[CVE-2026-64600][IMPORTANT] xfs: resample the data fork mapping after cycling ILOCK

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

CVE: CVE-2026-64600
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: xfs: resample the data fork mapping after cycling ILOCK
Commit: e705d81a7193dd19e69b8e2bad4696d78a4ea075
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e705d81a7193dd19e69b8e2bad4696d78a4ea075
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64600
Analysis date: Thu, 23 Jul 2026 02:52:03 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in the XFS reflink CoW direct I/O path can leave a stale data fork mapping after ILOCK cycling, allowing writes to proceed with outdated physical block information and potentially causing data corruption or CoW isolation violation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64600 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64600

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: IMPORTANT
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-2026-64600	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:H';CWE-362;CWE-367;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L';BEST CVSS score: '6.5';DESCR 'A race in the XFS reflink CoW direct I/O path can leave the data fork mapping stale after the inode ILOCK is dropped and reacquired. A racing O_DIRECT writer can complete a CoW cycle and remap the same file offset while the original path still uses the old physical block mapping. This can make the shared block recheck use the wrong blocks and can allow the write path to proceed with stale iomap state, causing data corruption or a CoW isolation violation. For the CVSS the PR:L is used because a local user or process with write access to suitable XFS reflink files can potentially trigger the race without full administrator privileges. The issue is not network reachable and requires local filesystem activity plus a timing window. Impact is primarily integrity loss and availability risk for filesystem data, with limited confidentiality impact possible only in a conservative review scenario.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	DISK LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	NO	guess

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* +1 Local unprivileged trigger: a local user with write access to suitable XFS reflink files can potentially exercise the O_DIRECT CoW path.
* +1 Filesystem/storage metadata or mapping corruption reachable by local user: stale data fork mapping can make the write path operate on the wrong physical blocks.
* +1 Important filesystem/storage path: XFS reflink CoW, extent mappings, and refcount btree logic are integrity sensitive.
* +1 Integrity impact plausible: the bug can cause data corruption or CoW isolation violation.
* +1 Dirty-Pipe-like/COW concern: this is not page-cache corruption, but it is an ownership/COW mapping violation class affecting shared physical blocks.
* +1 Availability impact realistic: persistent filesystem data corruption can lead to repair needs or filesystem shutdown behavior.
* -1 Hard race / timing required: the issue requires an ILOCK drop/reacquire window and a racing O_DIRECT writer.

Paranoid additions:

* +1 Confidentiality impact plausible: stale physical block use could theoretically expose or affect data outside the intended logical file mapping.
* +1 Stronger CoW isolation concern: the stale iomap can cross file/block ownership expectations, so manual-review sensitivity justifies the higher score.

Not counted:

* No generic kernel memory corruption score: the patch shows stale filesystem block mapping, not kernel heap UAF, OOB write, type confusion, or arbitrary write.
* No LPE score: privilege escalation is not strongly supported by the patch.

## 3. Reachability analysis

The likely trigger is local filesystem activity on XFS with reflink/CoW and O_DIRECT writes. Full administrator privileges are not required if the attacker can create or write suitable files on an affected XFS filesystem. Containers may matter if a container has writable access to a host XFS reflink-backed volume, because the trigger is file I/O rather than CAP_SYS_ADMIN-only control-plane setup. The issue is not network reachable. It requires specific concurrent write timing, but the affected subsystem is common enough that this should not be auto-closed as a low-priority niche bug.

Call-site confidence: medium-high. The patch includes the affected helpers and explains the direct I/O CoW race, but not the entire iomap caller chain.

## 4. Severity interpretation

This behaves more like an actionable Moderate or Important-candidate filesystem integrity bug than an ordinary crash-only Moderate. Realistic impact is data corruption or CoW isolation failure. Theoretical confidentiality impact exists only as a conservative review concern due to stale physical block mapping. There is no demonstrated kernel memory corruption or reliable privilege-escalation primitive in the provided patch.

## 5. One-sentence report phrase

A race in the XFS reflink CoW direct I/O path can leave a stale data fork mapping after ILOCK cycling, allowing writes to proceed with outdated physical block information and potentially causing data corruption or CoW isolation violation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: this is a race in an important filesystem mapping path with plausible persistent integrity impact, and the stale mapping may affect block ownership or CoW isolation even though kernel memory corruption is not shown.

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

Patch: xfs: resample the data fork mapping after cycling ILOCK
Commit: e705d81a7193dd19e69b8e2bad4696d78a4ea075
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e705d81a7193dd19e69b8e2bad4696d78a4ea075

Commit description:

xfs_reflink_fill_{cow_hole,delalloc} are both presented with an inode,
a data fork mapping, and a cow fork mapping.  Unfortunately, these two
helpers cycle the ILOCK to grab a transaction, which means that the
mappings are stale as soon as we reacquire the ILOCK.  Currently we
refresh the cow fork mapping by re-calling xfs_find_trim_cow_extent, but
we don't refresh the data fork mapping beforehand, which means that the
xfs_bmap_trim_cow in that function queries the refcount btree about the
wrong physical blocks and returns an inaccurate value in *shared.

If *shared is now false, the directio write proceeds with a stale data
fork mapping.  Fix this by querying the data fork mapping if the
sequence counter changes across the ILOCK cycle.

Cc: hch@lst.de
Cc: stable@vger.kernel.org # v4.11
Fixes: 3c68d44 ("xfs: allocate direct I/O COW blocks in iomap_begin")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/xfs/xfs_reflink.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=e705d81a7193dd19e69b8e2bad4696d78a4ea075

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

The original Linux kernel CVE announcement for CVE-2026-64600 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64600

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:H/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:H
  The Best / paranoid CVSS score: 6.5

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: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).