[CVE-2026-64357][MODERATE REGULAR] xfs: fix exchmaps reservation limit check

[CVE-2026-64357][MODERATE REGULAR] xfs: fix exchmaps reservation limit check

[AL-KERNEL]

CVE: CVE-2026-64357
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: xfs: fix exchmaps reservation limit check
Commit: c597c8580d50127fc1221b5a5b653a94d49e23e3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c597c8580d50127fc1221b5a5b653a94d49e23e3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64357
Analysis date: Sat, 25 Jul 2026 15:22:04 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended / Not Strong Important on current evidence
Manual review required: YES

Summary:
An incorrect XFS exchmaps reservation limit check may allow an oversized computed block reservation to be truncated before xfs_trans_alloc, potentially causing transaction under-reservation, filesystem shutdown, or limited metadata integrity impact under special local conditions.

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

The original Linux kernel CVE announcement for CVE-2026-64357 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64357

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64357	MODERATE	CHECK WITH IMPACT FROM ORIG NN 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:L/A:H';CWE-190;CWE-681;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'An incorrect limit check in xfs_exchmaps_estimate_overhead can allow the computed resblks value to exceed UINT_MAX after bmbt and rmapbt overhead is added. The oversized value is later stored back and passed toward xfs_trans_alloc where the block reservation argument is unsigned int, creating a possible truncation or under reservation condition in XFS transaction setup. For the CVSS the PR:L is used because a local user with access to suitable files on an XFS filesystem may be able to reach the exchmaps path, while full administrator privileges are not clearly required for the file operation itself. The issue is not network reachable and requires local filesystem activity with unusually large or complex mapping state. Impact is primarily denial of service through failed transaction setup or filesystem shutdown. A limited integrity impact is kept in the paranoid score because the bug affects filesystem metadata reservation accounting, but the patch does not support treating it as a direct memory corruption or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Not Strong Important on current evidence (with actual score 4)	YES	DISK SIMPLEFIX IMPROVEONLY LINUS 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended / Not Strong Important on current evidence**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local user with access to suitable files on an XFS filesystem may be able to reach the exchmaps path. Full host root is not clearly required for the file operation itself.

* Important filesystem/storage path: +1
  The bug is in XFS transaction reservation logic for file mapping exchange operations.

* Filesystem/storage metadata corruption reachable by a local user: +1
  The issue affects metadata transaction reservation accounting. An oversized computed reservation may be truncated when later passed to xfs_trans_alloc as unsigned int.

* Availability impact realistic: +1
  Under reservation or invalid transaction accounting can plausibly lead to transaction failure, filesystem shutdown, or local DoS.

* Hard or special triggering conditions: -1
  Triggering appears to require unusually large or complex extent and metadata overhead conditions, so this is not a simple reliable crash path.

Paranoid-only signal:

* Integrity impact plausible: +1
  Because the bug is in filesystem metadata reservation accounting, limited filesystem integrity impact is defensible for triage. There is no evidence of arbitrary write, page-cache corruption, or direct privilege escalation.

Not applied:

* Remote reachable: +0
* Generic strong memory corruption: +0
* Real lifetime corruption: +0
* Privilege escalation plausible: +0
* Dirty-Pipe-like primitive: +0

Call-site confidence: medium. The patch states the computed value is later passed to xfs_trans_alloc, but the full caller chain and exact failure mode are not included.

## 3. Reachability analysis

The affected path is local filesystem control/data-plane behavior in XFS exchmaps. A local user may be able to trigger it through file mapping exchange operations on files they can access, but practical triggering likely needs a specially shaped filesystem state with very large mapping and metadata overhead. Namespaces or containers can matter if XFS files are exposed to less privileged workloads, but this does not look network reachable. The subsystem is common and important, but the exact edge condition is narrow.

## 4. Severity interpretation

This behaves more like a borderline actionable Moderate than an Important-class kernel vulnerability. The theoretical concern is integer truncation or under-reservation in a filesystem metadata transaction, which can justify manual review and a paranoid integrity point. The realistic evidence supports local DoS or filesystem shutdown more strongly than privilege escalation. No concrete UAF, OOB write, arbitrary write, page-cache corruption, or reclaim primitive is shown.

## 5. One-sentence report phrase

An incorrect XFS exchmaps reservation limit check may allow an oversized computed block reservation to be truncated before xfs_trans_alloc, potentially causing transaction under-reservation, filesystem shutdown, or limited metadata integrity impact under special local conditions.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

This should not be auto-closed solely as a low-risk correctness fix because it affects XFS transaction reservation accounting, but current evidence does not justify Strong Important classification.

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

Patch: xfs: fix exchmaps reservation limit check
Commit: c597c8580d50127fc1221b5a5b653a94d49e23e3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c597c8580d50127fc1221b5a5b653a94d49e23e3

Commit description:

xfs_exchmaps_estimate_overhead() adds the bmbt and rmapbt
overhead to a local resblks variable, but the final UINT_MAX
check still tests req->resblks.  That is the reservation value
from before the overhead was added.

The computed value is stored back in req->resblks and later passed
to xfs_trans_alloc(), whose block reservation argument is unsigned
int.  Check the computed reservation so the existing limit applies
to the value that will be used.

Fixes: 966ceaf ("xfs: create deferred log items for file mapping exchanges")
Cc: stable@vger.kernel.org # v6.10
Signed-off-by: Yingjie Gao <gaoyingjie@uniontech.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/xfs/libxfs/xfs_exchmaps.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=c597c8580d50127fc1221b5a5b653a94d49e23e3

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

The original Linux kernel CVE announcement for CVE-2026-64357 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64357

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:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/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).