* [CVE-2026-93813][MODERATE 7.0] btrfs: tree-checker: validate INODE_REF's namelen [ Upstream
@ 2026-09-25 1:13 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-25 1:13 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-93813
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: tree-checker: validate INODE_REF's namelen [ Upstream
Commit: eeddee090b834a7d4b6a8214d2e3739e64c7a338
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=eeddee090b834a7d4b6a8214d2e3739e64c7a338
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93813
Analysis date: Thu, 24 Sep 2026 21:13:40 -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-2026-93813 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93813
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-2026-93813 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-93813 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:R/S:U/C:H/I:H/A:H';*CWE-787;*CWE-20;CWE-125;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.3';DESCR 'A crafted BTRFS image can contain an INODE_REF item with a namelen value larger than BTRFS_NAME_LEN while the item size remains structurally valid. Later name processing can copy the attacker controlled length into a buffer sized for a normal BTRFS name, resulting in an out of bounds write in kernel memory and a demonstrated kernel panic. For the CVSS the PR:L rating in the paranoid vector assumes a local user can supply a crafted filesystem image through removable media, a loop backed image, an automount path, or another trusted filesystem processing service. UI:R is used because a privileged user or service normally needs to mount or process that image. Impact is at least local denial of service through a kernel crash. The out of bounds write provides a plausible memory corruption path to confidentiality and integrity impact, so manual review is required. The issue is not directly network reachable.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) YES OOB DANGER DISK SKIP KERNEL_PANIC_PLUS_UAF HARDWARE KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK NO NO checked
======================================================================
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 floor: 4 -> 5. A concrete attacker-controlled kernel out of bounds write is a strong corruption primitive and should not be automatically closed below Actionable Moderate solely because the mounting and image-processing privilege boundary is unresolved.
* Applied paranoid floor: 4 -> 5. No additional paranoid bonus is awarded because controlled reclaim, arbitrary write target selection, disclosure, and privilege escalation are not established.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 Strong corruption primitive. The crafted INODE_REF namelen controls a copy length exceeding BTRFS_NAME_LEN while the destination name buffer is stated to be 255 bytes. The resulting read_extent_buffer and memcpy operation writes beyond that kernel buffer. The source data originates from attacker-controlled filesystem metadata.
* +1 Reliable kernel crash or strong DoS. The supplied reproducer demonstrates a supervisor-mode write page fault in memcpy followed by kernel panic.
* +1 Important filesystem path. The flaw affects BTRFS metadata validation and later inode-name processing during exportfs file-handle decoding. This is active filesystem metadata processing, not diagnostic-only parsing.
* +0 Local unprivileged trigger not awarded. The supplied evidence proves that a crafted image causes the fault when processed, but does not prove that an ordinary unprivileged user can independently mount the image and reach exportfs_decode_fh_raw without a privileged mount, automount service, or pre-existing filesystem setup.
* +0 Remote reachability not awarded. A remote NFS client may potentially reach exportfs processing only after a privileged actor has mounted and exported the malicious BTRFS image. The complete remote attacker path is not established.
* +0 Controlled reclaim or controlled replacement not demonstrated. The overflow destination is adjacent kernel memory after a fixed-size name allocation, but no controlled target object replacement or allocator placement is supplied.
* +0 Arbitrary or directed kernel write not demonstrated. The write length and payload are attacker-influenced, but the exact adjacent target is not attacker-selected in the supplied evidence.
* +0 Confidentiality and integrity consequence not separately awarded. Kernel memory corruption is concrete, but no read-back path, protected-object overwrite, credential modification, policy bypass, or demonstrated privilege escalation is supplied.
* +0 Extra availability point not awarded. The demonstrated kernel panic already accounts for the DoS consequence.
* No negative privilege penalty applied. Host-admin-equivalent privilege may be needed in common deployments to mount or export the crafted filesystem, but that requirement is not positively established as mandatory for every complete trigger path.
3. Reachability analysis
The immediate malicious input is a crafted local BTRFS filesystem image containing an oversized INODE_REF name length. The supplied stack trace shows that the malformed metadata reaches btrfs_get_name through reconnect_path and exportfs_decode_fh_raw after the filesystem is already being processed by the kernel.
A host administrator commonly mounts and exports BTRFS filesystems, but the complete privilege boundary is unresolved from the supplied material. Possible deployment paths include direct privileged mounting, an existing privileged service processing an attacker-supplied image, or an automount workflow. These possibilities are sufficient to require investigation, but do not prove an ordinary-user complete trigger.
The vulnerability is not directly network reachable from the patch alone. If a malicious BTRFS image is already mounted and exported through NFS, a network client could possibly cause the exportfs lookup path to run. However, that would depend on a separate privileged image-mount and export setup step.
No namespace-scoped capability path is established. No special hardware, boot option, race condition, or unusual timing is required. The core prerequisite is processing attacker-controlled BTRFS metadata.
4. Primitive and severity interpretation
The exact primitive is an attacker-influenced out of bounds write into kernel memory. The INODE_REF item passes the existing item-boundary validation because itemsize matches the malicious namelen, but namelen exceeds the semantic BTRFS name maximum of 255 bytes. Later code copies that oversized name into a buffer intended for a normal BTRFS name.
This is not a UAF, stale-object bug, refcount bug, or cleanup-only crash. It is a direct bounds-validation failure that permits an oversized copy from filesystem metadata into a smaller kernel buffer. The observed consequence is a kernel panic, but the write primitive is stronger than an invalid pointer dereference because attacker-controlled metadata influences both the copied bytes and the copy length.
PrimitiveClass is CLASS_A because a concrete attacker-influenced out of bounds kernel write is established. Controlled reclaim, precise overwrite targeting, arbitrary kernel read or write, and privilege escalation are not established. Therefore the evidence supports mandatory manual review rather than a claim of demonstrated local privilege escalation.
ImportantCandidate is YES because the patch establishes real kernel memory corruption rather than only malformed-input rejection or crash-only state handling. AutoDowngradeAllowed is NO because automatically treating the issue as an ordinary filesystem crash would discard the unresolved exploitation relevance of the confirmed out of bounds write.
Likely operational severity is Actionable Moderate at minimum. It is a Strong Important candidate if further review confirms a low-privilege image-processing path, a useful adjacent allocation target, controlled heap layout, or a practical NFS or automount delivery route.
5. One-sentence report phrase
A crafted BTRFS image can use an oversized INODE_REF name length to trigger an attacker-influenced kernel out of bounds write and kernel panic during filesystem metadata processing, requiring manual review for possible exploitation beyond denial of service.
6. Manual review recommendation
MANUAL CHECK REQUIRED
A concrete attacker-controlled out of bounds write in BTRFS metadata processing is established, while the exact mount privilege boundary, adjacent overwrite target, and possible escalation path remain unresolved.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: btrfs: tree-checker: validate INODE_REF's namelen [ Upstream
Commit: eeddee090b834a7d4b6a8214d2e3739e64c7a338
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=eeddee090b834a7d4b6a8214d2e3739e64c7a338
Changed files:
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=eeddee090b834a7d4b6a8214d2e3739e64c7a338
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-93813 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93813
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:H/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7.3
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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