[CVE-2026-93100][MODERATE 7.0] fs/resctrl: Prevent use-after-free in rdtgroup_kn_put() [ Upstream

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

CVE: CVE-2026-93100
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fs/resctrl: Prevent use-after-free in rdtgroup_kn_put() [ Upstream
Commit: ae69f936e8ffed553d899ba4c02f8a7461a89e54
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ae69f936e8ffed553d899ba4c02f8a7461a89e54
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93100
Analysis date: Fri, 18 Sep 2026 04:20:13 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93100 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93100

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-2026-93100	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-93100 CHECK WITH IMPACT FROM ORIG NN MODERATE	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-415;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 fs resctrl can cause a use after free in rdtgroup_kn_put() when rdtgroup::waitcount is decremented to zero outside rdtgroup_mutex and a concurrent teardown path frees the same rdtgroup before rdtgroup flags are inspected. The same race can also create a double free condition if the freed memory later satisfies the RDT_DELETED flag check. For the CVSS the PR:L is used for the paranoid score because practical triggering may be possible by a local user or delegated service account that can operate on resctrl groups, even if default deployments often restrict resctrl administration to root. The issue is not network reachable and requires local interaction with the resctrl filesystem or teardown paths. Impact is at least local denial of service via kernel crash. In the worst case it may allow confidentiality or integrity impact because use after free and double free are memory corruption primitives that can sometimes be developed into privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	LOCK DISK INIT RACE UAF KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Applied conservative floor/cap: REAL UAF FLOOR 5 applies because the patch establishes a concrete UAF plus a possible second free on the same rdtgroup after lifetime may already have ended.
* Raw paranoid score: 6
* Paranoid score: 6
* Applied paranoid floor/cap: none.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive conservative signals:

* +2 Real UAF / active lifetime violation: rdtgroup_kn_put() can decrement waitcount to zero, then a concurrent teardown path can free the same struct rdtgroup before rdtgroup_kn_put() reads rdtgrp->flags.
* +1 Reliable kernel crash / strong DoS: post-free access to rdtgrp->flags and possible stale rdtgroup_remove() can plausibly fault or corrupt lifetime state.
* +1 Concrete repeated release/free operation: after the concurrent bulk teardown frees the rdtgroup, the same rdtgrp reference remains in rdtgroup_kn_put(), and the old code can call rdtgroup_remove(rdtgrp) again if the stale flags check passes.

Positive paranoid-only signals:

* +1 Attacker-influenced repeatable lifetime manipulation: an actor able to operate resctrl groups can plausibly repeat group removal and teardown style transitions to hit the vulnerable lifetime window.
* +1 Paranoid repeated-refcount/free leverage: the second stale operation is not only a read. It can perform a real lifetime/free operation through storage whose original object lifetime may already have ended.

Non-awarded signals:

* +0 controlled reclaim: not demonstrated.
* +0 arbitrary or directed write: not demonstrated.
* +0 confidentiality and integrity direct impact: not awarded as direct evidence because no leak, arbitrary write, credential overwrite, or controlled replacement is shown.
* +0 structured stale-object leverage separately: not awarded because the relevant exploitation concern is already counted as concrete repeated release/free.
* +0 Weak LPE concern separately: not awarded to avoid double-counting the same repeated lifetime mutation.
* +0 extra Availability: not awarded because the crash/DoS is already counted once.
* +0 remote reachability: resctrl is a local filesystem/control-plane path, not packet-triggered.

Repeated release/refcount checklist:

* First lifetime-ending operation can occur: yes, free_all_child_rdtgrp() or rmdir_all_sub() can observe waitcount == 0 and kfree() the rdtgroup.
* Exact same reference survives: yes, rdtgroup_kn_put() still holds rdtgrp after atomic_dec_and_test().
* Second operation mutates lifetime/free state: yes, stale rdtgroup_remove(rdtgrp) may be called.
* +1 Concrete repeated release/free awarded: yes.
* Paranoid repeated-refcount/free leverage +1 awarded: yes.
* FLOOR 5 applied: yes.

Negative signals:

* No -2 host-admin penalty: host-admin-equivalent privilege is not positively established from supplied evidence.
* No -1 race penalty: the bug is a race, but the supplied evidence does not prove a one-shot or externally uncontrollable timing condition sufficient for a scoring penalty.
* No rare-subsystem penalty: resctrl is specialized, but specialization alone is not enough for a negative signal.

3. Reachability analysis

The vulnerable code is in fs/resctrl and is reached through resctrl resource group lifetime management. Triggering requires local interaction with resctrl directory removal, resctrl filesystem unmount, or related teardown paths while another caller uses rdtgroup_kn_get()/rdtgroup_kn_put() across mutex acquisition.

Default deployments commonly restrict resctrl management to privileged users, but the supplied evidence does not prove that host-admin-equivalent privilege is strictly required for every complete trigger. Delegated access to resctrl directories or a service account managing resource groups may be possible depending on system policy. Therefore HostAdminRequired is UNKNOWN rather than YES.

The operation is not meaningfully namespace-scoped. resctrl controls host CPU resource-management state and is not analogous to a per-network-namespace object. NamespaceScopedPrivilege is NO.

There is no network trigger. Prior setup may include mounted resctrl and existing resource groups. No special rare hardware beyond a platform supporting resctrl is established as a scoring penalty. The race is real, but repeated create/remove/teardown attempts appear plausible for an actor with resctrl management ability.

4. Primitive and severity interpretation

The exact primitive is a race-driven lifetime violation on struct rdtgroup. The object is an active structured kernel object, not fixed stale storage. The stale use is in a release/teardown path, but it is not merely an ordinary cleanup stale dereference because the old path can proceed from a post-free flags read into rdtgroup_remove() on the same potentially freed object.

This is category B: second lifetime/free mutation through a potentially dangling object. Controlled reclaim, arbitrary R/W, callback control, or demonstrated LPE are not established, so this is not CLASS_A. The demonstrated consequence is local kernel crash or DoS. The speculative consequence is stronger post-free lifetime corruption that could require Important-vs-Moderate manual review.

ImportantCandidate is YES because a concrete UAF plus possible double free leaves meaningful unresolved exploitation leverage. AutoDowngradeAllowed is NO because automatically treating this as ordinary crash-only cleanup would discard a real repeated lifetime/free primitive.

Likely operational severity is actionable Moderate at minimum, with Strong Important candidacy requiring manual review.

5. One-sentence report phrase

A race in resctrl rdtgroup_kn_put() can allow concurrent teardown to free a struct rdtgroup after waitcount reaches zero but before flags are inspected, leading to use-after-free and possible double free with local DoS and unresolved post-free corruption risk.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly establishes a real UAF and possible double free on an active structured kernel object, with a concrete second lifetime/free operation through a potentially dangling reference.

7. Call-site confidence

Call-site confidence: high

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

Patch: fs/resctrl: Prevent use-after-free in rdtgroup_kn_put() [ Upstream
Commit: ae69f936e8ffed553d899ba4c02f8a7461a89e54
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ae69f936e8ffed553d899ba4c02f8a7461a89e54

Changed files:
  fs/resctrl/rdtgroup.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=ae69f936e8ffed553d899ba4c02f8a7461a89e54

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

The original Linux kernel CVE announcement for CVE-2026-93100 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93100

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: 5
  Paranoid ActionableScore: 6

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).