[CVE-2026-74596][MODERATE 7.0] fs,fsverity: remove check for fsverity being enabled in setattr_prepare()

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

CVE: CVE-2026-74596
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fs,fsverity: remove check for fsverity being enabled in setattr_prepare()
Commit: 2d2b2ed7bdcc6df4942292318c5455b5e94ba525
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2d2b2ed7bdcc6df4942292318c5455b5e94ba525
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74596
Analysis date: Sat, 22 Aug 2026 15:46:01 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A local integrity bypass in fsverity handling allows truncate or other size changes on IS_VERITY files when the kernel is built without CONFIG_FS_VERITY, breaking the expected immutability of protected files.

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

The original Linux kernel CVE announcement for CVE-2026-74596 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74596

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 7.0
Manual review required: NO

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-2026-74596	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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:H/A:N';CWE-693;CWE-284;CWE-345;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '5.5';DESCR 'A protection bypass in fsverity related setattr_prepare logic can allow size changes on files that are marked as verity protected when the kernel is built without CONFIG_FS_VERITY. This breaks the expected immutability property of fsverity files and may allow a local actor with sufficient file operation rights to truncate protected content that should not be modifiable. For the CVSS the PR:L is used because reliable triggering requires a local user or process context with permission to perform the truncate or size change operation on the affected file. The issue is not directly network reachable and does not indicate kernel memory corruption. Impact is primarily integrity loss for protected file contents, with a higher paranoid interpretation when fsverity is part of a local trust or package integrity model.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-74596 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	DISK SIMPLEFIX IMPROVEONLY LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local user or local process with permission to truncate or resize the affected file can reach the vulnerable setattr path.

* Security or integrity policy bypass: +2 in paranoid scoring, +1 in conservative scoring
  fsverity is intended to make protected files immutable after verification is enabled. Allowing truncate on an inode with IS_VERITY breaks that integrity boundary.

* Practical exploit chain strongly implied: +1
  The commit explicitly states that truncate on an fsverity file can succeed when this check is incorrectly gated by CONFIG_FS_VERITY.

* Integrity impact plausible: +1
  The attacker can modify protected file state by changing file size. This is an integrity impact, not memory corruption.

* Important filesystem or storage path: +1
  The issue affects generic VFS setattr_prepare behavior for fsverity protected files.

Subtracted signals:

* Configuration dependent condition: -1
  The issue requires a filesystem containing verity marked files on a kernel built without CONFIG_FS_VERITY. This is plausible but not the most common deployment state.

Not triggered:

* No generic memory corruption.
* No UAF, OOB write, double free, or refcount lifetime issue.
* No remote or network trigger.
* No direct confidentiality impact.
* No direct availability impact.

## 3. Reachability analysis

The bug is locally reachable through truncate or other file size changing operations that call setattr_prepare. The attacker needs a local user or process context with sufficient file permissions to attempt the size change. This is PR:L in CVSS style reasoning, not PR:N, because local execution or a local process context is required.

Namespaces or containers may matter if a container workload has access to a mounted filesystem containing fsverity marked files and can perform file operations on them. The issue does not require CAP_SYS_ADMIN by itself once the filesystem and file permissions allow the operation.

The path is in common VFS code, but the vulnerable condition is configuration dependent because it matters specifically when IS_VERITY inodes exist while CONFIG_FS_VERITY is disabled.

## 4. Severity interpretation

This behaves like an actionable integrity protection bypass, not like memory corruption. The realistic impact is unauthorized size modification of files that should remain immutable under fsverity semantics. The paranoid score is higher because fsverity may be part of a local trust, package integrity, or protected content model, where truncating protected files could have operational security consequences.

This is not an Important class memory corruption candidate. It is best treated as Actionable Moderate at minimum because it bypasses a filesystem integrity boundary and should not be auto closed without checking product exposure.

## 5. One-sentence report phrase

A local integrity bypass in fsverity handling allows truncate or other size changes on IS_VERITY files when the kernel is built without CONFIG_FS_VERITY, breaking the expected immutability of protected files.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a security feature bypass in fsverity semantics. It is not memory corruption, but exposure depends on whether affected products can mount or ship filesystems with verity marked files while CONFIG_FS_VERITY is disabled.

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

Patch: fs,fsverity: remove check for fsverity being enabled in setattr_prepare()
Commit: 2d2b2ed7bdcc6df4942292318c5455b5e94ba525
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2d2b2ed7bdcc6df4942292318c5455b5e94ba525

Commit description:

The check that fs-verity is available in the kernel is not necessary
here. Filesystems could have fsverity files even without fs-verity
enabled. In that case, truncate on fsverity file will succeed, what this
check is trying to prevent.

Fixes: e973465 ("fs,fsverity: reject size changes on fsverity files in setattr_prepare")
Cc: stable@vger.kernel.org
Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260727094352.1734826-1-aalbersh@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/attr.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=2d2b2ed7bdcc6df4942292318c5455b5e94ba525

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

The original Linux kernel CVE announcement for CVE-2026-74596 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74596

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:L/PR:L/UI:N/S:U/C:N/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N
  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: 4
  Paranoid ActionableScore: 5

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 7.0

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).