[CVE-2026-97415][MODERATE 7.0] btrfs: tree-checker: validate names in ROOT_REF and ROOT_BACKREF [ Upstream

[CVE-2026-97415][MODERATE 7.0] btrfs: tree-checker: validate names in ROOT_REF and ROOT_BACKREF [ Upstream

[AL-KERNEL]

CVE: CVE-2026-97415
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: tree-checker: validate names in ROOT_REF and ROOT_BACKREF [ Upstream
Commit: 9154542070ca7eadb3c764a882f39a127677854b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9154542070ca7eadb3c764a882f39a127677854b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97415
Analysis date: Thu, 24 Sep 2026 12:51:22 -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-2026-97415 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97415

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-2026-97415	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97415 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:H/I:H/A:H';*CWE-787;*CWE-20;CWE-125;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A malformed Btrfs ROOT_REF or ROOT_BACKREF item can provide an invalid on disk name length that is trusted by BTRFS_IOC_GET_SUBVOL_INFO. The ioctl can then copy more bytes than fit in its fixed size name buffer, causing an out-of-bounds kernel memory write and potentially a kernel crash or memory corruption. The attack is local and requires a crafted or corrupted Btrfs filesystem to be mounted and processed by the target. For the CVSS the PR:L value is used for the paranoid score because a local user may access an already mounted untrusted Btrfs filesystem and invoke the ioctl, although preparing and mounting a malicious filesystem is commonly restricted. The issue is not directly network reachable. Impact is at least local denial of service and may include confidentiality integrity and privilege escalation consequences if the bounded overwrite can be made reliably controllable. This issue should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	OOB DISK SKIP  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	NO	NO	guess

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

ActionableScore=5
PrimitiveClass=CLASS_A
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 and paranoid floor: 4 -> 5 because the patch establishes an attacker-influenced kernel out-of-bounds write into a fixed-size UAPI buffer.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Strong corruption primitive. `BTRFS_IOC_GET_SUBVOL_INFO` copies `item_len`, derived from corrupted on-disk item metadata, into the fixed-size `subvol_info->name` buffer. The source bytes and oversized item layout are attacker-influenced through crafted Btrfs metadata.
* +1 Reliable kernel crash or strong DoS. A kernel heap buffer overflow can plausibly corrupt adjacent memory or trigger an immediate fault.
* +1 Important filesystem/storage path. The issue affects Btrfs on-disk metadata validation and a Btrfs ioctl path processing subvolume metadata.

Important non-awarded signals:

* Controlled reclaim, target selection, and replacement of the adjacent overwritten kernel object are not demonstrated.
* Arbitrary or directed kernel write is not demonstrated. The destination begins at a fixed field in the ioctl result allocation.
* Confidentiality impact is not awarded because no kernel-memory disclosure path to userspace is established.
* Integrity impact against a protected external object is not separately awarded because the patch does not demonstrate corruption of another users data, filesystem contents, credentials, or policy state.
* Privilege escalation is not awarded as an established consequence. It remains a possible consequence of a controllable kernel heap overflow and is the reason manual review is required.
* Remote reachability is not established.
* No race or timing penalty applies. This is malformed metadata processing, not a timing-dependent race.
* No host-admin penalty applies because mounting or accessing the crafted filesystem may require differing privileges depending on the deployment, and the complete privilege boundary is not established.

3. Reachability analysis

The triggering input is a crafted or corrupted Btrfs filesystem containing an invalid `ROOT_REF` or `ROOT_BACKREF` item whose item size does not match the embedded name length. The vulnerable copy occurs when a process invokes `BTRFS_IOC_GET_SUBVOL_INFO` on a mounted filesystem and the relevant root-backreference metadata is read.

The attack is local. It is not directly network reachable. No special hardware, boot option, or race condition is required.

The complete attacker privilege model is unresolved. Creating and mounting an attacker-controlled Btrfs block-device image is commonly an administrative operation, but an attacker may also interact with a filesystem that was already mounted by another actor, including removable-media or storage-processing workflows. The supplied patch does not show the ioctl permission checks or the mount policy. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.

Namespace-scoped reachability is also unresolved. The supplied material does not establish that an attacker can mount the required Btrfs filesystem and complete the ioctl trigger entirely from an attacker-controlled user namespace.

4. Primitive and severity interpretation

The exact primitive is an attacker-influenced out-of-bounds kernel write. Before the patch, `btrfs_ioctl_get_subvol_info()` computed the copy length from the on-disk item size rather than from the validated `btrfs_root_ref` name length. A malformed item can therefore cause `read_extent_buffer()` to write beyond the fixed-size `subvol_info->name` destination.

This is not a UAF, refcount issue, race, or cleanup-only stale-pointer case. It is a direct memory-layout validation failure involving an on-disk metadata length and a fixed-size kernel destination buffer.

The overwrite target starts at a known fixed buffer, but the patch does not establish attacker control over the adjacent heap-object type, allocator placement, callback fields, or an arbitrary destination. Thus the issue is CLASS_A because an attacker-influenced kernel buffer overflow is directly established, but strong exploitation leverage beyond the overwrite itself is not established.

The demonstrated practical consequence is local kernel memory corruption and likely kernel crash or DoS. Privilege escalation, confidentiality impact, and integrity impact are plausible but speculative because no controlled reclaim, protected-object overwrite, or disclosure chain is present in the supplied evidence.

ImportantCandidate is YES because direct attacker-influenced kernel memory corruption should not be automatically treated as an ordinary malformed-filesystem handling error. AutoDowngradeAllowed is NO because closing this issue automatically would discard a concrete heap-overwrite primitive whose exploitation properties require manual allocator and call-path review.

Likely operational severity is Actionable Moderate at minimum. It is a strong Important candidate if manual review confirms that a below-host-admin attacker can cause mounting or processing of crafted Btrfs metadata, or that the heap overwrite can reliably affect useful adjacent kernel objects.

5. One-sentence report phrase

A crafted Btrfs ROOT_REF or ROOT_BACKREF item can cause BTRFS_IOC_GET_SUBVOL_INFO to copy an oversized metadata payload into a fixed-size kernel name buffer, resulting in local kernel memory corruption and potential denial of service.

6. Manual review recommendation

MANUAL CHECK REQUIRED

A direct attacker-influenced kernel out-of-bounds write is established, but the complete mounting privilege model, adjacent allocation layout, overwrite controllability, and possible privilege-escalation consequences require manual review.

7. Call-site confidence

Call-site confidence: medium

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

Patch: btrfs: tree-checker: validate names in ROOT_REF and ROOT_BACKREF [ Upstream
Commit: 9154542070ca7eadb3c764a882f39a127677854b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9154542070ca7eadb3c764a882f39a127677854b

Changed files:
  fs/btrfs/ioctl.c
  fs/btrfs/tree-checker.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=9154542070ca7eadb3c764a882f39a127677854b

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

The original Linux kernel CVE announcement for CVE-2026-97415 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97415

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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).