* [CVE-2026-97923][LOW] tracing: Free histogram the var ref when its initialization fails
@ 2026-09-25 12:25 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-25 12:25 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-97923
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: tracing: Free histogram the var ref when its initialization fails
Commit: 74375caa20fe5165807b4334c783d1790b01f02a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=74375caa20fe5165807b4334c783d1790b01f02a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97923
Analysis date: Fri, 25 Sep 2026 08:25:40 -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-2026-97923 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97923
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-2026-97923 LOW https://www.kernelcve.org/list/?q=CVE-2026-97923 CHECK WITH IMPACT FROM ORIG NN LOW 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 failed histogram variable reference initialization can leak a hist_field object because the object is not yet present in the trigger var_refs array and destroy_hist_field does not free VAR_REF fields on this error path. Repeated failed histogram trigger configuration attempts can accumulate allocated kernel memory and may eventually cause resource exhaustion. For the CVSS the PR:H because creating or modifying tracing histogram triggers normally requires administrative tracing access such as CAP_SYS_ADMIN. The issue is not network reachable and is triggered through the local tracing control plane. Impact is denial of service through memory exhaustion. No memory corruption, information disclosure, integrity impact, or privilege escalation primitive is indicated by the affected allocation and cleanup paths.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1) YES TRACE LEAK UAF LINUS YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=1
PrimitiveClass=NONE
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.
2. Signal breakdown
* +1 Concrete persistent kernel resource leak. `create_var_ref()` allocates `ref_field`, `init_var_ref()` can fail, and the old call to `destroy_hist_field()` intentionally skips freeing fields marked `HIST_FIELD_FL_VAR_REF` before the object has been added to `hist_data->var_refs`.
* No reliable kernel crash or strong DoS point awarded. The patch establishes a leak, but not an immediate crash, unbounded per-operation allocation, host-wide exhaustion, or an accounting bypass.
* No additional generic resource-exhaustion point awarded. Repetition can accumulate leaked objects, but the supplied evidence does not establish a realistic host-wide exhaustion rate or cross-container resource impact.
* No UAF, double free, stale dereference, or repeated lifetime/refcount mutation is established. The patch changes cleanup from a flag-aware destructor to `__destroy_hist_field()` specifically to free an otherwise leaked allocation.
* Controlled reclaim, arbitrary write, information disclosure, integrity impact, privilege escalation, and security-boundary bypass are not demonstrated.
* Remote reachability is not established.
* No privilege penalty was applied. The supplied context says tracing histogram configuration normally requires administrative tracing access, but does not positively establish the complete host-admin versus delegated tracefs access boundary for the evaluated deployment.
3. Reachability analysis
The vulnerable path is reached by locally configuring tracing histogram triggers and causing `init_var_ref()` to fail. It is a tracing control-plane operation, not a packet-processing or remotely reachable path.
In normal Linux deployments, writing histogram trigger configuration through tracefs is generally restricted to an administrator or a delegated tracing service. However, the supplied patch does not show the exact tracefs permission checks, mount policy, user namespace behavior, or whether the complete trigger sequence can be performed by a delegated non-host-admin actor. Therefore, `HostAdminRequired=UNKNOWN` and `NamespaceScopedPrivilege=UNKNOWN`.
No special hardware, boot option, race condition, firmware interaction, or prior privileged topology is relevant. Repetition is possible in principle through repeated failed configuration attempts, but the patch does not establish a practical rate sufficient for reliable host-wide memory exhaustion.
4. Primitive and severity interpretation
The exact primitive is a missing free of a `struct hist_field` allocation on one initialization-failure path. The allocated `VAR_REF` field is neither inserted into the trigger `var_refs` ownership array nor freed by the flag-aware `destroy_hist_field()` helper. This is a concrete resource leak.
This is not an active structured-object UAF, fixed stale storage bug, destructor UAF, or repeated release/refcount case. No first release may destroy the object, no stale reference survives a release, and no second lifetime-mutating operation occurs. The corrected call invokes `__destroy_hist_field()` to free the still-owned allocation.
The demonstrated consequence is incremental kernel memory consumption under repeated failed trace histogram configuration. The speculative worst case is resource exhaustion after sustained abuse, but no evidence establishes a practical host-wide DoS, memory corruption, confidentiality impact, integrity impact, or privilege escalation mechanism.
`PrimitiveClass=NONE` is appropriate because the patch establishes only a resource-management leak and no meaningful corruption, lifetime, authorization, ownership, or security-boundary primitive. `ImportantCandidate=NO` and `AutoDowngradeAllowed=YES` are appropriate because no unresolved corruption or boundary-bypass leverage remains.
5. One-sentence report phrase
A failed tracing histogram variable reference initialization leaks a hist_field allocation, allowing repeated privileged tracing configuration failures to consume kernel memory over time.
6. Manual review recommendation
NO MANUAL CHECK NEEDED
The patch establishes a straightforward local kernel memory leak with no UAF, memory corruption, authorization bypass, remote trigger, or demonstrated host-wide exhaustion mechanism.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: tracing: Free histogram the var ref when its initialization fails
Commit: 74375caa20fe5165807b4334c783d1790b01f02a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=74375caa20fe5165807b4334c783d1790b01f02a
Commit description:
create_var_ref() allocates a VAR_REF hist_field and then calls
init_var_ref() to fill it in. When that fails the field is leaked.
commit 656fe2b ("tracing: Use hist trigger's var_ref array to destroy
var_refs") made destroy_hist_field() return early for
HIST_FIELD_FL_VAR_REF, since var refs are freed by walking the trigger's
var_refs[] array instead. create_var_ref() adds the field to that array
only after init_var_ref() has succeeded, so on this path the field is in
neither place and nothing frees it. The call was correct when it was
written, before var refs were taken out of destroy_hist_field().
init_var_ref() cannot free it either. The caller owns the field, so
init_var_ref() undoes only its own string allocations and leaves the
field alone. Freeing it there would leave create_var_ref() passing freed
memory to destroy_hist_field(), which reads its flags.
Call __destroy_hist_field(), which frees the field without consulting
the flag.
Cc: [email protected]
Link: https://patch.msgid.link/[email protected]
Fixes: 656fe2b ("tracing: Use hist trigger's var_ref array to destroy var_refs")
Signed-off-by: Donggeun Yoo <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
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=74375caa20fe5165807b4334c783d1790b01f02a
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-97923 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97923
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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