* [CVE-2026-90260][MODERATE 7.0] btrfs: zoned: don't clobber the extent buffer when zeroing it out [ Upstream
@ 2026-09-18 12:46 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-18 12:46 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90260
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: zoned: don't clobber the extent buffer when zeroing it out [ Upstream
Commit: 92484ad014f2e2b0e0e7bbc1610951e978cf557c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=92484ad014f2e2b0e0e7bbc1610951e978cf557c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90260
Analysis date: Fri, 18 Sep 2026 08:46:43 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90260 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90260
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: YES
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-90260 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90260 CHECK WITH IMPACT FROM ORIG NN NONE 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-664;CWE-362;*CWE-20;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H';BEST CVSS score: '6.5';DESCR 'On a zoned Btrfs filesystem, a freed but still dirty tree block can be scheduled for a zero write only to advance the zone write pointer. The old path zeroed the folios owned by the live extent buffer, even though later code could still read its header. A later call to btrfs_free_tree_block can therefore observe a zero bytenr, create an invalid delayed reference, corrupt the extent tree, trigger an assertion or abort the filesystem. For the CVSS the PR:L reflects that an unprivileged local user with write access to an already mounted affected filesystem may be able to generate the required filesystem workload. Creating and mounting the zoned Btrfs filesystem still normally requires administrative setup. The issue is not directly network reachable. The demonstrated impact includes filesystem metadata corruption and denial of service. Limited confidentiality impact is retained only in the paranoid score because corrupted extent metadata could theoretically cause incorrect data references, but the supplied code does not establish a reliable disclosure primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) SKIP CVE-2026-90260 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE WRITE OOB DISK INIT SKIP KPANIC MEMORY KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 4
* Paranoid score: 5
* Applied conservative floor: 4 -> 5 because the patch establishes corruption of a live structured filesystem metadata object, subsequent consumption of its corrupted header, and extent-tree corruption with unresolved consequences beyond the observed abort.
* Applied paranoid floor: 4 -> 5 for the same concrete Class B primitive. No speculative confidentiality, arbitrary-write, or privilege-escalation points were added.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Positive signals:
* +1 Constrained kernel buffer overwrite. The kernel deliberately zeroes the folios of a still-referenced extent buffer. The payload is fixed zeros and no arbitrary target selection is established.
* +1 Weak exploitation concern from meaningful invalid structured-object interpretation. `btrfs_free_tree_block()` later consumes the destroyed header, interprets the zeroed bytenr, and creates an invalid delayed-reference operation against bytenr 0.
* +1 Reliable strong DoS. The established outcomes include an assertion, transaction abort, and filesystem shutdown.
* +1 Integrity impact. The commit explicitly establishes extent-tree corruption rather than a passive bookkeeping discrepancy.
* +1 Important filesystem path. The affected state belongs to Btrfs tree-block writeback and delayed-reference processing.
Negative signals:
* -1 Rare and difficult-to-reach configuration. The vulnerable path specifically requires a zoned Btrfs filesystem, which normally requires prior administrative provisioning and zoned storage or an equivalent setup.
Not awarded:
* Real UAF was not awarded. The extent buffer remains alive but its contents are destructively modified while references remain.
* Strong corruption +2 was not awarded because the payload is fixed zeros, the destination is the selected extent buffer itself, and arbitrary target or payload control is not demonstrated.
* Local unprivileged trigger +1 was not awarded because ordinary filesystem workload is a plausible trigger, but the supplied evidence does not establish that an unprivileged writer can complete every required operation.
* No race penalty was applied. Although reproduction is described as flaky, it occurs under repeatable fsstress workloads, and the evidence does not establish a narrow one-shot race outside attacker influence.
* Confidentiality was not awarded. Incorrect extent references could theoretically expose data, but no read-back or disclosure path is established.
* Arbitrary write, controlled reclaim, COW ownership bypass, cross-file modification, and privilege escalation are not demonstrated.
* No second Availability point was awarded because the filesystem abort and loss of availability are the same demonstrated consequence.
* No repeated release or refcount signal applies. There is no first lifetime-ending release followed by a second mutation through the same dangling reference.
* No remote-reachability point was awarded.
3. Reachability analysis
The affected system must have a zoned Btrfs filesystem mounted and active. Creating and mounting that filesystem normally requires a separate privileged administrator and suitable zoned storage or an equivalent configured device.
Once the filesystem exists, ordinary file creation, deletion, writeback, and tree-block churn may contribute to the vulnerable state. The fsstress reproductions indicate that the path is workload-driven and can be exercised repeatedly. However, the supplied material does not prove that an ordinary unprivileged user with write access can independently force the complete `EXTENT_BUFFER_ZONED_ZEROOUT` sequence. Therefore, `HostAdminRequired=UNKNOWN` rather than YES or NO.
No namespace capability is identified, and mounting or provisioning zoned Btrfs is not shown to be fully available inside an attacker-created namespace. Therefore, `NamespaceScopedPrivilege=UNKNOWN`.
There is no direct network trigger. A network service writing attacker-controlled files to the affected filesystem could provide an indirect workload path, but that is not established by this patch.
4. Primitive and severity interpretation
The exact primitive is destructive zeroing of the folios backing a live, structured Btrfs extent buffer while later kernel code can still consume its metadata header. This is an active-object consistency and ownership violation, not fixed stale storage and not a literal post-free UAF.
The later use is broader than an immediate invalid dereference. `btrfs_free_tree_block()` uses the corrupted header to derive a bytenr and perform delayed-reference processing, resulting in an invalid operation against bytenr 0 and corruption of the extent tree.
This is `CLASS_B` because a concrete active structured-object corruption primitive exists, but attacker-directed payload, arbitrary target selection, controlled reclaim, arbitrary kernel read or write, and privilege escalation are not established. The demonstrated practical consequence is filesystem corruption followed by assertion or abort.
The speculative worst case includes persistent metadata damage or incorrect extent ownership decisions. Confidentiality impact, cross-file data modification, and privilege escalation remain unproven and were not scored.
`ImportantCandidate=YES` because active filesystem metadata is corrupted and subsequently used to mutate the extent-reference state. The complete security consequences and unprivileged reachability require manual analysis.
`AutoDowngradeAllowed=NO` because automatically treating this as an ordinary crash would discard the established structured metadata-corruption primitive and its unresolved downstream effects.
5. One-sentence report phrase
A zoned Btrfs writeback path zeroed a still-referenced extent buffer, causing later delayed-reference processing to consume a destroyed header, corrupt the extent tree, and abort the filesystem.
6. Manual review recommendation
MANUAL CHECK REQUIRED
The issue establishes active filesystem metadata corruption beyond a simple assertion, while the extent of persistent damage, below-host-admin reachability, and possible cross-object effects remains unresolved.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: btrfs: zoned: don't clobber the extent buffer when zeroing it out [ Upstream
Commit: 92484ad014f2e2b0e0e7bbc1610951e978cf557c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=92484ad014f2e2b0e0e7bbc1610951e978cf557c
Changed files:
fs/btrfs/disk-io.c
fs/btrfs/extent_io.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=92484ad014f2e2b0e0e7bbc1610951e978cf557c
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90260 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90260
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:H
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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