT image with an invalid parent directory link count can let local `rmdir()` drive `i_nlink` to zero and trigger a WARN, causing possible DoS when `panic_on_warn` is enabled.

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

The original Linux kernel CVE announcement for CVE-2026-45915 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45915

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: LOW
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-2026-45915	LOW	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-191;CWE-682;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A corrupted FAT or VFAT filesystem image can contain a parent directory inode with an invalid low link count. When a local user removes a subdirectory, msdos_rmdir() or vfat_rmdir() used to call drop_nlink() unconditionally and could drive the parent i_nlink to zero, triggering a WARN in the VFS link count path. For the CVSS the PR:L is used because the attacker needs local ability to access a mounted writable FAT filesystem and issue rmdir, but full administrator privileges are not required once the crafted image is mounted. The issue is not network reachable. Impact is denial of service through WARN or possible panic_on_warn behavior. No supported evidence indicates information disclosure, integrity impact beyond the already corrupted filesystem image, kernel memory corruption, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2) 	MAYBE	OOB DISK SIMPLEFIX IMPROVEONLY LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

* Local unprivileged trigger after mount: +1
  Once a corrupted FAT or VFAT image is mounted writable and accessible, a local user can issue `rmdir()` on the affected directory.

* Availability impact realistic: +1
  The demonstrated impact is a WARN in `drop_nlink()`. This can become stronger DoS if `panic_on_warn` is enabled.

* Important filesystem/storage path: +1 in paranoid scoring
  FAT/VFAT is widely used for removable media, but the bug requires an already corrupted image and affects link-count accounting rather than kernel memory safety.

* Requires special crafted/corrupted filesystem state: -1 conservative
  The issue depends on a corrupted FAT image with inconsistent parent link count.

* No memory corruption: +0
  The patch shows an `i_nlink` underflow/WARN prevention, not UAF, OOB write, arbitrary write, or heap corruption.

* No strong integrity impact: +0
  The filesystem is already corrupt. The kernel is not shown to create broad persistent metadata corruption beyond the invalid input state.

## 3. Reachability analysis

The attacker needs a corrupted FAT/VFAT filesystem image to be mounted writable, then needs local access to remove a subdirectory. If users can mount arbitrary images or insert removable media that is automounted writable, the practical trigger is easier. If mounting removable or loopback filesystems is restricted to trusted administrators, reachability is much lower.

Namespaces may matter if unprivileged mounts or container-managed removable media are allowed, but by default this is not a network-reachable issue. Realistic exploitation is WARN or `panic_on_warn` DoS, not privilege escalation.

## 4. Severity interpretation

This behaves like ordinary Moderate or Low-like DoS, with a borderline case for environments that automatically mount untrusted FAT media writable. The theoretical impact is availability via WARN/panic. The realistic evidence does not support memory corruption, data disclosure, LPE, or kernel-created filesystem corruption.

## 5. One-sentence report phrase

A corrupted FAT/VFAT image with an invalid parent directory link count can let local `rmdir()` drive `i_nlink` to zero and trigger a WARN, causing possible DoS when `panic_on_warn` is enabled.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

No manual check is needed for normal server deployments. Consider manual review only for products that auto-mount untrusted removable FAT media writable or enable `panic_on_warn`.

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

Patch: fat: avoid parent link count underflow in rmdir [ Upstream
Commit: 7fe0de287e931e07cb96ecf1f449b2ebdb0e1115
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7fe0de287e931e07cb96ecf1f449b2ebdb0e1115

Changed files:
  fs/fat/namei_msdos.c
  fs/fat/namei_vfat.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=7fe0de287e931e07cb96ecf1f449b2ebdb0e1115

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

The original Linux kernel CVE announcement for CVE-2026-45915 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45915

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:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 1
  Paranoid ActionableScore: 3

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).