[CVE-2026-99175][MODERATE REGULAR] fsnotify: prevent mark additions during connector teardown

[CVE-2026-99175][MODERATE REGULAR] fsnotify: prevent mark additions during connector teardown

[AL-KERNEL]

CVE: CVE-2026-99175
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fsnotify: prevent mark additions during connector teardown
Commit: 952501bfc6e5e654ac4a17ec9d90c1ec614bd9f1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=952501bfc6e5e654ac4a17ec9d90c1ec614bd9f1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99175
Analysis date: Wed, 28 Oct 2026 22:22:52 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
An unprivileged inotify teardown race can leave a mark attached to a detached connector, causing a concurrent fdinfo read to invoke `igrab(NULL)` and crash the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-99175 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99175

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.

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AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-99175	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99175 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, 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:N/I:N/A:H';CWE-362;**CWE-476;*CWE-664;BEST CVSS score: '4.7';DESCR 'A race in fsnotify connector teardown allows a replacement mark to be inserted before the destroy walk cursor after IN_IGNORED. The walk can miss that mark and detach the connector while the mark remains reachable through its group list and inotify IDR. A concurrent unprivileged fdinfo read can then pass a NULL object to igrab and cause a kernel oops or panic. For the CVSS the PR:L value is selected because triggering requires a local user or process that can create inotify watches and read fdinfo. AC:H reflects the required timing race. The demonstrated primitive is a NULL pointer dereference and does not establish UAF, OOB access, information disclosure, or a controlled write. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the manual review requirement was applied. ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	KASAN NULLPTR DISK RACE LEAK ERRORPATH LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 3; no floor or cap applied.
* Raw paranoid score: 3.
* Paranoid score: 3; no additional exploit primitive is technically supported.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +1 local unprivileged trigger: inotify watch manipulation and the racing fdinfo read are explicitly described as unprivileged.
* +1 concrete invalid active-state condition: a replacement mark remains in the group list/IDR after its connector is detached from the object.
* +1 reliable kernel crash concern: the supplied trace shows `igrab(NULL)` causing a null-pointer write fault.
* +1 broad exposure: inotify/fsnotify and fdinfo are commonly available local interfaces.
* -1 hard timing race: successful triggering requires re-addition and fdinfo access around final connector teardown; the supplied evidence explicitly characterizes this as AC:H.

Not awarded: UAF, OOB write, controlled target or payload, repeated lifetime mutation, disclosure, authorization bypass, or integrity impact. The null-address write is a faulting dereference, not evidence of an attacker-directed kernel write. No second availability point is added for the same crash.

3. Reachability analysis

A normal local process can create and re-add inotify watches and read fdinfo without host UID 0 or host-administrator capabilities. No namespace capability or prior privileged setup is identified. The race requires a replacement mark to sort before the teardown cursor and a concurrent fdinfo read; repeat attempts appear possible, but the timing remains specifically described as difficult.

4. Primitive and severity reading

The concrete primitive is an inconsistent fsnotify connector/mark state followed by `igrab(NULL)`. This is neither a real UAF nor stale reclaimed-object consumption: the demonstrated terminal operation is a NULL-pointer dereference. It is therefore CLASS_C with crash/DoS as the shown effect. Stronger memory corruption, LPE, or protected-state modification is unsupported.

ImportantCandidate is NO because no useful non-crash corruption or trust-boundary primitive remains. AutoDowngradeAllowed is YES, although manual review is recommended because the race is unprivileged and affects common kernel infrastructure.

5. One-sentence report phrase

An unprivileged inotify teardown race can leave a mark attached to a detached connector, causing a concurrent fdinfo read to invoke `igrab(NULL)` and crash the kernel.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm product-specific panic behavior and practical race reproducibility; no Important-preserving primitive is currently shown.

7. Call-site confidence

Call-site confidence: high

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

Patch: fsnotify: prevent mark additions during connector teardown
Commit: 952501bfc6e5e654ac4a17ec9d90c1ec614bd9f1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=952501bfc6e5e654ac4a17ec9d90c1ec614bd9f1

Commit description:

fsnotify_destroy_marks() keeps a reference to its current mark while it
drops conn->lock to destroy the mark. This stabilizes the forward-walk
cursor, but does not prevent a new mark from being inserted before it.
For example, an inotify watch re-added after IN_IGNORED sorts before the
old, detached mark for the same group. The walk can miss the replacement
and then detach the entire connector from the object.

The replacement is still on its group's marks_list and, for inotify, in
the watch IDR. The group mutex no longer protects the connector's object
from disappearing. An unprivileged fdinfo read racing the final detach
can consequently call igrab(NULL):

  BUG: KASAN: null-ptr-deref in igrab+0x24/0x100
  Write of size 4 at addr 0000000000000148
  Call Trace:
   igrab+0x24/0x100
   inotify_show_fdinfo+0x1dc/0x5e0
   seq_show+0x397/0x690

Set a DESTROYING flag under conn->lock before starting the walk, and
reject new marks on that connector with -ENOENT. The existing add-mark
error paths undo the temporary group-list entry and the backend's watch
allocation. Do not wait for teardown while holding the group mutex,
which the destroyer needs. Once the old connector is detached, an add
can install a fresh connector as before.

This closes the insertion race for all object types using the common
connector teardown, without changing the mark reference or SRCU lifetime
rules.

Fixes: 6b3f05d ("fsnotify: Detach mark from object list when last reference is dropped")
Cc: stable@vger.kernel.org
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/notify/mark.c
  include/linux/fsnotify_backend.h

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=952501bfc6e5e654ac4a17ec9d90c1ec614bd9f1

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

The original Linux kernel CVE announcement for CVE-2026-99175 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99175

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).