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.
Prioritize with Active Threat Intelligence
With curated Threat Intelligence, you can see which vulnerabilities truly put you at risk, prioritize what matters most, and act before attackers do.
Explore Intelligence Hub