[CVE-2026-93795][LOW] blk-cgroup: fix leaks and online flag on radix_tree_insert failure [ Upstream

[CVE-2026-93795][LOW] blk-cgroup: fix leaks and online flag on radix_tree_insert failure [ Upstream

[AL-KERNEL]

CVE: CVE-2026-93795
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: blk-cgroup: fix leaks and online flag on radix_tree_insert failure [ Upstream
Commit: 6e85f02d45b5977c0326a8d854c6db878aa6b34a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e85f02d45b5977c0326a8d854c6db878aa6b34a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93795
Analysis date: Thu, 24 Sep 2026 17:50:46 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93795 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93795

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: LOW
Manual review required: NO

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

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

CVE-2026-93795	LOW	https://www.kernelcve.org/list/?q=CVE-2026-93795 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:N/A:H';*CWE-401;*CWE-772;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A blk cgroup resource leak can occur if radix_tree_insert fails while creating a blkcg_gq object. The failed path can retain the blkcg_gq object, per CPU I O statistics, policy data, parent references, and a cgroup css reference which may prevent cgroup destruction. Repeated triggering requires a local workload capable of reaching the blk cgroup path and inducing the uncommon radix tree insertion failure condition, typically under memory pressure. For the CVSS the PR:L reflects a local user or delegated cgroup context rather than full system administration. The issue is not network reachable. Impact is local denial of service through accumulated resource consumption and stuck cgroup lifetime. No memory corruption, information disclosure, integrity impact, or privilege escalation primitive is indicated by the patch.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	SIMPLEFIX LEAK ERRORPATH HARDWARE 	NO	NO	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap was applied. The supplied patch establishes a persistent resource and reference leak, but does not establish memory corruption, an attacker-controlled lifetime primitive, a reliable host-wide DoS, or below-host-admin reachability.

2. Signal breakdown

* +1 Concrete persistent kernel resource/reference leak. When `radix_tree_insert()` fails, `blkg_put()` runs while `blkg->refcnt` remains in percpu mode. The decrement does not invoke `blkg_release()`, permanently retaining the `blkcg_gq`, per CPU I O statistics, policy data, parent `blkcg_gq` reference, and cgroup css reference.
* +0 Resource exhaustion impact. The leak is concrete and persistent, and a retained css reference can prevent cgroup destruction. However, the supplied evidence does not demonstrate that a small number of operations causes host-wide exhaustion or a reliable system-wide failure.
* +0 Local unprivileged trigger. The supplied patch does not establish that an ordinary local user can complete the full trigger sequence, including reaching the relevant blk cgroup path and causing `radix_tree_insert()` to fail.
* +0 Remote reachability. No network-reachable path is established.
* +0 Memory corruption. No UAF, stale dereference, double release, out of bounds access, invalid free, or write through freed memory is established.
* +0 Confidentiality and integrity. The patch establishes only retained references and leaked allocations. It does not establish disclosure, unauthorized modification, page ownership corruption, or privilege escalation.
* +0 Strong DoS. The retained references may accumulate under repeated failures, but the supplied material does not establish a reliable immediate kernel crash, panic, filesystem shutdown, or host-wide resource exhaustion.
* No privilege penalty was applied. Host-admin-equivalent privilege is not positively established, but below-host-admin reachability is also not positively established.

3. Reachability analysis

The vulnerable path is in `blkg_create()` and requires `radix_tree_insert()` to fail after a `blkcg_gq` has been allocated and partially initialized. The supplied commit identifies the failure handling mechanics but does not provide the complete call chain, capability checks, cgroup delegation conditions, block controller configuration, or a proof that an ordinary user can repeatedly induce the allocation or insertion failure.

A local workload operating through a configured blk cgroup environment may plausibly reach this path, especially under memory pressure. That is not sufficient evidence to classify the complete attack path as ordinary-user reachable. Likewise, the presence of cgroups does not prove that all required operations are available in an attacker-controlled cgroup namespace.

No network trigger, special hardware requirement, boot parameter, or direct race condition is established. The error condition is uncommon because it depends on `radix_tree_insert()` failure, but no timing penalty is applied because this is an ordered error path rather than a demonstrated narrow race.

4. Primitive and severity interpretation

The exact primitive is a persistent resource and reference leak caused by incorrect percpu reference-count shutdown during an error path. `blkg_put()` does not complete normal object release while the reference remains in percpu mode. As a result, `blkg_release()` is not called and the object graph remains retained.

This is not an active structured-object UAF and not fixed stale storage. There is no first release that may free an object followed by a second release through the same dangling reference. The patch replaces an ineffective reference decrement with `percpu_ref_kill()` so that the normal release path can execute. Therefore, the concrete repeated release/refcount rule does not apply.

The incorrectly set `blkg->online` flag is an invalid-state issue for an object that was never successfully inserted. The available evidence does not show that this flag exposes an active object to later unsafe dereference or corruption.

PrimitiveClass is CLASS_C because the demonstrated result is invalid lifecycle state and persistent resource retention without an independently useful corruption primitive. ImportantCandidate is NO because no unresolved UAF, controlled corruption, authorization bypass, cross-boundary impact, or active post-free mutation is established. AutoDowngradeAllowed is YES because this is a constrained leak and error-path cleanup defect with no demonstrated exploitation leverage beyond possible gradual resource consumption.

The likely operational severity is Low or ordinary Moderate. Systems that heavily create blk cgroup associations while repeatedly hitting allocation failures may experience retained cgroups and accumulated kernel memory use, but the patch does not support an Important-class interpretation.

5. One-sentence report phrase

A failed blk cgroup creation can permanently leak blkcg_gq resources and cgroup references because the percpu reference is decremented without first being killed, potentially causing gradual local resource exhaustion under repeated failure conditions.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a resource leak and blocked cgroup cleanup, but does not establish memory corruption, a security-boundary bypass, reliable host-wide DoS, or a concrete below-host-admin exploit path.

7. Call-site confidence

Call-site confidence: medium

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

Patch: blk-cgroup: fix leaks and online flag on radix_tree_insert failure [ Upstream
Commit: 6e85f02d45b5977c0326a8d854c6db878aa6b34a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e85f02d45b5977c0326a8d854c6db878aa6b34a

Changed files:
  block/blk-cgroup.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=6e85f02d45b5977c0326a8d854c6db878aa6b34a

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

The original Linux kernel CVE announcement for CVE-2026-93795 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93795

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:L
  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: 1

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

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).