[PATCH 1/2] tracing: hist: take the reference before publishing the named trigger

[Donggeun Yoo <donggeunyoo.kernel@gmail.com>]

event_hist_trigger_named_init() puts the trigger on the global
named_triggers list and only then takes the reference on the trigger it
shares its histogram with:

	data->ref++;

	save_named_trigger(data->named_data->name, data);

	ret = event_hist_trigger_init(data->named_data);
	if (ret < 0) {
		kfree(data->cmd_ops);
		data->cmd_ops = &trigger_hist_cmd;
	}

	return ret;

event_hist_trigger_init() fails when alloc_hist_pad() cannot allocate, and
nothing takes the trigger back off the list on the way out.
event_hist_trigger_parse() frees it, and the next lookup by name reads the
freed object:

 BUG: KASAN: slab-use-after-free in find_named_trigger+0xac/0xc0
 Read of size 8 at addr ffff888009346860 by task init/1
  find_named_trigger+0xac/0xc0
  hist_register_trigger+0xc1/0xa00
  event_hist_trigger_parse+0x3146/0x6af0
  event_trigger_write+0xce/0x160
 Freed by task 67:
  kfree+0x154/0x420
  trigger_kthread_fn+0xfd/0x160

Do the reference first and publish once it has succeeded, so that nothing
which can fail runs after the trigger becomes findable.

Reported-by: Sashiko AI <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/linux-trace-kernel/20260907092944.3950E1F00A3D@smtp.kernel.org/
Fixes: 7ab0fc61ce73 ("tracing: Move histogram trigger variables from stack to per CPU structure")
Cc: stable@vger.kernel.org
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
---
 kernel/trace/trace_events_hist.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 963e0d6b61fd..c6c04926bdf0 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -6383,17 +6383,18 @@ static int event_hist_trigger_named_init(struct event_trigger_data *data)
 {
 	int ret;
 
-	data->ref++;
-
-	save_named_trigger(data->named_data->name, data);
-
 	ret = event_hist_trigger_init(data->named_data);
 	if (ret < 0) {
 		kfree(data->cmd_ops);
 		data->cmd_ops = &trigger_hist_cmd;
+		return ret;
 	}
 
-	return ret;
+	data->ref++;
+
+	save_named_trigger(data->named_data->name, data);
+
+	return 0;
 }
 
 static void event_hist_trigger_named_free(struct event_trigger_data *data)

base-commit: df2908090cda368b01ff43709f51890076c56157
-- 
2.53.0