[CVE-2026-90263][MODERATE 7.0] btrfs: check if root is readonly when setting posix acl [ Upstream

[CVE-2026-90263][MODERATE 7.0] btrfs: check if root is readonly when setting posix acl [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90263
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: check if root is readonly when setting posix acl [ Upstream
Commit: d5602f96ff1e266f069b4d0d94f441050e057e06
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d5602f96ff1e266f069b4d0d94f441050e057e06
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90263
Analysis date: Thu, 17 Sep 2026 16:42:46 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90263 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90263

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.

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

CVE-2026-90263	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90263 CHECK WITH IMPACT FROM ORIG NN NONE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N';CWE-284;CWE-862;CWE-732;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '4.4';DESCR 'Btrfs POSIX ACL updates could bypass the read only property of a Btrfs root or subvolume because btrfs_set_acl did not check whether the root was readonly before changing ACL metadata. A local user who is otherwise allowed to set ACLs on an inode could modify access control metadata even though write operations on that readonly root should be rejected. For the CVSS the PR:L is selected because triggering requires a local user or local process with permission to attempt ACL changes, but not full administrative privileges in the vulnerable path. The issue is not network reachable and does not indicate memory corruption or kernel code execution. Impact is mainly a local integrity policy bypass. In the paranoid interpretation, confidentiality is limited because changed ACLs may affect who can read files, but there is no evidence of direct privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5)	YES	DISK SIMPLEFIX SKIP IMPROVEONLY  KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	YES	NO	checked

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

ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=RECOMMENDED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate**::

No floor or cap was applied.

2. Signal breakdown

Positive signals:

* +2 Concrete authorization decision bypass. A Btrfs root with the read-only property should deny write-like metadata updates, but the set ACL path missed the readonly-root check and allowed ACL modification.
* +1 Integrity plausible. The attacker can modify POSIX ACL metadata despite a filesystem policy that should reject the operation with EROFS.
* +1 Local below-host-admin trigger. A local user or process that is otherwise allowed to set ACLs on an inode can reach the vulnerable path without host-admin-equivalent privilege.
* +1 Important filesystem metadata path. The affected operation changes Btrfs access-control metadata on a root or subvolume where write operations are expected to be blocked.

Non-awarded signals:

* +0 RealUAF. No use-after-free, stale reference, double release, or refcount lifetime mutation is established.
* +0 Memory corruption. No OOB access, arbitrary write, type confusion, or kernel object corruption is shown.
* +0 Confidentiality. ACL changes can weaken access policy, but the supplied evidence does not establish a concrete protected-data read by the attacker.
* +0 Availability. No crash, hang, filesystem shutdown, or resource exhaustion is indicated.
* +0 Remote reachability. The issue is triggered through local filesystem metadata operations, not network traffic.
* +0 Privileged FD or resource theft. No downstream protected FD, credential, or root escalation chain is supplied.
* +0 Practical exploit chain beyond policy bypass. The established consequence is ACL modification on a readonly Btrfs root, not demonstrated LPE.

Negative signals:

* No negative privilege penalty is applied. Host-admin-equivalent privilege is not positively established for the complete vulnerable ACL operation.
* No rare-subsystem penalty is applied. Btrfs is optional but common enough that rarity is not a concrete reducing signal here.

3. Reachability analysis

The vulnerable operation is local. The attacker must be able to invoke POSIX ACL modification on a Btrfs inode. In normal VFS ACL semantics this is available to a file owner or a process with suitable local permissions, while the missing Btrfs-specific readonly-root check is the policy gate that should have rejected the operation.

HostAdminRequired=NO because the complete vulnerable operation does not require host root when the attacker is otherwise permitted to set ACLs on the target inode. NamespaceScopedPrivilege=NO because this is not a namespace capability issue. A prior administrator may have configured the Btrfs root or subvolume as read-only, but that is the protected state being bypassed rather than a separate attacker privilege requirement.

No network reachability, special hardware, race timing, sysctl, boot parameter, or device prerequisite is established.

4. Primitive and severity interpretation

The exact primitive is a local filesystem policy bypass: POSIX ACL metadata can be changed even though the Btrfs root has its read-only property set. This is not memory corruption and not a lifetime bug.

PrimitiveClass=CLASS_A because the patch restores a concrete mandatory authorization condition: readonly Btrfs roots should reject write-like ACL operations, and the vulnerable path incorrectly allowed them. The demonstrated consequence is unauthorized access-control metadata modification. The speculative consequence is broader local policy weakening if ACL changes expose or alter access to files in readonly snapshots or subvolumes, but supplied evidence does not show direct privilege escalation.

ImportantCandidate=YES follows from the concrete authorization boundary bypass, but the numeric score remains Actionable Moderate because the impact is limited to ACL metadata and no LPE, protected FD theft, arbitrary write, or memory corruption is established.

AutoDowngradeAllowed=NO because automatically closing a concrete filesystem authorization bypass could miss environments that rely on readonly Btrfs roots or snapshots as a security or integrity boundary.

Likely operational severity: actionable Moderate, generally not urgent as a memory-corruption issue, but not suitable for automatic downgrade without considering local Btrfs readonly-subvolume security assumptions.

5. One-sentence report phrase

A local Btrfs user who is otherwise allowed to set POSIX ACLs on an inode could modify ACL metadata on a readonly Btrfs root or subvolume because btrfs_set_acl missed the readonly-root check, resulting in a local integrity policy bypass.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is not memory corruption and has no demonstrated LPE, but it is a concrete filesystem authorization-policy bypass affecting readonly Btrfs roots or subvolumes.

7. Call-site confidence

Call-site confidence: high

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

Patch: btrfs: check if root is readonly when setting posix acl [ Upstream
Commit: d5602f96ff1e266f069b4d0d94f441050e057e06
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d5602f96ff1e266f069b4d0d94f441050e057e06

Changed files:
  fs/btrfs/acl.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=d5602f96ff1e266f069b4d0d94f441050e057e06

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

The original Linux kernel CVE announcement for CVE-2026-90263 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90263

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:L/I:L/A:N
  The Best / paranoid CVSS score: 4.4

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: 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: YES
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).