* [CVE-2026-90266][LOW] btrfs: zoned: don't force read-only on transient -EAGAIN from reloc merge [ Upstream
@ 2026-09-17 22:10 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-17 22:10 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90266
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: btrfs: zoned: don't force read-only on transient -EAGAIN from reloc merge [ Upstream
Commit: 8249dfe46337d599e2087b772a2e32ef9f77282b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8249dfe46337d599e2087b772a2e32ef9f77282b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90266
Analysis date: Thu, 17 Sep 2026 18:10:58 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
A transient metadata reservation failure during zoned Btrfs relocation can be misclassified as fatal and force the filesystem read-only, allowing a local writer to cause availability loss under specific reclaim and metadata-pressure conditions.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90266 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90266
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-2026-90266 LOW https://www.kernelcve.org/list/?q=CVE-2026-90266 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:N/I:N/A:H';CWE-703;CWE-755;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A zoned Btrfs filesystem can be forced into read only mode because a transient EAGAIN returned during relocation root merging is handled as a fatal filesystem error instead of a retryable reclaim condition. The trigger requires a zoned filesystem, block group reclaim or relocation, and metadata reservation pressure that exceeds the currently usable space after zone unusable bytes are considered. For the CVSS the PR:L reflects a local user or process with write access that can generate filesystem and metadata activity, although reliable triggering depends on filesystem state and storage configuration. The issue is not network reachable by itself, but it may be reached indirectly through a network service that permits an attacker to create or modify files on the affected filesystem. Impact is denial of service because the filesystem can become read only and reject further writes. There is no supported UAF, OOB access, information leak, or arbitrary write primitive, so confidentiality and integrity remain unaffected.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) SKIP CVE-2026-90266 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE LOCK DISK INIT SKIP INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
## 1. ActionableScore
* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::
## 2. Signal breakdown
Positive signals:
* +1 Local unprivileged trigger. A user with write access to an existing affected filesystem can generate metadata activity and contribute to the reservation pressure that reaches reclaim.
* +1 Reliable strong DoS. Misclassifying transient `-EAGAIN` as fatal can abort the transaction and force the filesystem read-only.
* +1 Important filesystem or storage path. The defect affects Btrfs relocation, metadata reservation, and block-group reclaim.
Negative signals:
* -1 Rare and difficult-to-instantiate configuration. The defect is specific to zoned Btrfs, which must already have been configured and mounted by an administrator. An ordinary attacker cannot create this prerequisite independently.
Not awarded:
* No memory-corruption or lifetime signal. The patch establishes no UAF, OOB access, stale pointer, double release, or invalid free.
* No confidentiality or integrity points. Forced read-only operation does not itself disclose data or permit unauthorized modification.
* No extra Availability point. The read-only transition is the same DoS already counted.
* No remote-reachability point. A network file service could expose the underlying write activity indirectly, but direct network triggering is not established.
* No resource-leak, ownership/COW, authorization-bypass, or privilege-escalation signal.
* The paranoid score is not raised merely because metadata pressure may be repeatable.
## 3. Reachability analysis
The evaluated attacker is an ordinary local user or process with permission to write to an already mounted zoned Btrfs filesystem. No capability is needed for ordinary filesystem writes, so host-admin-equivalent privilege is not required for the runtime trigger.
A host administrator must previously create and mount the zoned filesystem, but that setup is separate from the vulnerable reclaim operation. No namespace-scoped capability is relevant to the complete trigger.
The attacker cannot ordinarily instantiate the required zoned Btrfs configuration. No race or narrow timing window is established. Triggering instead depends on filesystem state, relocation or block-group reclaim, and sufficient metadata reservation pressure. Direct remote reachability is unsupported, although a writable network file service could provide an indirect path to the same filesystem workload.
## 4. Primitive and severity interpretation
The exact defect is incorrect error handling: a transient and retryable `-EAGAIN` from relocation-root merging is treated as a fatal filesystem error. The consequence is transaction abort and an unnecessary read-only transition.
There is no stale storage or active structured-object UAF. There is no repeated release or refcount mutation. The new `abort_reloc_roots()` logic performs orderly rollback for deferred relocation rather than repairing demonstrated memory corruption.
This is `CLASS_C` because the established behavior is availability loss through invalid error-state handling. The demonstrated consequence is filesystem write denial. Data corruption, disclosure, LPE, and kernel-memory exploitation are not supported.
`ImportantCandidate=NO` because no corruption, ownership violation, authorization bypass, or unresolved exploitation primitive exists. `AutoDowngradeAllowed=YES` because the impact is configuration-dependent DoS on a specialized filesystem mode, with no security primitive that would be lost through ordinary Moderate handling.
Likely operational severity is Moderate for affected zoned-storage deployments and low outside those deployments.
## 5. One-sentence report phrase
A transient metadata reservation failure during zoned Btrfs relocation can be misclassified as fatal and force the filesystem read-only, allowing a local writer to cause availability loss under specific reclaim and metadata-pressure conditions.
## 6. Manual review recommendation
NO MANUAL CHECK NEEDED
The patch establishes a configuration-dependent filesystem DoS without memory corruption, persistent ownership violation, authorization bypass, or privilege-escalation leverage.
## 7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: btrfs: zoned: don't force read-only on transient -EAGAIN from reloc merge [ Upstream
Commit: 8249dfe46337d599e2087b772a2e32ef9f77282b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8249dfe46337d599e2087b772a2e32ef9f77282b
Changed files:
fs/btrfs/block-group.c
fs/btrfs/relocation.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=8249dfe46337d599e2087b772a2e32ef9f77282b
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90266 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90266
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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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