[CVE-2026-99174][MODERATE 7.0] ext4: free the sbi after a grace period [ Upstream

[CVE-2026-99174][MODERATE 7.0] ext4: free the sbi after a grace period [ Upstream

[AL-KERNEL]

CVE: CVE-2026-99174
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ext4: free the sbi after a grace period [ Upstream
Commit: 80d5db1686b05fad6f384be017714297d828a714
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=80d5db1686b05fad6f384be017714297d828a714
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99174
Analysis date: Wed, 28 Oct 2026 22:31:40 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
An ext4 RCU pathwalk can outlive superblock teardown and access or update freed sbi/percpu-counter state, causing a demonstrated kernel fault and an unresolved post-lifetime corruption risk.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-99174 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99174

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-2026-99174	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99174 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=5 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; 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-672;**CWE-476;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race between ext4 RCU pathwalk and superblock teardown can leave ext4_get_link accessing sb s_fs_info after ext4_put_super has destroyed the extent status counters or freed the ext4 sbi. The demonstrated outcomes include a NULL pointer dereference and use after free. Updating a destroyed percpu counter may also access freed storage and creates a plausible kernel memory corruption risk, although reliable privilege escalation is not demonstrated and would require control over race timing and memory reuse. For the CVSS the PR:L value is selected because a local user or process must perform pathwalk operations and participate in the mount reference lifetime. AC:H reflects the narrow race involving the final reference to a lazily unmounted ext4 mount. The issue is not directly network reachable. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and LOWERED_FROM_HIGH_BASED_ON_NULLPTR. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 5 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	NULLPTR DISK INIT RACE LEAK UAF LINUS  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC LOWERED_FROM_HIGH_BASED_ON_NULLPTR	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 4.
* Conservative score: 5. Applied the active structured RCU-lifetime calibration floor, 4 -> 5.
* Raw paranoid score: 5.
* Paranoid score: 5. No additional floor or cap.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF: `ext4_put_super()` can free `ext4_sb_info` while an RCU pathwalker still accesses it through `sb->s_fs_info`.
* +1 Concrete RCU referenced-object lifetime mismatch: the superblock state remains available to a legitimate RCU reader, but the referenced sbi and counters were destroyed before the grace period completed.
* +1 Reliable kernel crash: the supplied trace demonstrates a NULL dereference; freed-sbi or destroyed-percpu-counter access also plausibly faults.
* +1 Important filesystem path: the stale state is consumed during ext4 block mapping and extent-status accounting.
* -1 Hard race: the commit explicitly says it has been hard to hit and requires the last reference to disappear outside the ordinary unmount grace-period path.
* Paranoid +1 reading: the stale path increments destroyed percpu counters, so it may perform a write-like operation through post-lifetime state rather than only read a stale pointer. This replaces the conservative uncertainty suppression; it does not stack above score 5.
* No controlled reclaim, attacker-selected target/payload, arbitrary write, disclosure, repeated release/refcount operation, or proven filesystem-data corruption is shown.
* No extra availability point: it would duplicate the demonstrated crash.
* NEW52/B5-R does not apply: no retry-accumulated index or confirmed heap OOB write exists.

3. Reachability analysis

A local process can invoke the pathwalk through operations such as `stat`, but the complete race also requires final mount-reference teardown, exemplified by closing a file or leaving a lazily unmounted directory. The evidence does not establish whether one ordinary user can arrange every stage, whether prior privileged mount/lazy-unmount setup is necessary, or whether a user/mount namespace suffices. Therefore host-admin and namespace-scoped privilege remain unknown. No network or special-hardware reachability exists.

4. Primitive and severity reading

This is an active structured-object RCU UAF, not fixed stale storage. The stale reader accesses sbi fields and updates extent-status counters during normal pathwalk processing; the fix moves counter destruction and sbi freeing into an RCU callback. The shown effect is a kernel fault. A post-free counter mutation is credible, but useful reclaim, directed corruption, confidentiality/integrity impact, and privilege escalation remain unproven.

Class B is appropriate. ImportantCandidate is YES because a legitimate reader can consume and mutate active filesystem state after its lifetime ends, leaving non-crash leverage unresolved. Auto-downgrade is disallowed pending review of the destroyed percpu-counter write and attainable mount-lifetime sequence.

5. One-sentence report phrase

An ext4 RCU pathwalk can outlive superblock teardown and access or update freed sbi/percpu-counter state, causing a demonstrated kernel fault and an unresolved post-lifetime corruption risk.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm below-host-admin triggerability and whether counter updates can reach reclaimed storage before faulting.

7. Call-site confidence

Call-site confidence: high

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

Patch: ext4: free the sbi after a grace period [ Upstream
Commit: 80d5db1686b05fad6f384be017714297d828a714
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=80d5db1686b05fad6f384be017714297d828a714

Changed files:
  fs/ext4/ext4.h
  fs/ext4/extents_status.c
  fs/ext4/extents_status.h
  fs/ext4/super.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=80d5db1686b05fad6f384be017714297d828a714

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

The original Linux kernel CVE announcement for CVE-2026-99174 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99174

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).