* [CVE-2026-98245][MODERATE 7.0] btrfs: take commit root semaphore when iterating in mark_block_group_to_copy() [ Upstream
@ 2026-10-11 6:13 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-10-11 6:13 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98245
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: take commit root semaphore when iterating in mark_block_group_to_copy() [ Upstream
Commit: 8e2007b263ea250251ec3021a0a197da990ee1a8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8e2007b263ea250251ec3021a0a197da990ee1a8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98245
Analysis date: Sun, 11 Oct 2026 02:13:17 -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-2026-98245 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98245
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-2026-98245 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98245 CHECK WITH IMPACT FROM ORIG NN NONE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC) 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-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in Btrfs zoned device replacement allows mark_block_group_to_copy() to iterate the commit root without holding the commit root semaphore. A concurrent transaction commit can replace and free that root while the search path still references its extent buffers, creating a use-after-free dereference that can crash the kernel and may expose a memory corruption path if the freed storage is reused. For the CVSS the PR:L value is used for the paranoid score because a local user may be able to generate filesystem activity and transaction commits while an administrator initiated device replacement is active, although initiating the replacement normally requires administrative privileges. The issue is not network reachable. Impact is at least a local denial of service via kernel crash and in the worst case may include confidentiality or integrity impact due to the use-after-free condition.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) MAYBE LOCK DISK UAF SKIP HARDWARE KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 4
* Paranoid score: 5
* Applied conservative floor: 4 -> 5 under REAL UAF FLOOR 5 because freed extent buffers remain subject to active B-tree traversal rather than a shallow cleanup dereference.
* Applied paranoid floor: 4 -> 5 for the same established active structured UAF. No speculative exploitation bonus was added.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 Real UAF. A transaction commit can replace and free the commit root while `mark_block_group_to_copy()` continues accessing its extent buffers.
* +1 Reliable kernel crash or strong DoS. Traversal of freed extent buffers can realistically produce an invalid kernel-memory access, although the commit does not document a specific panic trace.
* +1 Important filesystem and storage path. The stale objects belong to the Btrfs commit-root extent tree and are consumed while selecting block groups for zoned device replacement.
* 0 Remote reachability. No network trigger is established.
* 0 Structured stale-object leverage. The stale extent buffer is actively traversed, but attacker-controlled reclaim, replacement contents, pointers, or offsets are not established.
* 0 Strong or weak LPE leverage. UAF alone does not demonstrate arbitrary write, controlled reclaim, callback control, disclosure, or privilege escalation.
* 0 Race penalty. The patch establishes concurrency with transaction commit but does not show that the window is narrow, one-shot, or externally uncontrollable.
* 0 Privilege penalty. Device replacement needs prior privileged initiation, but the evidence does not resolve whether a lower-privileged writer can complete the trigger by inducing concurrent transactions after that setup.
* No extra availability point was awarded because it would duplicate the same potential kernel crash.
* No confidentiality or integrity points were awarded because post-free disclosure or modification is not demonstrated.
* No concrete repeated release or refcount operation exists. There is a stale traversal after free, not a second `put`, `free`, or refcount mutation.
* FLOOR 5 was applied because this is an active structured UAF in nontrivial filesystem-tree processing, not cleanup-only stale storage.
* NEW52 B5-R does not apply. There is no retry-accumulated heap OOB write.
* NEW53 B3-PENDING does not apply. There is no shared-backing write.
* NEW54 B4-PENDING does not apply. No authorization decision is changed from DENY to ALLOW.
3. Reachability analysis
The vulnerable search is entered during Btrfs zoned device replacement. Starting device replacement normally requires host-level filesystem and device-management privilege, creating a genuine prior privileged setup requirement.
A process with write access to the mounted filesystem may generate transactions while an administrator is starting replacement. The supplied evidence does not prove that such a process can reliably coordinate the complete race without controlling the privileged replacement operation. Therefore `HostAdminRequired=UNKNOWN`, and no local-unprivileged bonus or host-admin penalty is applied.
The complete device-replacement operation is not available through an attacker-created user namespace alone, so `NamespaceScopedPrivilege=NO`. No network path, special external hardware input, or remote trigger is established. Zoned Btrfs and active device replacement are prerequisites, but specialization alone is not scored as a rarity penalty.
4. Primitive and severity interpretation
The exact primitive is a real post-lifetime dereference of freed Btrfs extent buffers. The transaction commit can swap and free the commit root while `mark_block_group_to_copy()` retains a search path into the old root.
The affected extent buffer is active structured storage used by B-tree search logic, not a fixed asynchronous buffer and not a cleanup-only pointer. Continued traversal may consume tree metadata from freed storage. However, the supplied patch does not establish controlled slab reclaim, attacker-shaped replacement data, arbitrary kernel read or write, callback control, or a completed privilege-escalation chain.
This is therefore CLASS_B rather than CLASS_A. The demonstrated practical consequence is kernel crash or filesystem-operation failure. Stronger memory-corruption consequences remain unresolved rather than established.
`ImportantCandidate=YES` and `AutoDowngradeAllowed=NO` are appropriate because an active filesystem-tree UAF would lose meaningful lifetime and structured-processing risk if automatically reduced to an ordinary crash-only issue. The likely operational severity is Actionable Moderate, with Important classification requiring manual call-path and allocator-reuse analysis.
5. One-sentence report phrase
A missing commit-root semaphore allows Btrfs device replacement to traverse freed extent buffers during a concurrent transaction commit, creating an active structured use-after-free with kernel-crash impact and unresolved post-free exploitation leverage.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Review the complete device-replacement call path, privilege split, stale extent-buffer accesses, allocator reuse possibilities, and whether unprivileged filesystem activity can reliably drive transaction commits during the vulnerable search.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: btrfs: take commit root semaphore when iterating in mark_block_group_to_copy() [ Upstream
Commit: 8e2007b263ea250251ec3021a0a197da990ee1a8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8e2007b263ea250251ec3021a0a197da990ee1a8
Changed files:
fs/btrfs/dev-replace.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=8e2007b263ea250251ec3021a0a197da990ee1a8
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98245 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98245
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: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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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