[PATCH v3] tracing: Fix use-after-free 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 synchronization 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
synchronization 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 histogram teardown must stay synchronous: remove_hist_vars() and
unregister_field_var_hists() have to detach a synthetic event from the
histogram before the trigger-removal write returns, otherwise a following
command races in and the synthetic-event removal fails with -EBUSY, as the
trigger-synthetic-eprobe.tc selftest catches. Make those callbacks wait
with the correct barrier - tracepoint_synchronize_unregister(), matching
the free kthread - before freeing.

The enable trigger has no such synchronous requirement, and a blocking
synchronize there would re-serialize the path that commit deliberately
deferred. Give it an optional free_private() callback that the free kthread
runs after its grace period, and free enable_data from there.

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.h                |  1 +
 kernel/trace/trace_events_hist.c    |  2 ++
 kernel/trace/trace_events_trigger.c | 18 +++++++++++++++---
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 80fe152af1dd..fceef65b9601 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1941,6 +1941,7 @@ struct event_trigger_data {
 	struct list_head		named_list;
 	struct event_trigger_data	*named_data;
 	struct llist_node		llist;
+	void                            (*free_private)(struct event_trigger_data *data);
 };
 
 /* Avoid typos */
diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 82ce492ab268..58d28cd1afa3 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -6349,6 +6349,7 @@ static void event_hist_trigger_free(struct event_trigger_data *data)
 
 		trigger_data_free(data);
 
+		tracepoint_synchronize_unregister();
 		remove_hist_vars(hist_data);
 
 		unregister_field_var_hists(hist_data);
@@ -6388,6 +6389,7 @@ static void event_hist_trigger_named_free(struct event_trigger_data *data)
 
 		del_named_trigger(data);
 		trigger_data_free(data);
+		tracepoint_synchronize_unregister();
 		kfree(cmd_ops);
 	}
 }
diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
index 655db2e82513..328d9657bcf2 100644
--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -38,6 +38,13 @@ static void trigger_create_kthread_locked(void)
 	}
 }
 
+static void trigger_data_free_one(struct event_trigger_data *data)
+{
+	if (data->free_private)
+		data->free_private(data);
+	kfree(data);
+}
+
 static void trigger_data_free_queued_locked(void)
 {
 	struct event_trigger_data *data, *tmp;
@@ -52,7 +59,7 @@ static void trigger_data_free_queued_locked(void)
 	tracepoint_synchronize_unregister();
 
 	llist_for_each_entry_safe(data, tmp, llnodes, llist)
-		kfree(data);
+		trigger_data_free_one(data);
 }
 
 /* Bulk garbage collection of event_trigger_data elements */
@@ -75,7 +82,7 @@ static int trigger_kthread_fn(void *ignore)
 		tracepoint_synchronize_unregister();
 
 		llist_for_each_entry_safe(data, tmp, llnodes, llist)
-			kfree(data);
+			trigger_data_free_one(data);
 	}
 
 	return 0;
@@ -1717,6 +1724,11 @@ int event_enable_trigger_print(struct seq_file *m,
 	return 0;
 }
 
+static void enable_trigger_free_private(struct event_trigger_data *data)
+{
+	kfree(data->private_data);
+}
+
 void event_enable_trigger_free(struct event_trigger_data *data)
 {
 	struct enable_trigger_data *enable_data = data->private_data;
@@ -1728,9 +1740,9 @@ void event_enable_trigger_free(struct event_trigger_data *data)
 	if (!data->ref) {
 		/* Remove the SOFT_MODE flag */
 		trace_event_enable_disable(enable_data->file, 0, 1);
+		data->free_private = enable_trigger_free_private;
 		trace_event_put_ref(enable_data->file->event_call);
 		trigger_data_free(data);
-		kfree(enable_data);
 	}
 }
 
-- 
2.53.0