Linux kernel: failed tracing command leaves histogram trigger active

In the Linux kernel, the following vulnerability has been resolved:

tracing: Unregister field variable histograms when the command fails

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.