[CVE-2026-98446][LOW] xfs: don't let hidden_space go negative in xfs_metafile_resv_init

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

CVE: CVE-2026-98446
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: xfs: don't let hidden_space go negative in xfs_metafile_resv_init
Commit: 930a9c1c321efdaa3e7f8caecb0479cee3baf342
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=930a9c1c321efdaa3e7f8caecb0479cee3baf342
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98446
Analysis date: Sat, 24 Oct 2026 09:19:27 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
An XFS reservation arithmetic underflow can severely distort free-block accounting and cause false ENOSPC, without evidence of memory corruption or a security-boundary bypass.

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

The original Linux kernel CVE announcement for CVE-2026-98446 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98446

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: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-98446	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98446 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE AEGIS_AS=1 LEGACY=LOW RESULT=DIFF (final Legacy and Aegis differ; FINAL: LOW, Aegis: MODERATE)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-191;CWE-682;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'An arithmetic underflow in xfs_metafile_resv_init can occur when dblocks_avail is below the space already used by a metadata btree file. The resulting hidden_space value can become extremely large and xfs_dec_fdblocks can subtract that value from free block accounting, potentially causing false ENOSPC results, reservation initialization failure, or loss of availability for the affected XFS filesystem. For the CVSS the PR:L value in the paranoid vector represents a local user who can consume space on a writable filesystem and help create the low space condition. Reliable triggering can still require a specific filesystem state or an administrative operation that runs reservation initialization, so AC:H is retained. There is no evidence of UAF, OOB access, information disclosure, or an arbitrary write primitive, and the supported impact is limited to availability. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	MAYBE	OOB DISK SIMPLEFIX LINUS DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 2.
* Conservative score: 2; no floor or cap applied.
* Raw paranoid score: 2.
* Paranoid score: 2; no defensible additional primitive is shown.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Important filesystem/storage path: XFS global free-block and metadata-btree reservation accounting is affected.
* +1 Availability impact: unsigned underflow makes `hidden_space` extremely large, causing `xfs_dec_fdblocks()` to over-decrement free-space accounting and potentially produce false ENOSPC or reservation failure.
* No crash point: panic, filesystem shutdown, or host-wide fatal failure is not shown.
* No corruption point: the patch shows an arithmetic/accounting error, not an OOB write, UAF, arbitrary write, persistent on-disk metadata corruption, or cross-file ownership breach.
* No reachability bonus: a local user may help exhaust writable space, but the complete reservation-initialization trigger is not shown as directly user-invocable.
* No AC penalty: this requires a specific filesystem state, not a proven narrow race.
* B5-R and B3-PENDING do not apply; there is no heap OOB write or shared-backing write.

3. Reachability analysis

A writable-filesystem user can influence available-space pressure, but the evidence does not establish who invokes `xfs_metafile_resv_init()` or whether an administrative lifecycle operation is required. Consequently, effective host-admin requirements and UID 0 requirements remain unknown. No namespace capability, special hardware, prior delegation, or timing race is demonstrated.

4. Primitive and severity reading

The exact defect is an unsigned subtraction underflow after `target` is reduced below `used`, followed by consumption of the huge value by free-block accounting. This is passive accounting corruption with availability consequences, classified CLASS_C. The fix enforces `target >= used`. No security authorization decision changes from DENY to ALLOW, and no protected resource is exposed. Important-grade exploit leverage is unsupported, so automatic downgrade is allowed.

5. One-sentence report phrase

An XFS reservation arithmetic underflow can severely distort free-block accounting and cause false ENOSPC, without evidence of memory corruption or a security-boundary bypass.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch-supported impact is constrained to filesystem availability/accounting.

7. Call-site confidence

Call-site confidence: medium

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

Patch: xfs: don't let hidden_space go negative in xfs_metafile_resv_init
Commit: 930a9c1c321efdaa3e7f8caecb0479cee3baf342
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=930a9c1c321efdaa3e7f8caecb0479cee3baf342

Commit description:

LOLLM points out that if the amount of fdblocks that we can reserve for
a metadata btree file goes below the space already used by that file,
then the hidden_space subtraction can underflow, causing
xfs_dec_fdblocks to subtract a huge amount of space.  We never want the
target to be less than the used sapce, so fix the logic that adjusts
dblocks_avail downwards.

Also fix an error in the adjacent comment.

Cc: stable@vger.kernel.org # v6.15
Fixes: 1df8d75 ("xfs: make metabtree reservations global")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Assisted-by: LOLLM # finding obvious bugs
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/xfs/libxfs/xfs_metafile.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=930a9c1c321efdaa3e7f8caecb0479cee3baf342

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

The original Linux kernel CVE announcement for CVE-2026-98446 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98446

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:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: unknown
  Paranoid ActionableScore: 2

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).