[CVE-2026-99171][IMPORTANT] ntfs3: free the sbi after a grace period [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-99171
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: ntfs3: free the sbi after a grace period [ Upstream
Commit: c8b1a4840467930e01f1a9374d991d7a8a2896c1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c8b1a4840467930e01f1a9374d991d7a8a2896c1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99171
Analysis date: Fri, 30 Oct 2026 11:58:23 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
NTFS3 can free nocase mount state before an RCU pathname lookup finishes, causing a confirmed active-state slab UAF read and possible kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-99171 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99171

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: IMPORTANT
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-99171	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-99171 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: 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:L/I:L/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in NTFS3 filesystem teardown can free ntfs_sb_info while an RCU pathwalk is still executing ntfs_d_hash or ntfs_d_compare for a nocase mount. The confirmed primitive is a slab use after free read in ntfs_nls_to_utf16 that can crash the kernel. For the CVSS the PR:L value reflects that a local user needs access to perform path lookups while the final mount reference is concurrently removed. AC:H reflects the required race with filesystem teardown and the masking grace period in the normal umount path. Base impact is C:N/I:N/A:H because the demonstrated result is a kernel crash. The paranoid C:L/I:L/A:H assessment accounts for possible reuse of freed sbi or table memory affecting name processing, but no arbitrary write or reliable privilege escalation primitive is established. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 5.8 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 IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied and KP specialist did not establish a qualifying panic. 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 produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and TEMP_KPANIC_MERGE_IMPORTANT_VETO and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 3.4 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	READ LOCK DEADLOCK DISK INIT LEAK UAF LINUS INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_VETO KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4
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: 3.
* Conservative score: 4. REAL UAF FLOOR 4 changes 3 -> 4.
* Raw paranoid score: 4.
* Paranoid score: 5. Concrete RCU active-structured-object calibration changes 4 -> 5.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF: `ntfs_sb_info`, mount options, and shared tables can be freed while an RCU pathwalk still uses them in `ntfs_d_hash()`/`ntfs_d_compare()`.
* +1 Concrete RCU referenced-object lifetime mismatch: the fix defers `sbi` and referenced-table destruction with `call_rcu()`.
* +1 Reliable kernel crash: KASAN confirms a slab UAF read in `ntfs_nls_to_utf16()`.
* -1 Hard race: pathwalk must overlap final mount teardown, and the ordinary unmount path is currently masked by `namespace_unlock()`'s grace period.
* Paranoid +1: stale active structured state supplies NLS/upcase pointers and conversion state to continued pathname processing, so behavior under reuse is unresolved.

No controlled reclaim, post-free write, disclosure, arbitrary write, or LPE is shown. Confidentiality and integrity points are not awarded. The same crash is not counted again as availability.

3. Reachability analysis

A local actor can perform pathname lookups, but the supplied evidence does not establish who can cause the final mount reference to be dropped or whether one below-host-admin principal controls the complete race. Mount/user-namespace capability scope and prior privileged setup are unresolved. No remote path or special hardware is involved.

4. Primitive and severity reading

This is an active structured-object RCU UAF, not fixed stale storage and not merely a teardown-side dereference: teardown frees the state, while an already-running pathwalk continues consuming its pointers and tables. The shown result is a host kernel fault. Useful reuse, attacker-shaped replacement contents, and non-crash exploitation remain unproven.

CLASS_B is appropriate. ImportantCandidate is retained because a concrete cross-object RCU lifetime invariant is broken during active pathname processing and downstream behavior after reuse is unknown. Auto-downgrade is disallowed pending that review, but IMPORTANT severity is not proven.

There is no authorization decision bypass; B4-PENDING does not apply.

5. One-sentence report phrase

NTFS3 can free nocase mount state before an RCU pathname lookup finishes, causing a confirmed active-state slab UAF read and possible kernel crash.

6. Manual review advice

MANUAL CHECK REQUIRED

Review allocator reuse and whether stale NLS/upcase pointers can produce disclosure, writes, or persistent pathname-policy effects beyond the demonstrated crash.

7. Call-site confidence

Call-site confidence: high

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

Patch: ntfs3: free the sbi after a grace period [ Upstream
Commit: c8b1a4840467930e01f1a9374d991d7a8a2896c1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c8b1a4840467930e01f1a9374d991d7a8a2896c1

Changed files:
  fs/ntfs3/ntfs_fs.h
  fs/ntfs3/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=c8b1a4840467930e01f1a9374d991d7a8a2896c1

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

The original Linux kernel CVE announcement for CVE-2026-99171 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99171

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:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 4
  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: IMPORTANT
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): IMPORTANT

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).