[PATCH 2/2] tracing: hist: undo the registration when enabling the trigger fails

[Donggeun Yoo]

Commit 6f86bdeab633 ("tracing: Fix bad hist from corrupting named_triggers
list") described how a trigger that is registered but not on file->triggers
ends up freed while still on the global named_triggers list, and moved the
registration down so that hist_trigger_enable() follows it immediately. One
path still gets there. hist_trigger_enable() adds the trigger and takes it
straight back out when the event cannot be enabled:

	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--;
	}

so the list walk in hist_unregister_trigger() matches nothing, test stays
NULL, and the ->free() that would call del_named_trigger() is skipped.
out_unreg falls through to out_free, which frees the trigger anyway:

 BUG: KASAN: slab-use-after-free in find_named_trigger+0xac/0xc0
 Read of size 8 at addr ffff8880091d3160 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 69:
  kfree+0x154/0x420
  trigger_kthread_fn+0xfd/0x160

Leave the trigger where hist_unregister_trigger() can find it and let that
undo the registration, which is the only code that knows all of what
cmd_ops->init() took: the named list entry, the hist_pad reference, the
reference on the trigger a named histogram is shared with, and the copied
cmd_ops. It also pairs the failed trace_event_trigger_enable_disable(),
whose sm_ref and buffered event reference are otherwise left behind.

Since ->free() releases trigger_data and, for a trigger that does not share
its histogram, hist_data with it, out_unreg can no longer fall through to
out_free. For a trigger that does share, hist_register_trigger() has
already destroyed the caller's hist_data, so the fall-through was reading
freed memory there as well.

Move the enable_timestamps check in hist_unregister_trigger() above the
->free() call for the same reason: hist_data does not outlive it once the
trigger being removed is the one that owns it.

Reported-by: Sashiko AI <sashiko-bot@xxxxxxxxxx>
Closes: https://lore.kernel.org/linux-trace-kernel/20260907092944.3950E1F00A3D@xxxxxxxxxxxxxxx/
Fixes: 067fe038e70f ("tracing: Add variable reference handling to hist triggers")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@xxxxxxxxx>
---
 kernel/trace/trace_events_hist.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index c6c04926bdf0..1de224a5a2bb 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -6679,11 +6679,12 @@ static int hist_trigger_enable(struct event_trigger_data *data,
 
 	update_cond_flag(file);
 
-	if (trace_event_trigger_enable_disable(file, 1) < 0) {
-		list_del_rcu(&data->list);
-		update_cond_flag(file);
+	/*
+	 * On failure the caller undoes the registration, and
+	 * hist_unregister_trigger() can only find the trigger here.
+	 */
+	if (trace_event_trigger_enable_disable(file, 1) < 0)
 		ret--;
-	}
 
 	return ret;
 }
@@ -6761,13 +6762,13 @@ static void hist_unregister_trigger(char *glob,
 		}
 	}
 
-	if (test && test->cmd_ops->free)
-		test->cmd_ops->free(test);
-
 	if (hist_data->enable_timestamps) {
 		if (!hist_data->remove || test)
 			tracing_set_filter_buffering(file->tr, false);
 	}
+
+	if (test && test->cmd_ops->free)
+		test->cmd_ops->free(test);
 }
 
 static bool hist_file_check_refs(struct trace_event_file *file)
@@ -6972,6 +6973,8 @@ static int event_hist_trigger_parse(struct event_command *cmd_ops,
 	return ret;
  out_unreg:
 	event_trigger_unregister(cmd_ops, file, glob+1, trigger_data);
+	/* The unregister frees trigger_data, skip out_free */
+	goto out;
  out_free:
 	remove_hist_vars(hist_data);
 
-- 
2.53.0