tracing: Fix use-after-free freeing trigger private data

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 private_data_free() callback that the free kthread runs after its grace period, and free enable_data from there.

---
 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(-)

index 80fe152af1dd..bf77331f56a4 100644
@@ -1941,6 +1941,7 @@ struct event_trigger_data {
 	struct list_head		named_list;
 	struct event_trigger_data	*named_data;
 	struct llist_node		llist;
+	void				(*private_data_free)(struct event_trigger_data *data);
 };
 
 /* Avoid typos */

index 82ce492ab268..58d28cd1afa3 100644
@@ -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);
 	}
 }

index 655db2e82513..46e60b70a4bb 100644
@@ -38,6 +38,13 @@ static void trigger_create_kthread_locked(void)
 	}
 }
 
+static void trigger_data_free_one(struct event_trigger_data *data)
+{
+	if (data->private_data_free)
+		data->private_data_free(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_private_data_free(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->private_data_free = enable_trigger_private_data_free;
 		trace_event_put_ref(enable_data->file->event_call);
 		trigger_data_free(data);
-		kfree(enable_data);
 	}
 }
 

2.53.0

[ Replying to Sashiko ]

Where was it published to file->triggers?

I see it happening as the last operation that can fail. If that fails, then we hit the error path where it is not registered. Is there someplace it can become visible and then fail that I'm not seeing?

From what I see, the last command before "success" is:

  ret = hist_trigger_enable(trigger_data, file);

And that does:

static int hist_trigger_enable(struct event_trigger_data *data,
struct trace_event_file *file)

{
	int ret = 0;

	list_add_tail_rcu(&data->list, &file->triggers);

	update_cond_flag(file);

	if (trace_event_trigger_enable_disable(file, 1) < 0) {
		list_del_rcu(&data->list);
		update_cond_flag(file);
		ret--;
	}

	return ret;
}

Now it adds to the file trigger and then tries to enable that event. If it fails to enable it, it removes it from the list. But if the event is not enabled, it can't be racing with this.

Hmm, this does look like it can be an issue. I'll write up a patch on top of this one.

-- Steve