* [CVE-2026-100073][MODERATE REGULAR] ext4: fix transaction overflow during writeback [ Upstream
@ 2026-09-25 14:27 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-25 14:27 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-100073
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ext4: fix transaction overflow during writeback [ Upstream
Commit: a22464c5df5827e884a5a7d3d8fb03870e5bb6c8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a22464c5df5827e884a5a7d3d8fb03870e5bb6c8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100073
Analysis date: Fri, 25 Sep 2026 10:27:03 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
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-2026-100073 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100073
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-2026-100073 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-100073 CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H';CWE-682;**CWE-400;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'An ext4 journal transaction can run out of reserved credits during writeback because unwritten extent conversion may split a large extent into many smaller written extents and may allocate additional extent tree nodes. The issue requires a writable ext4 filesystem using the affected writeback path together with a suitable unwritten extent layout and sparse dirtying of a large folio, so reliable triggering depends on nontrivial filesystem state and timing. For the CVSS the PR:L value is used because a local user with permission to write files on the affected filesystem may be able to create the required workload without administrative control. The issue is not network reachable by itself, although a remote file service could indirectly expose the vulnerable filesystem write path. Impact is primarily denial of service through transaction failure, journal abort, failed writeback, or a filesystem becoming read only. The paranoid score includes limited integrity impact because interrupted writeback may prevent expected file or metadata updates, but there is no demonstrated memory corruption or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) SKIP CVE-2026-100073 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DISK BPF SIMPLEFIX LINUS INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
Positive signals:
* +1 Local unprivileged trigger. A user with write access to a file on an already mounted affected ext4 filesystem can plausibly create unwritten extents and sparse dirtying that reaches this writeback path.
* +1 Reliable strong DoS. Once the corner-case extent layout exists, insufficient journal credits can cause transaction failure, journal abort, failed writeback, or filesystem shutdown/read-only operation.
* +1 Important filesystem/storage path. The defect affects ext4 writeback, journal credit reservation, extent conversion, and possible extent-tree growth.
Negative signals:
* No negative points. The nontrivial filesystem state is not a race and does not establish materially uncontrollable timing.
* No host-admin penalty. Mount configuration may require prior administrative setup, but the subsequent vulnerable writeback can be caused by an ordinary user with file-write permission.
* No rare-subsystem penalty. ext4 is broadly deployed, although the precise large-folio and unwritten-extent conditions are narrower.
Material non-awarded signals:
* No filesystem metadata-corruption point. The patch establishes insufficient transaction credits, not persistent on-disk corruption or cross-file modification.
* No integrity point. Failed or incomplete expected writes do not by themselves establish unauthorized modification of protected data.
* No confidentiality point, UAF, OOB access, arbitrary write, controlled corruption, ownership violation, or privilege-escalation primitive.
* No additional Availability point. Filesystem shutdown or read-only operation is part of the same transaction-failure consequence already counted as strong DoS.
* No remote-reachability point. A network file service might indirectly expose writes to ext4, but direct network triggering and the complete remote conditions are not established.
3. Reachability analysis
An ordinary local user can plausibly reach the vulnerable operation by writing and sparsely dirtying a suitable file on an affected writable ext4 filesystem. The affected branch additionally requires `ext4_should_dioread_nolock()` and a layout where conversion of unwritten extents may split extents and allocate new extent-tree nodes.
Administrative action is required to mount and configure the filesystem, but it is prior environmental setup rather than a privilege required for each trigger. No capability or namespace operation is needed once the attacker has ordinary write access. The trigger is not namespace-specific, requires no special hardware, and is not a timing race. It depends on a corner-case extent and folio layout, but that alone does not justify a race or rarity penalty.
4. Primitive and severity interpretation
The exact defect is an underestimated upper bound for ext4 journal transaction credits. The previous calculation accounted for converting blocks represented by a large folio but did not fully account for extent splitting and allocation of additional extent-tree nodes.
There is no stale object, post-lifetime access, repeated release, memory overwrite, or authorization bypass. The appropriate classification is CLASS_C because the established behavior is transaction exhaustion and filesystem availability failure without an independently useful corruption primitive.
The demonstrated practical consequence is writeback or journal failure. Persistent metadata corruption, cross-file modification, information disclosure, and privilege escalation remain unsupported. ImportantCandidate is therefore NO, and automatic downgrade is allowed because no unresolved Class A or Class B primitive would be discarded. Operational severity is ordinary to borderline Moderate, with manual review useful for confirming the exact JBD2 failure behavior.
5. One-sentence report phrase
Insufficient ext4 journal credit reservation during sparse large-folio writeback can overflow the transaction when unwritten extent conversion requires extent splitting and new tree nodes, potentially causing journal failure or filesystem denial of service for a local user with ordinary file-write access.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Confirm whether credit exhaustion on affected kernel versions only fails writeback or can deterministically abort the journal and force the entire filesystem into read-only or shutdown state.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: ext4: fix transaction overflow during writeback [ Upstream
Commit: a22464c5df5827e884a5a7d3d8fb03870e5bb6c8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a22464c5df5827e884a5a7d3d8fb03870e5bb6c8
Changed files:
fs/ext4/inode.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=a22464c5df5827e884a5a7d3d8fb03870e5bb6c8
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-100073 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100073
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:N/I:N/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H
The Best / paranoid CVSS score: 5.3
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: 3
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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