tracing: Set the trace clock before registering the histogram trigger

From: Donggeun Yoo <[email protected]>

[ Upstream commit 6ede78d0563a2a3ae3e46f9c07cedb5d79645429 ]

hist_register_trigger() puts the trigger on the global named_triggers list in cmd_ops->init(), and only then sets the trace clock:

	if (data->cmd_ops->init) {
		ret = data->cmd_ops->init(data);
		if (ret < 0)
			goto out;
	}

	if (hist_data->enable_timestamps) {
		ret = tracing_set_clock(file->tr, hist_data->attrs->clock);
		if (ret) {
			hist_err(tr, HIST_ERR_SET_CLOCK_FAIL, errpos(clock));
			goto out;
		}

The clock string is not checked anywhere before that call, so a named trigger using common_timestamp with an unknown clock fails after it has already become findable. event_hist_trigger_parse() then frees it without taking it off the list, and the next lookup by name reads the freed object:

~# cd /sys/kernel/tracing/events/sched/sched_switch
~# echo 'hist:name=foo:keys=common_pid:ts=common_timestamp:clock=bogus' > trigger
bash: echo: write error: Invalid argument
~# echo 'hist:name=foo:keys=common_pid' > trigger

  BUG: KASAN: slab-use-after-free in find_named_trigger+0xac/0xc0
  Read of size 8 at addr ffff88800915d760 by task init/1
   find_named_trigger+0xac/0xc0
   hist_register_trigger+0xc1/0x900
   event_hist_trigger_parse+0x3146/0x6af0
   event_trigger_write+0xce/0x160
  Freed by task 63:
   kfree+0x154/0x420
   trigger_kthread_fn+0xfd/0x160

Set the clock before the trigger is registered, so that nothing which can fail runs after it is published, the way commit 6f86bdeab633 ("tracing: Fix bad hist from corrupting named_triggers list") moved the registration below the rest of the setup.

tracing_set_filter_buffering() is reference counted, so the init failure path has to drop the reference that the clock block now takes first.

[Lee: Call data->ops->init(data->ops, data) as b052d70f7c9c ("tracing:
Merge struct event_trigger_ops into struct event_command") and 476705419518 ("tracing: Remove redundant trigger_ops params") are not present]

---
 kernel/trace/trace_events_hist.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

index 69a7e83926a2..3105208e517c 100644
@@ -5641,12 +5641,6 @@ static int hist_register_trigger(char *glob, struct event_trigger_ops *ops,
 		data->ops = &event_hist_trigger_named_ops;
 	}
 
-	if (data->ops->init) {
-		ret = data->ops->init(data->ops, data);
-		if (ret < 0)
-			goto out;
-	}
-
 	if (hist_data->enable_timestamps) {
 		char *clock = hist_data->attrs->clock;
 

@@ -5659,6 +5653,15 @@ static int hist_register_trigger(char *glob, struct event_trigger_ops *ops,
 		tracing_set_filter_buffering(file->tr, true);
 	}
 
+	if (data->ops->init) {
+		ret = data->ops->init(data->ops, data);
+		if (ret < 0) {
+			if (hist_data->enable_timestamps)
+				tracing_set_filter_buffering(file->tr, false);
+			goto out;
+		}
+	}
+
 	if (named_data)
 		destroy_hist_data(hist_data);
 

2.56.0.385.gd3acb90ef8-goog

Queued for 5.15, thanks.

Thanks,
Sasha