* [CVE-2026-98091][MODERATE REGULAR] btrfs: detach failed sprout device from transaction update list [ Upstream
@ 2026-09-25 14:06 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-25 14:06 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98091
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: btrfs: detach failed sprout device from transaction update list [ Upstream
Commit: e9e7e37afa85db770e6084360b21a21a8b3a583f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e9e7e37afa85db770e6084360b21a21a8b3a583f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98091
Analysis date: Fri, 25 Sep 2026 10:06:10 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98091 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98091
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 REGULAR
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-2026-98091 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98091 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:H/I:H/A:H';*CWE-416;CWE-825;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '6.4';DESCR 'A use after free condition can occur in Btrfs device initialization when a failed sprout operation releases a device while its post_commit_list entry remains linked in the transaction dev_update_list. Later transaction processing can dereference the stale list entry and may cause a kernel crash or memory corruption. For the CVSS the PR:H because reliably triggering the condition normally requires CAP_SYS_ADMIN level control over Btrfs device management and a failure during system chunk creation. The issue is not network reachable. Impact is at least local denial of service through a kernel crash. In the worst case the stale reference may allow confidentiality or integrity impact through use after free memory corruption and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) YES DISK INIT UAF SKIP HARDWARE INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::
No floor or cap was applied.
2. Signal breakdown
* +2 Real UAF or active lifetime violation. The patch and commit establish that `device` is released while `device->post_commit_list` remains linked in the transaction `dev_update_list`. The transaction list therefore retains a reference into freed device storage.
* +1 Important filesystem/storage path. The stale object is a Btrfs device participating in transaction device-update handling during metadata and system chunk setup.
* +1 paranoid only Reliable kernel fault concern. A later traversal or manipulation of the transaction update list can consume the list node embedded in freed device memory and can plausibly produce a kernel fault or list corruption. A direct crash trace is not supplied, so this point is withheld from the conservative score.
Important non-awarded signals:
* Controlled reclaim or attacker-controlled replacement of the freed `struct btrfs_device` is not demonstrated.
* Arbitrary write, directed write, callback control, refcount takeover, confidentiality impact, integrity impact, and privilege escalation are not demonstrated.
* No concrete repeated release, put, free, or refcount mutation through the same dangling device reference is visible in the supplied patch.
* Structured stale-object leverage is not awarded. The stale object is structured, but the supplied evidence does not show that a later stale use consumes attacker-controlled device fields beyond the retained list linkage.
* Reliable kernel crash is not conservatively awarded because the supplied commit reports a warning and stale transaction-list reference, not an observed crash.
* Remote reachability is not established.
* No race penalty is applied. The described sequence is an ordered error-unwind path, not a demonstrated narrow timing race.
* No host-admin penalty is applied because the supplied context does not show the exact capability checks or prove that the complete trigger requires host-admin-equivalent authority.
3. Reachability analysis
The vulnerable sequence is local. It requires a Btrfs sprout or new-device initialization attempt in which creation of the first metadata chunk succeeds enough to link the device into `dev_update_list`, followed by failure while creating the system chunk.
Btrfs device-add and device-management operations are normally administrative operations and commonly involve `CAP_SYS_ADMIN` plus access to an appropriate block device. However, the supplied patch does not include the ioctl call path, capability checks, mount ownership conditions, device permissions, or namespace behavior. Therefore, host-admin-equivalent privilege is not positively established for the complete sequence.
Namespace-scoped reachability is also unresolved. A capability check that may be namespace-scoped would not by itself prove that an attacker can perform the full device-management and Btrfs sprout sequence against the affected filesystem and storage device from a namespace.
A preexisting Btrfs filesystem and a candidate device are operational prerequisites. The evidence does not establish whether an attacker can provision these independently, whether a privileged administrator must prepare them, or whether a delegated storage-management service can invoke the affected path.
4. Primitive and severity interpretation
The exact primitive is a real lifetime violation. A `struct btrfs_device` is freed while its embedded `post_commit_list` node remains linked in an active transaction update list. This creates a stale post-lifetime list entry that can later be discovered by transaction-list processing.
This is an active structured-object UAF, not fixed asynchronous stale storage. However, the supplied patch shows only the failure cleanup and the stale list membership. It does not show a concrete later traversal, controlled reuse, attacker-shaped replacement object, function-pointer or callback consumption, arbitrary write, or a second release/refcount operation on the freed device.
This is therefore CLASS_B. The demonstrated consequence is a stale transaction list referencing freed memory and a warning during device release. A later kernel fault or list corruption is technically plausible and is included only in the paranoid score. Privilege escalation or confidentiality and integrity impact remain speculative because no exploitation-relevant post-free object reuse or write primitive is established.
ImportantCandidate is NO because the available evidence supports a constrained error-path lifetime defect but does not establish meaningful exploitation leverage beyond stale-list consumption and possible crash behavior. AutoDowngradeAllowed is YES because no concrete repeated lifetime mutation, active attacker-controlled reuse, security-boundary bypass, or controlled corruption primitive remains unresolved. Manual review is still recommended to inspect transaction cleanup and post-commit list traversal call sites.
5. One-sentence report phrase
A failed Btrfs sprout device initialization can free a device while its post_commit_list entry remains linked in the transaction dev_update_list, creating a local use after free risk during later transaction list handling.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
The patch establishes a real stale reference to freed Btrfs device storage, but the supplied context does not show the subsequent transaction-list consumers needed to determine whether the outcome is limited to warning and crash behavior or can cause stronger list or memory corruption.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: btrfs: detach failed sprout device from transaction update list [ Upstream
Commit: e9e7e37afa85db770e6084360b21a21a8b3a583f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e9e7e37afa85db770e6084360b21a21a8b3a583f
Changed files:
fs/btrfs/volumes.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=e9e7e37afa85db770e6084360b21a21a8b3a583f
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98091 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98091
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:L/I:L/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 6.4
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: 3
Paranoid ActionableScore: 4
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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR
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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