* [CVE-2026-99234][LOW] tracing: Unregister field variable histograms when the command fails
@ 2026-11-03 2:34 21% AL-KERNEL
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From: AL-KERNEL @ 2026-11-03 2:34 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99234
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: tracing: Unregister field variable histograms when the command fails
Commit: f21351d1c4cdcc6a68f03636a407c2af1a5e1e10
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f21351d1c4cdcc6a68f03636a407c2af1a5e1e10
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99234
Analysis date: Mon, 02 Nov 2026 22:34:55 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
A failed trace histogram command can leave an untracked histogram active, causing persistent resource use and event-processing overhead without demonstrated memory corruption.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99234 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99234
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-2026-99234 LOW https://www.kernelcve.org/list/?q=CVE-2026-99234 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=1 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';CWE-459;CWE-404;*CWE-772;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '3.3';DESCR 'A failed histogram trigger command can leave an automatically created field variable histogram registered on another trace event. The orphaned histogram remains active and continues counting events because the error cleanup frees its tracking entry lacking first unregistering the trigger. This is a cleanup and resource retention bug rather than demonstrated UAF or other memory corruption. For the CVSS the PR:H value reflects that writing tracefs histogram triggers normally requires root or CAP_SYS_ADMIN. For the CVSS the paranoid PR:L value covers deployments that delegate tracing access to a service or restricted operator. The supported impact is limited availability degradation from retained memory and event processing overhead. Reliable host wide exhaustion is not established and the issue is not remotely reachable. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 3.3 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE INIT TRACE LEAK UAF LINUS - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 2.
* Conservative score: 2; no floor or cap applied.
* Raw paranoid score: 2.
* Paranoid score: 2; no additional technically supported primitive.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Concrete persistent kernel resource retention: failed command cleanup destroys `field_var_hists[]` tracking entries without unregistering the already active field-variable histogram.
* +1 Limited availability impact: the orphaned histogram continues counting events, retaining memory and imposing event-processing overhead independently of the bookkeeping leak.
* No privilege penalty: standard tracefs trigger writes normally require root or CAP_SYS_ADMIN, but supplied evidence also mentions possible delegated write access; the complete privilege boundary is therefore unresolved.
* No UAF or memory-corruption score: the patch shows an active trigger becoming untracked, not the trigger dereferencing the freed tracking entry.
* No reliable crash, host-wide exhaustion, disclosure, integrity impact, arbitrary write, or authorization bypass is shown.
* B5-R does not apply: there is no heap OOB write, retry-accumulated index, or later consumption of corrupted structured state.
* B4-PENDING does not apply: no mandatory authorization guard changes from DENY to ALLOW.
3. Reachability analysis
The trigger requires writing crafted histogram commands to tracefs. Ordinary deployments restrict this to root or CAP_SYS_ADMIN, while delegated tracefs permissions could permit a restricted service or operator to invoke it. The supplied evidence does not establish either host-admin-only or concrete below-host-admin reachability, so `HostAdminRequired` and host UID requirements remain unknown. No namespace-scoped capability path, remote path, special hardware, or timing race is involved. Repeated failed commands plausibly create additional orphaned histograms, but reliable host-wide exhaustion is not demonstrated.
4. Primitive and severity reading
The exact defect is failed-command cleanup that frees registration-tracking entries without unregistering the corresponding active histogram triggers. This is persistent resource retention and active processing overhead, classified as CLASS_C. It is not a stale-object dereference, repeated release, OOB access, or protected-state corruption. The shown impact is limited availability degradation; stronger exhaustion or memory-safety effects are unsupported. Consequently, no Important review candidate remains and automatic Moderate/Low-like downgrade is permissible.
5. One-sentence report phrase
A failed trace histogram command can leave an untracked histogram active, causing persistent resource use and event-processing overhead without demonstrated memory corruption.
6. Manual review advice
NO MANUAL CHECK NEEDED
The patch supports only a constrained cleanup/resource-retention defect with no unresolved corruption or security-boundary primitive.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: tracing: Unregister field variable histograms when the command fails
Commit: f21351d1c4cdcc6a68f03636a407c2af1a5e1e10
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f21351d1c4cdcc6a68f03636a407c2af1a5e1e10
Commit description:
An action that names a field on another event needs that field as a
variable, so create_field_var_hist() registers a second hist trigger on
that event to provide it, and records it in
target_hist_data->field_var_hists[].
If a later part of the same command fails, create_actions() returns an
error and event_hist_trigger_parse() jumps to out_free. The write fails
with -EINVAL and sched_switch gets no trigger, but:
# echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> \
events/sched/sched_waking/trigger
# echo 'my_synth u64 lat; int a; int b' >> synthetic_events
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:\
onmatch(sched.sched_waking).my_synth($wakeup_lat,prio,nosuchfld)'\
>> events/sched/sched_switch/trigger
# cat events/sched/sched_waking/trigger
hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
hist:keys=pid:vals=hitcount:synthetic_prio=prio:sort=hitcount:size=2048 [active]
The second histogram is the one created for 'prio'. It is still active and
still counting.
out_free reaches destroy_field_var_hists(), which frees the
field_var_hists[] entries, but never unregister_field_var_hists(), which is
what removes the triggers and runs only from event_hist_trigger_free(). The
entry is freed and the trigger is left with nothing tracking it.
Unregister the field variable histograms in out_free as well, and record
each one as soon as its trigger is registered rather than after the lookup
that follows it, so no exit can leave a registered trigger untracked. The
entry then belongs to field_var_hists[] and is released by
destroy_field_var_hists(), so freeing it on that exit would be a double
free.
Cc: [email protected]
Link: https://patch.msgid.link/[email protected]
Fixes: 02205a6 ("tracing: Add support for 'field variables'")
Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Donggeun Yoo <[email protected]>
Assisted-by: Claude:claude-fable-5
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=f21351d1c4cdcc6a68f03636a407c2af1a5e1e10
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99234 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99234
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:L/UI:N/S:U/C:N/I:N/A:L
The Best / paranoid CVSS score: 3.3
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: 2
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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