* [CVE-2026-99233][MODERATE 7.0] tracing/histograms: Verify event_call identity when updating synth_event refcount
@ 2026-11-03 2:53 21% AL-KERNEL
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From: AL-KERNEL @ 2026-11-03 2:53 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99233
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tracing/histograms: Verify event_call identity when updating synth_event refcount
Commit: 381f543ed35c434d5b707369aa5dcd69ecf440af
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=381f543ed35c434d5b707369aa5dcd69ecf440af
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99233
Analysis date: Mon, 02 Nov 2026 22:53:13 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A trace-event basename collision can underflow synthetic-event lifetime accounting and leave an active histogram action holding a freed structured kernel object.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99233 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99233
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-2026-99233 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99233 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 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:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-911;CWE-706;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A trace event basename collision can make histogram registration and removal update the reference count of an unrelated synthetic event because the lookup checks only the event name and not the trace_event_call identity. A crafted operation order can decrement se ref without a matching increment and allow the synthetic event to be freed while an active histogram action still holds its pointer. This provides a concrete kernel use-after-free candidate with likely denial of service and a plausible memory corruption or privilege escalation path if the freed object can be reclaimed and its later use influenced. For the CVSS the PR:H base case reflects the usual requirement for privileged write access to tracefs. For the CVSS the PR:L paranoid case covers delegated tracing access or a service that permits a low privileged user to configure synthetic events and histogram triggers. The issue is local and is not directly reachable through network traffic. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7.8 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 1 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 LEAK UAF LINUS KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
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: 4.
* Conservative score: 4; the Real-UAF floor 4 is satisfied but does not change the arithmetic.
* Raw paranoid score: 5.
* Paranoid score: 5; no cap or additional floor applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 real UAF/lifetime violation: basename-only lookup can decrement the wrong synthetic event reference, freeing `synth_event` while an active histogram action retains its pointer.
* +1 attacker-influenced lifetime manipulation: a tracing-control actor can arrange registration, synthetic-event creation, and later removal to produce the unmatched decrement.
* +1 plausible kernel crash/DoS from subsequent use of the freed active object.
* Paranoid +1 structured stale-object leverage: the retained pointer belongs to an active synthetic-event action, and attacker-configured event state may be consumed after free. Exact consumed fields and reclaim behavior are not supplied, so this is not Class A.
* No privilege penalty: host-admin-equivalent access is not positively established. Conversely, delegated or ordinary-user tracing access is not proven, so no local-unprivileged bonus applies.
* No controlled reclaim, directed write, callback control, disclosure, LPE, or C/I impact is shown.
* No concrete repeated release/refcount operation on the same dangling reference is shown; its dedicated +1 and floor 5 were not applied.
* No extra availability point: it would duplicate the same potential crash.
3. Reachability analysis
The trigger uses tracefs histogram and synthetic-event configuration. The supplied hints describe privileged tracefs access as usual and delegated tracing as possible, but do not establish the complete permission path for either deployment. Thus host-admin and namespace-scoped privilege remain unknown. The sequence is ordered rather than a hard race, and no special hardware, network reachability, or retry-accumulated B5-R mechanism is involved.
4. Primitive and severity reading
This is an active structured-object UAF, not fixed stale storage: erroneous identity matching corrupts `synth_event` lifetime accounting and can free it while an active histogram action retains the pointer. The shown effect supports crash risk; useful post-reclaim consumption remains unresolved. It is therefore CLASS_B, not CLASS_A. A credible delegated tracing boundary and privileged kernel consumer remain insufficiently resolved, making this an Important candidate under active-state trust-boundary review. Auto-downgrade is unsafe until stale-action consumption and effective tracing permissions are checked.
There is no authorization DENY-to-ALLOW defect, so B4-PENDING does not apply.
5. One-sentence report phrase
A trace-event basename collision can underflow synthetic-event lifetime accounting and leave an active histogram action holding a freed structured kernel object.
6. Manual review advice
MANUAL CHECK REQUIRED
Confirm tracefs delegation reachability, the stale histogram-action consumer, fields dereferenced after free, and whether attacker-shaped reclaim is practical.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: tracing/histograms: Verify event_call identity when updating synth_event refcount
Commit: 381f543ed35c434d5b707369aa5dcd69ecf440af
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=381f543ed35c434d5b707369aa5dcd69ecf440af
Commit description:
In event_hist_trigger_parse() and hist_unreg_all(), the histogram
registration and unregistration paths look up find_synth_event() using
trace_event_name(file->event_call) and adjust se->ref if a match is
found.
Because find_synth_event() only compares the event basename, registering
or removing a histogram on a non-synthetic trace event whose basename
matches a synthetic event erroneously modifies the synthetic event's
reference count. If a histogram is registered on the non-synthetic
event before the synthetic event is created and removed afterwards,
se->ref is decremented without a matching increment, allowing the
synthetic event to be freed while an active histogram action still holds
a pointer to it.
Introduce find_synth_event_call() to verify that &se->call matches
file->event_call before adjusting se->ref.
Fixes: 4b14793 ("tracing: Add support for 'synthetic' events")
Cc: [email protected]
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Lee Jones <[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
kernel/trace/trace_events_synth.c
kernel/trace/trace_synth.h
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=381f543ed35c434d5b707369aa5dcd69ecf440af
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99233 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99233
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:L/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7.8
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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