* [CVE-2026-99140][LOW] tracing: Fix hist trigger timestamps for buffered events
@ 2026-10-29 2:38 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-10-29 2:38 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99140
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: tracing: Fix hist trigger timestamps for buffered events
Commit: 09c7cce2e59e6b3b1807520bcb1980c561f15d1e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=09c7cce2e59e6b3b1807520bcb1980c561f15d1e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99140
Analysis date: Wed, 28 Oct 2026 22:38:36 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
A privileged tracefs configuration race can emit a WARN_ON_ONCE and record an incorrect histogram timestamp without a demonstrated memory-corruption or security-boundary primitive.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99140 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99140
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-2026-99140 LOW https://www.kernelcve.org/list/?q=CVE-2026-99140 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=0 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:H/PR:H/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-682;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.1';DESCR 'A race can occur when a filtered trace event is stored in the per CPU temporary buffer while a hist trigger is concurrently attached. The buffered event has no active ring buffer commit nesting, so ring_buffer_event_time_stamp can raise WARN_ON_ONCE and produce an invalid histogram timestamp. This is a race and timestamp logic bug with no demonstrated UAF, OOB access, information leak, or write primitive. For the CVSS the PR:H value is selected because configuring tracefs event filters and histogram triggers normally requires administrative tracing access. AC:H reflects the need to hit the concurrent trigger registration window. The normal impact is limited to incorrect tracing data and a warning, while availability can become high only on systems configured to panic on warnings. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.1 and the production ActionableScore returned 0 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 0.0 and ActionableScore 0 with primitive class CLASS_C 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 0) MAYBE WARNONLY TRACE RACE LEAK HARDWARE LINUS - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=0
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 0.
* Conservative score: 0.
* Raw paranoid score: 0.
* Paranoid score: 0.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +0 WARN_ON_ONCE: the race calls `ring_buffer_event_time_stamp()` without active commit nesting, producing a warning and bogus histogram timestamp. WARN alone does not establish fatal availability impact.
* +0 constrained metadata error: the incorrect value affects tracing histogram timestamps, not active security or ownership state.
* No UAF, OOB access, write primitive, authorization bypass, disclosure, or protected-state corruption is shown.
* No crash point is awarded: panic requires a separate panic-on-warning configuration and is not established by the patch.
* B5-R and B3-PENDING do not apply: there is no heap overwrite or shared-backing write.
* V33/V34 do not apply: no attacker-influenced active structured state is consumed across a trust boundary.
3. Reachability analysis
The demonstrated trigger requires configuring tracefs event filters and histogram triggers while scheduler events race concurrently. These controls normally require host administrative tracing access and are not shown to be user-namespace scoped. Numeric host UID 0 is not specifically required because delegated tracefs access is possible in principle, but no below-host-admin complete trigger is evidenced. The race is probabilistic, although syzkaller can reproduce it.
4. Primitive and severity reading
The defect is a race-induced timestamp API mismatch: a temporary buffered event lacks ring-buffer commit nesting, causing a warning and an invalid tracing timestamp. This is passive diagnostic-state corruption, not kernel memory corruption or an authorization decision failure. The fix selects the API designed to handle both temporary and ring-buffer events. No non-crash exploitation leverage or security boundary is shown; therefore `PrimitiveClass=NONE`, `ImportantCandidate=NO`, and automatic downgrade is allowed.
5. One-sentence report phrase
A privileged tracefs configuration race can emit a WARN_ON_ONCE and record an incorrect histogram timestamp without a demonstrated memory-corruption or security-boundary primitive.
6. Manual review advice
NO MANUAL CHECK NEEDED
The patch and call trace fully explain a warning-only tracing timestamp bug with no Important-preserving primitive.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: tracing: Fix hist trigger timestamps for buffered events
Commit: 09c7cce2e59e6b3b1807520bcb1980c561f15d1e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=09c7cce2e59e6b3b1807520bcb1980c561f15d1e
Commit description:
Syzkaller triggered a WARN_ON_ONCE(!nest) warning:
WARNING: kernel/trace/ring_buffer.c:821 at ring_buffer_event_time_stamp
Call trace:
ring_buffer_event_time_stamp
hist_field_timestamp
hist_fn_call
event_hist_trigger
event_triggers_call
__event_trigger_test_discard
trace_event_buffer_commit
do_trace_event_raw_event_sched_switch
trace_event_raw_event_sched_switch
The warning is probabilistically reproducible and is related to syzkaller
executing the following commands:
echo 'prev_pid == 999999' > events/sched/sched_switch/filter
echo 'hist:keys=common_timestamp' > events/sched/sched_switch/trigger
The specific triggering process is as follows:
CPU 0 CPU 1
(context switching) (echo 'hist:keys=common_timestamp' > \
events/sched/sched_switch/trigger)
do_trace_event_raw_event_##call
trace_event_buffer_lock_reserve
if (!tr->no_filter_buffering_ref &&
trace_file->flags & EVENT_FILE_FL_FILTERED)
return entry; // return directly
event_hist_trigger_parse
hist_register_trigger
tracing_set_filter_buffering(file->tr, true);
tr->no_filter_buffering_ref++;
__trace_buffer_lock_reserve
rb_start_commit
local_inc(&cpu_buffer->committing); // skipped and not executed
trace_event_buffer_commit
__event_trigger_test_discard
event_triggers_call
event_hist_trigger
hist_fn_call
hist_field_timestamp
ring_buffer_event_time_stamp
nest = local_read(&cpu_buffer->committing);
WARN_ON_ONCE(!nest) // trigger warning
The root cause is that when an event file has a filter attached and the
hist trigger has not yet been attached, trace_event_buffer_lock_reserve()
writes the event into the per-CPU trace_buffered_event temp buffer instead
of reserving it in the ring buffer, so cpu_buffer->committing is not
incremented. When the event is later processed at commit time by a hist
trigger, hist_field_timestamp() -> ring_buffer_event_time_stamp() sees a
zero committing count, triggering WARN_ON_ONCE(!nest).
tracing_event_time_stamp() was added by commit d8279bf ("tracing:
Add tracing_event_time_stamp() API") for exactly this case: if the event
is the per-CPU trace_buffered_event, it returns the current ring buffer
timestamp. But it never gained a caller. Use it in hist_field_timestamp(),
so that a hist trigger that races onto a buffered event records the
current time, which is only marginally later than when the event was
recorded, instead of warning and recording a bogus timestamp.
Cc: [email protected]
Link: https://patch.msgid.link/[email protected]
Fixes: b94bc80 ("tracing: Use a no_filter_buffering_ref to stop using the filter buffer")
Signed-off-by: Tengda Wu <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
kernel/trace/ring_buffer.c
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=09c7cce2e59e6b3b1807520bcb1980c561f15d1e
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99140 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99140
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:H/PR:H/UI:N/S:U/C:N/I:N/A:L
The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS score: 4.1
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
^ permalink raw reply [relevance 21%]
Results 1-1 of 1 | reverse | sort options + mbox downloads above
-- links below jump to the message on this page --
2026-10-29 2:38 21% [CVE-2026-99140][LOW] tracing: Fix hist trigger timestamps for buffered events AL-KERNEL
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox