[CVE-2026-97922][LOW] tracing: Free histogram var refs regardless of how often they are referenced

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

CVE: CVE-2026-97922
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: tracing: Free histogram var refs regardless of how often they are referenced
Commit: e7b6d67b43caee9183a4374d23641578ea556e6c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e7b6d67b43caee9183a4374d23641578ea556e6c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97922
Analysis date: Fri, 25 Sep 2026 10:01:53 -0400
ActionableScore: 0
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-2026-97922 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97922

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-2026-97922	LOW	https://www.kernelcve.org/list/?q=CVE-2026-97922 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.4';DESCR 'A memory leak in tracing histogram triggers occurs when the same variable is referenced three or more times in one trigger. Removing the trigger can leave the shared var_ref object and its associated strings allocated because its reference count reflects created references rather than remaining owners. For the CVSS the PR:H rating reflects that configuring and removing histogram triggers through tracefs normally requires administrative tracing privileges. The issue is not network reachable. Impact is local denial of service through accumulated memory consumption after repeated trigger creation and removal. No memory corruption or privilege escalation primitive is indicated. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)	YES	WRITE OOB TRACE LEAK UAF LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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

ActionableScore=0
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: -1
* Conservative score: 0
* Raw paranoid score: -1
* Paranoid score: 0
* Applied score lower bound: -1 -> 0
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Concrete persistent kernel resource leak. The shared var_ref object and its allocated name strings are not released when one variable is referenced three or more times in a histogram trigger and the trigger is removed.
* -2 Host-admin-equivalent privilege requirement. Creating and removing histogram triggers requires write access to tracefs tracing control files, which is normally restricted to host administrators or equivalent tracing-management privilege.
* No additional resource-exhaustion point. The patch establishes a small repeatable leak, but does not establish host-wide exhaustion, a cgroup accounting bypass, cross-container impact, or immediate allocation failure.
* No memory-corruption signal. The defect is an ownership and reference-count accounting error that suppresses freeing. It does not free an object too early.
* No UAF signal. The commit explicitly states that var_refs is the sole owner, external references are checked by check_var_refs, and removal is refused with EBUSY while references remain.
* No repeated-release or refcount-mutation signal. The faulty refcount caused a missing free, not a second put, decrement, free, or mutation through dangling storage.
* No confidentiality or integrity signal. No disclosure, unauthorized modification, authorization bypass, or attacker-controlled memory corruption is established.
* No remote reachability signal. Triggering requires local tracefs control-plane operations.
* No special race/timing penalty. This is deterministic reference-accounting behavior, not a race.

3. Reachability analysis

The trigger requires creating a tracing histogram trigger containing repeated references to the same variable and then removing that trigger. Histogram trigger configuration is performed through tracefs tracing control files and normally requires host-administrative tracing privileges, such as root or an equivalent host-level tracing-management authority.

The supplied patch does not establish that an ordinary local user, a container user, or a user with namespace-scoped capabilities can complete the full trigger sequence. User and network namespaces do not normally grant write access to the host tracefs tracing control interface. Therefore, HostAdminRequired is YES and NamespaceScopedPrivilege is NO.

The issue also requires histogram-trigger support to be present and tracing to be accessible, but no additional hardware, network access, boot option, firmware interaction, or race timing is required. A privileged actor can repeat creation and removal operations, but the leak is limited to var_ref-related allocations per affected trigger.

4. Primitive and severity interpretation

The exact primitive is a persistent memory leak caused by incorrect ownership accounting for shared histogram variable-reference objects. When the same variable is used three or more times, create_var_ref increments a counter for a single shared object, while the normal destruction path does not perform matching decrements. The object and its strings can therefore remain allocated after trigger removal.

This is neither an active structured-object UAF nor fixed stale storage. It is not a cleanup-time stale dereference. The patch removes the unnecessary refcount because the trigger var_refs array is the sole owner of each var_ref, while check_var_refs prevents destruction if references are still active.

No first lifetime operation may free the object before a later stale operation. No second lifetime or refcount mutation through a potentially dangling reference exists. The bug class is resource leakage only, so PrimitiveClass is NONE and RealUAF is NO.

The demonstrated consequence is gradual kernel-memory consumption after repeated privileged histogram-trigger operations. A host-wide denial of service is theoretically possible only after sustained privileged abuse, but the patch does not establish broad exhaustion, isolation bypass, controlled corruption, information disclosure, integrity impact, or privilege escalation. The issue is suitable for automatic deferred handling rather than urgent security remediation.

ImportantCandidate is NO because no meaningful corruption, lifetime, authorization, or security-boundary primitive remains unresolved. AutoDowngradeAllowed is YES because the established behavior is a constrained privileged memory leak without a UAF or exploitation-leverage mechanism.

5. One-sentence report phrase

A tracing histogram trigger can leak var_ref allocations and related strings when the same variable is referenced three or more times, causing limited memory consumption after repeated privileged trigger creation and removal.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a deterministic privileged memory leak with no early free, dangling reference, memory corruption, security-boundary bypass, or below-host-admin trigger.

7. Call-site confidence

Call-site confidence: high

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

Patch: tracing: Free histogram var refs regardless of how often they are referenced
Commit: e7b6d67b43caee9183a4374d23641578ea556e6c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e7b6d67b43caee9183a4374d23641578ea556e6c

Commit description:

Using the same variable three or more times in one hist trigger leaks the
variable reference and its strings when the trigger is removed.

commit 656fe2b ("tracing: Use hist trigger's var_ref array to destroy
var_refs") made a trigger's var_refs[] array the only owner of a var ref:
destroy_hist_field() returns early for HIST_FIELD_FL_VAR_REF, so the field
expressions never destroy one. One entry, freed once, no count needed.

commit 8bcebc7 ("tracing: Fix histogram code when expression has same
var as value") then made repeated references share one object and added a
count of them. Only the increment side exists, since those expressions
still return early and never drop a reference, so __destroy_hist_field()
sees how many references were created rather than how many are left. It
frees when the decremented count is 0 or 1, so two references work and
three or more leak.

Sharing kept one array entry per object, and create_var_ref() searches and
appends within a single trigger, so nothing outside it holds the object.
Removing a trigger whose variables are still referenced is already refused
by check_var_refs() with -EBUSY. Drop the count and free unconditionally.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260906124025.3550596-1-donggeunyoo.kernel@gmail.com
Fixes: 8bcebc7 ("tracing: Fix histogram code when expression has same var as value")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/trace/trace_events_hist.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=e7b6d67b43caee9183a4374d23641578ea556e6c

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

The original Linux kernel CVE announcement for CVE-2026-97922 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97922

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:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.4

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: 0

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).