[CVE-2026-93255][LOW] btrfs: make sure EXTENT_BUFFER_READING is cleared under refs_lock [ Upstream

[CVE-2026-93255][LOW] btrfs: make sure EXTENT_BUFFER_READING is cleared under refs_lock [ Upstream

[AL-KERNEL]

CVE: CVE-2026-93255
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: btrfs: make sure EXTENT_BUFFER_READING is cleared under refs_lock [ Upstream
Commit: 2073f3d97b0c5d34fe64812d1237e37bd79c17ec
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2073f3d97b0c5d34fe64812d1237e37bd79c17ec
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93255
Analysis date: Thu, 24 Sep 2026 18:29:44 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93255 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93255

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-2026-93255	LOW	https://www.kernelcve.org/list/?q=CVE-2026-93255 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:H/UI:N/S:U/C:N/I:N/A:L';CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:N';BEST CVSS score: '1.9';DESCR 'A race in Btrfs cleanup allowed invalidate_and_check_btree_folios to observe an extent buffer after EXTENT_BUFFER_READING was cleared but before the read reference was dropped. This inconsistent snapshot could trigger WARN_ON_ONCE and an unable to release extent buffer message during a failed mount or unmount. The available code indicates a false diagnostic alert rather than a use after free because the extra reference remains valid and is released by the read completion path. For the CVSS the PR:H value is used because mounting and unmounting the affected filesystem normally requires CAP_SYS_ADMIN. AC:H is used because the observation depends on a narrow race window between metadata read completion and Btrfs teardown. The issue is not network reachable. Confirmed confidentiality and integrity impact is none and confirmed availability impact is none. A low paranoid availability impact can be retained only to cover operational disruption from warnings or failed cleanup.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)  SKIP CVE-2026-93255 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	LOCK DISK INIT WARNONLY RACE SKIP HARDWARE 	-	-	checked

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

ActionableScore=0
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 0.
* Conservative score: 0.
* Raw paranoid score: 0.
* Paranoid score: 0.
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +0 False diagnostic warning. The inconsistent snapshot can trigger WARN_ON_ONCE and an unable to release extent buffer message, but WARN_ON continuation has no score without a fatal consequence.
* +0 No lifetime violation. The read path still owns a valid reference when teardown observes refs equal to 3, and it subsequently drops that reference exactly once.
* +0 No reliable DoS. The failed mount is caused by the missing seed filesystem and chunk-tree error, not by the warning or refcount observation.
* +0 No filesystem-integrity signal. The patch changes synchronization of a diagnostic refcount check and does not establish metadata corruption, filesystem shutdown, or cross-file ownership damage.
* The narrow race and host-admin mount requirement are concrete restrictions, but no negative points are needed because the raw score is already zero.
* Controlled reclaim, UAF, OOB access, arbitrary write, disclosure, repeated release, persistent leak, and authorization bypass are not demonstrated.
* Confidentiality, integrity, and availability bonuses are not awarded.
* No paranoid point is awarded merely for the theoretical possibility that a warning could disrupt an unusually configured system.

Repeated-release check:

* The read completion owns one legitimate reference.
* `free_extent_buffer_clear_reading()` drops that reference once.
* The same reference does not survive and reach a second lifetime-mutating operation.
* Concrete repeated release or refcount mutation: not established.
* Dedicated repeated-release bonus: not awarded.
* Paranoid repeated-refcount bonus: not awarded.
* UAF floor 4 or 5: not applicable.

3. Reachability analysis

The observed path requires mounting or tearing down a block-backed Btrfs filesystem. The supplied trace shows a UID 0 mount operation, and normal Btrfs block-device mounting requires host-admin-equivalent `CAP_SYS_ADMIN`. Btrfs does not provide an attacker-created user-namespace mount path for the complete operation, so the required privilege is not namespace scoped.

The race requires metadata read completion to clear `EXTENT_BUFFER_READING` while teardown waits for that flag and reads the reference count before the read reference is dropped. The window is narrow, but this affects only whether teardown emits a warning. No network trigger, special physical hardware, attacker-controlled replacement object, or prior delegation path is established.

4. Primitive and severity interpretation

The exact defect is a synchronization race between clearing a state flag and dropping the associated valid read reference. Teardown can observe a transient but legitimate reference count and report a false alert.

This is not a UAF. The extent buffer is protected by a successful `refcount_inc_not_zero()` during iteration, while the read completion also retains its own valid reference. The fix makes flag clearing and reference dropping atomic with respect to `refs_lock`, ensuring that the diagnostic reader receives a consistent snapshot.

There is no ordinary stale cleanup dereference and no second lifetime mutation through a dangling object. The old code does not bypass an object-retirement protocol, leave a secondary index stale, or permit an object to outlive its required protection.

Primitive classification is `NONE`. The demonstrated consequence is a warning during an already failing mount or teardown. No patch-supported non-crash exploitation leverage or security-boundary effect exists. `ImportantCandidate=NO` and `AutoDowngradeAllowed=YES` because no unresolved corruption or ownership primitive remains.

5. One-sentence report phrase

A narrow Btrfs teardown race could produce a false extent buffer reference warning, but the object remained valid and no UAF, memory corruption, or independent denial of service is established.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch corrects a false diagnostic observation without establishing a security primitive or independent availability impact.

7. Call-site confidence

Call-site confidence: high

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

Patch: btrfs: make sure EXTENT_BUFFER_READING is cleared under refs_lock [ Upstream
Commit: 2073f3d97b0c5d34fe64812d1237e37bd79c17ec
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2073f3d97b0c5d34fe64812d1237e37bd79c17ec

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=2073f3d97b0c5d34fe64812d1237e37bd79c17ec

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

The original Linux kernel CVE announcement for CVE-2026-93255 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93255

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:H/UI:N/S:U/C:N/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS score: 1.9

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: 0

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).