CVE-2026-99140 in Linux
Summary
by MITRE • 10/28/2026
In the Linux kernel, the following vulnerability has been resolved:
tracing: Fix hist trigger timestamps for buffered events
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 d8279bfc5e959 ("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.
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Analysis
by VulDB Data Team • 10/28/2026
The Linux kernel tracing subsystem contains a race condition vulnerability related to the handling of timestamps for events processed by histogram triggers on buffered data. This issue manifests as a probabilistic WARN_ON_ONCE warning within the ring_buffer_event_time_stamp function when specific trace event filters and hist triggers are configured concurrently. The root cause lies in the interaction between the per-CPU buffered event mechanism used during filter evaluation and the subsequent processing of those events by dynamic tracing tools like histogram triggers. When an event file has a filter attached, the kernel optimizes performance by writing initial event data into a temporary per-CPU buffer rather than immediately committing it to the main ring buffer. This optimization is designed to avoid unnecessary overhead for events that might be discarded later if they do not match the filter criteria. However, this buffering mechanism alters the state of the cpu_buffer->committing counter, which tracks active commit operations within the ring buffer subsystem.
The vulnerability occurs when a histogram trigger is registered or activated while an event is in this buffered state but has not yet been fully committed to the ring buffer structure that normal tracing functions expect. Specifically, during the execution of trace_event_buffer_lock_reserve, if the system determines that filtering is active and buffering should occur, it returns early without incrementing the cpu_buffer->committing counter via local_inc. Later, when the event trigger logic processes this buffered entry through hist_field_timestamp to record its timestamp for histogram aggregation, it calls ring_buffer_event_time_stamp. This function expects a non-zero nesting count in the committing variable as an indicator of valid buffer context. Because the commit path was bypassed due to buffering, the counter remains zero, causing the WARN_ON_ONCE(!nest) condition to trigger. While this primarily results in a kernel warning and potentially incorrect timestamp data rather than a system crash or privilege escalation, it indicates a flaw in how temporal metadata is handled across different tracing subsystems during concurrent operations.
From an operational perspective, this vulnerability affects systems relying on detailed process scheduling analysis using the sched_switch trace point combined with histogram triggers for performance profiling or debugging context switching behavior. The presence of such warnings can clutter kernel logs and potentially obscure other critical issues if not addressed. Furthermore, recording a bogus timestamp due to accessing uninitialized or incorrectly contextualized buffer data could lead to inaccurate forensic conclusions regarding event timing sequences. Although the immediate impact is limited to diagnostic noise and potential data integrity issues within trace outputs, it represents a violation of internal API contracts between different parts of the tracing infrastructure. The issue highlights the complexity of maintaining consistency in lock-free, per-CPU buffering mechanisms when dynamic instrumentation tools interact with them asynchronously.
The resolution involves updating the hist_field_timestamp function to utilize the tracing_event_time_stamp API instead of directly calling ring_buffer_event_time_stamp. This API was originally introduced to handle cases where events reside in temporary buffers rather than committed ring buffer entries. By using this specialized accessor, the kernel correctly retrieves the current timestamp for buffered events without relying on the committing counter state that is invalid during buffering phases. This change ensures that histogram triggers can safely process filtered and buffered events by recording a valid time stamp that reflects when the event was actually captured, rather than triggering warnings or producing erroneous data due to structural mismatches in buffer handling logic.
This vulnerability aligns with CWE-362, which describes concurrent execution issues leading to race conditions where shared resources are accessed without proper synchronization or state validation. In this context, the shared resource is the ring buffer commit state and its associated metadata counters. The interaction between the filter buffering mechanism and the histogram trigger processing logic creates a window where assumptions about buffer states become invalid. Additionally, from an ATT&CK perspective, while not directly exploitable for malicious purposes by external attackers in most standard configurations, such kernel-level inconsistencies can be relevant to techniques involving system discovery or defense evasion if an attacker were attempting to manipulate trace data integrity or obscure their activities within the tracing subsystem. However, its primary classification remains a stability and correctness defect within the operating system core.
To mitigate this issue, users should ensure that Linux kernels are updated with patches addressing this specific hist trigger timestamp handling logic. For administrators unable to update immediately, avoiding the combination of complex sched_switch filters with histogram triggers on high-frequency systems may reduce the likelihood of triggering the warning, though complete avoidance is difficult given the nature of context switching events. The fix ensures robustness in dynamic tracing operations by decoupling timestamp retrieval from strict ring buffer commit state requirements for buffered entries. This maintains data integrity and prevents kernel log pollution during intensive profiling scenarios involving filtered event streams.