[CVE-2026-99142][MODERATE 7.0] tracing: Check operator stack capacity before pushing

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

CVE: CVE-2026-99142
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tracing: Check operator stack capacity before pushing
Commit: a866a8a753922cdf7710b8eb3f686dc0efc06505
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a866a8a753922cdf7710b8eb3f686dc0efc06505
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99142
Analysis date: Fri, 30 Oct 2026 11:27:58 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A sufficiently nested tracing filter causes a confirmed one-entry heap overwrite beyond the operator stack, with credible crash impact and unresolved adjacent-object exploitability.

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

The original Linux kernel CVE announcement for CVE-2026-99142 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99142

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-99142	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99142 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	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-787;CWE-193;CWE-129;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'predicate_parse reserves the first operator stack entry and increments top before storing an opening parenthesis. The old capacity test permits a push from the final allocated entry and causes a one entry heap out of bounds write when a sufficiently nested tracing filter is parsed. For the CVSS the PR:L value reflects deployments where a tracefs instance or tracing control service is delegated to a less privileged local user. A kernel crash is credible and privilege escalation is plausible because a concrete heap write occurs, but exploitation beyond denial of service depends on the overwritten object and allocator layout. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore threshold rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4. The independent weaker Aegis shadow evaluated CVSS 2.3 and ActionableScore 0 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	TRACE SIMPLEFIX LEAK IMPROVEONLY LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 5. Manual-review safety floor applied: 3 -> 5 for a confirmed attacker-influenced heap OOB write with unresolved adjacent-object corruption.
* Raw paranoid score: 3.
* Paranoid score: 5. No additional speculative exploit points.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 strong corruption primitive: nested opening parentheses advance `top` and write one entry beyond the allocated operator stack. Input controls whether the invalid push occurs.
* +1 reliable kernel crash/strong DoS: corruption of adjacent heap storage makes a kernel fault credible.
* Payload selection, adjacent victim, arbitrary target, controlled reclaim, disclosure, and LPE are not shown; no C/I points awarded.
* No extra availability point: it would duplicate the same prospective crash.
* NEW52 B5-R does not apply: no retry/re-entry accumulation or attacker-influenced retry sequence is shown.
* The patch does not establish later consumption of a particular corrupted adjacent object, so no downstream structured-state bonus is awarded.
* No authorization decision is involved; B4-PENDING is inapplicable.

3. Reachability analysis

The parser input controls nesting depth, but the supplied source does not establish the complete permissions for writing the affected tracefs filter. The legacy summary mentions delegated tracefs instances or tracing services, but does not identify a concrete delegation path or prove that the complete operation is available below host administration. Therefore host-admin and namespace privilege remain unknown, with neither a privilege penalty nor an unprivileged-trigger bonus. No special hardware or race is required.

4. Primitive and severity reading

The exact primitive is a one-entry heap OOB write caused by checking capacity without accounting for the reserved first entry and pre-incremented push. This is not a UAF. The write value and adjacent victim are not shown to be attacker-selected, limiting claims beyond corruption and crash. Nevertheless, the confirmed heap overwrite is a Class A corruption primitive whose exploit relevance cannot safely be auto-downgraded without allocator-layout and call-path review. Important severity is a candidate, not proven LPE or arbitrary kernel access.

5. One-sentence report phrase

A sufficiently nested tracing filter causes a confirmed one-entry heap overwrite beyond the operator stack, with credible crash impact and unresolved adjacent-object exploitability.

6. Manual review advice

MANUAL CHECK REQUIRED

Review the overwritten entry width, allocator adjacency, filter-write permissions, and whether corrupted neighboring state is subsequently consumed.

7. Call-site confidence

Call-site confidence: medium

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

Patch: tracing: Check operator stack capacity before pushing
Commit: a866a8a753922cdf7710b8eb3f686dc0efc06505
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a866a8a753922cdf7710b8eb3f686dc0efc06505

Commit description:

predicate_parse() uses the first operator stack entry before processing
any opening parentheses. A push then increments top before writing.
The existing check still permits that write when top already points to
the last allocated entry, which can access beyond the operator stack.

Reject the push when top reaches the last allocated entry.

Cc: stable@vger.kernel.org
Fixes: 39f7c41 ("tracing/filters: Enable filtering a cpumask field by another cpumask")
Link: https://patch.msgid.link/atELO9L0I9kg5CpC@Taeyangs-MacBook-Pro.local
Signed-off-by: Taeyang Lee <0wn@theori.io>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/trace/trace_events_filter.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=a866a8a753922cdf7710b8eb3f686dc0efc06505

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

The original Linux kernel CVE announcement for CVE-2026-99142 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99142

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:L/I:L/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: unknown
  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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).