[PATCH v2] tracing: Restore synchronize_rcu() before freeing trigger private data

[David Carlier <devnexen@gmail.com>]

Commit 61d445af0a7c ("tracing: Add bulk garbage collection of freeing
event_trigger_data") moved the kfree() of event_trigger_data to a kthread
that runs tracepoint_synchronize_unregister() before freeing. That removed
the synchronize_rcu() the trigger .free callbacks used to get implicitly and
inline from trigger_data_free().

event_hist_trigger_free(), event_hist_trigger_named_free() and
event_enable_trigger_free() free their satellite data (hist_data, cmd_ops,
enable_data) right after trigger_data_free() returns. With the synchronize
now deferred to the kthread, a concurrent tracepoint handler can still reach
that data through the list_del_rcu()'d trigger, causing a use-after-free.

The teardown must also stay synchronous for correctness: the histogram
cleanup (remove_hist_vars(), unregister_field_var_hists()) has to detach a
synthetic event from the histogram before the trigger-removal write returns.
Deferring it lets a following command race in and fail the synthetic-event
removal with -EBUSY, as caught by
tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-synthetic-eprobe.tc.

Call synchronize_rcu() directly before freeing the satellite data instead.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Fixes: 61d445af0a7c ("tracing: Add bulk garbage collection of freeing event_trigger_data")
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 kernel/trace/trace_events_hist.c    | 3 +++
 kernel/trace/trace_events_trigger.c | 1 +
 2 files changed, 4 insertions(+)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 82ce492ab268..ddd2f70dac4f 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -6349,6 +6349,8 @@ static void event_hist_trigger_free(struct event_trigger_data *data)
 
 		trigger_data_free(data);
 
+		synchronize_rcu();
+
 		remove_hist_vars(hist_data);
 
 		unregister_field_var_hists(hist_data);
@@ -6388,6 +6390,7 @@ static void event_hist_trigger_named_free(struct event_trigger_data *data)
 
 		del_named_trigger(data);
 		trigger_data_free(data);
+		synchronize_rcu();
 		kfree(cmd_ops);
 	}
 }
diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
index 655db2e82513..c3f54f2540b6 100644
--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -1730,6 +1730,7 @@ void event_enable_trigger_free(struct event_trigger_data *data)
 		trace_event_enable_disable(enable_data->file, 0, 1);
 		trace_event_put_ref(enable_data->file->event_call);
 		trigger_data_free(data);
+		synchronize_rcu();
 		kfree(enable_data);
 	}
 }
-- 
2.53.0