* [CVE-2026-93099][MODERATE 7.0] fs/resctrl: Fix UAF from worker threads when domains are removed [ Upstream
@ 2026-09-17 20:16 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-17 20:16 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-93099
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fs/resctrl: Fix UAF from worker threads when domains are removed [ Upstream
Commit: b86dbfe4583bf134932a6cc45bf25b12c10a0b2f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b86dbfe4583bf134932a6cc45bf25b12c10a0b2f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93099
Analysis date: Thu, 17 Sep 2026 16:16:39 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-93099 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93099
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-2026-93099 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-93099 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-667;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 resctrl CPU hotplug handling can leave mbm_handle_overflow or cqm_handle_limbo workers running after the rdt_l3_mon_domain containing their delayed_work has been removed. If a worker is blocked before it can protect the domain lifetime, the hotplug path may free the domain and the later container_of() on the embedded work pointer can dereference freed kernel memory. For the CVSS the PR:L is used for the paranoid score because triggering may be possible from a local actor with delegated CPU hotplug or resctrl control, reduced root capabilities, or a service account that can offline CPUs or manage resctrl state. The issue is not network reachable and requires local control plane actions. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel use-after-free, so manual review is needed.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) YES LOCK DANGER DISK UAF HARDWARE KPANIC NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 4
* Applied conservative floor/cap: Real UAF floor 4 applied because a real stale post-lifetime object use is established, the stale object is not fixed passive storage, and host-admin-equivalent reachability is not fully resolved from the supplied evidence.
* Raw paranoid score: 5
* Paranoid score: 5
* Applied paranoid floor/cap: none
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Conservative signals:
* +2 Real UAF / active lifetime violation. The delayed_work is embedded in struct rdt_l3_mon_domain, and the domain can be freed while a worker is blocked before later using container_of() on the embedded work pointer.
* +1 Reliable kernel crash / strong DoS. Dereferencing freed kernel memory from mbm_handle_overflow() or cqm_handle_limbo() can plausibly fault or panic the kernel.
* +1 Workqueue / RCU lifetime synchronization mismatch. The fix drains pending and in-flight work after the domain is unlinked from RCU-visible lists and before architecture code frees the domain, showing a concrete lifetime ordering invariant was broken.
* -1 Hard timing / CPU hotplug interleaving. The demonstrated path requires a worker to be blocked on cpus_read_lock() while CPU hotplug teardown proceeds under cpus_write_lock().
Paranoid additional signal:
* +1 Attacker-influenced repeatable lifetime manipulation, only under a defensible local control-plane model where an actor can repeatedly drive CPU offline or resctrl-related state transitions and cause the affected workers to race domain teardown.
Important non-awarded signals:
* Controlled reclaim is not demonstrated.
* Arbitrary kernel read or write is not demonstrated.
* Callback or function-pointer control is not demonstrated.
* Remote reachability is not established.
* Confidentiality and integrity are not awarded as direct signals merely because UAF theoretically can lead to stronger exploitation.
* Concrete repeated release/refcount mutation is not established. The bug is a stale dereference/use of an embedded work item after domain free, not a second put/free/refcount operation on the same dangling reference.
* Structured stale-object leverage is not separately awarded beyond the workqueue/RCU lifetime mismatch because attacker-controlled replacement contents are not shown.
* Extra Availability is not awarded because the crash/DoS is already counted once.
* FLOOR 5 from the repeated-release/refcount rule is not applied because there is no concrete second lifetime/refcount mutation.
Repeated-release/refcount checklist:
* First release can end lifetime: the architecture can free the rdt_l3_mon_domain during CPU hotplug teardown.
* Exact same reference survives: the embedded work_struct pointer can remain queued/running and later be used for container_of().
* Second operation is a real lifetime/refcount mutation: no.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 via repeated-release rule applied: no.
3. Reachability analysis
The trigger is local control-plane activity around CPU hotplug and resctrl monitoring domains. The vulnerable workers are scheduled internally as delayed_work for resctrl monitoring, while the freeing side is architecture CPU hotplug domain teardown under cpus_write_lock().
Ordinary unprivileged reachability is not established. CPU offline/hotplug and resctrl control are normally privileged operations, but the supplied evidence does not fully resolve whether the complete trigger requires full host-admin-equivalent privilege in every evaluated deployment or could be reachable through delegated management service accounts. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.
NamespaceScopedPrivilege is NO. No supplied evidence indicates that CPU hotplug domain removal or the relevant resctrl teardown can be fully performed with attacker-created namespace-scoped capabilities.
Prior host setup may exist in practice because resctrl monitoring must be available and relevant hardware support must exist. This is not treated as a rare hardware penalty because resctrl/RDT is a normal x86 server feature class, not a physical attacker peripheral requirement.
Race repeatability is uncertain. The conservative score treats the cpus_read_lock() versus cpus_write_lock() window as a hard interleaving. The paranoid score assumes a local actor with suitable delegated control could repeatedly exercise CPU offline and worker scheduling enough that the lifetime issue should not be auto-closed.
4. Primitive and severity interpretation
The exact primitive is a real use-after-free of struct rdt_l3_mon_domain through delayed_work embedded inside the freed domain. The stale storage is an active structured kernel object, not a fixed passive buffer. The stale use is not merely a destructor freeing a pointer. The worker can resume and derive the containing domain from the stale work pointer, then enter resctrl monitoring logic.
This is CLASS_B: a real kernel lifetime primitive exists and exploitation-leverage analysis is required. It is not CLASS_A because controlled reclaim, directed corruption, arbitrary write, function-pointer control, or demonstrated privilege escalation is not established.
Demonstrated consequence is local DoS via kernel crash or fault. Speculative consequence is stronger exploitation from kernel UAF, but the supplied patch does not establish enough control over replacement contents or stale structured fields to score it as strong exploitation leverage.
ImportantCandidate is YES because this is an active workqueue/domain lifetime UAF with unresolved post-lifetime behavior, not a simple NULL dereference or resource leak. AutoDowngradeAllowed is NO because automatically treating it as ordinary Moderate would discard a concrete active-object UAF that still needs manual exploitation-leverage review.
Likely operational severity is Actionable Moderate, with Strong Important consideration depending on whether a below-host-admin actor can actually trigger the complete CPU hotplug/resctrl teardown sequence or influence object reuse.
5. One-sentence report phrase
A race in resctrl CPU hotplug teardown can free an rdt_l3_mon_domain while delayed workers embedded in that domain are still pending or blocked, leading to a kernel use-after-free and at least a local DoS, with stronger impact requiring manual review.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: real active-object kernel UAF is established, the stale worker path is not purely passive fixed-storage cleanup, and the complete privilege boundary and exploitation leverage beyond crash remain unresolved.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: fs/resctrl: Fix UAF from worker threads when domains are removed [ Upstream
Commit: b86dbfe4583bf134932a6cc45bf25b12c10a0b2f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b86dbfe4583bf134932a6cc45bf25b12c10a0b2f
Changed files:
fs/resctrl/monitor.c
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=b86dbfe4583bf134932a6cc45bf25b12c10a0b2f
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-93099 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93099
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: 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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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