[for-next][PATCH 4/6] tracing: Save the event file instead of the compatible histogram

[Steven Rostedt]

From: Donggeun Yoo <donggeunyoo.kernel@xxxxxxxxx>

create_field_var_hist() creates a histogram on the matched event to
supply a field variable, and records it on the target so it can be torn
down later:

	hist_data = find_compatible_hist(target_hist_data, file);
	...
	/* Save the compatible histogram information */
	var_hist->hist_data = hist_data;

hist_data is not the histogram it creates. It is one that was already
there, whose keys the new one copies. The saved pointer has a single
user, unregister_field_var_hists(), which runs when the target is
removed:

	file = hist_data->field_var_hists[i]->hist_data->event_file;

find_compatible_hist() matches on keys alone, so it can pick one with no
variables of its own. check_var_refs() then returns false and the user
can remove it while the target still points at it:

 BUG: KASAN: slab-use-after-free in event_hist_trigger_free+0x2b2/0x320
 Read of size 8 at addr ffff8880093b90e0 by task init/1

 Freed by task 1:
  kfree+0x154/0x420
  event_hist_trigger_free+0x1d5/0x320
  event_hist_trigger_parse+0x35f3/0x69e0
  trigger_process_regex+0x1a6/0x250

Save the event file instead. It is all the dereference ever wanted, and
it does not go away when the user removes a trigger.

Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Link: https://lore.kernel.org/all/20260914110753.221E61F000FF@xxxxxxxxxxxxxxx/
Link: https://patch.msgid.link/20260914234754.3676905-1-donggeunyoo.kernel@xxxxxxxxx
Fixes: 02205a6752f2 ("tracing: Add support for 'field variables'")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@xxxxxxxxx>
Assisted-by: Claude:claude-fable-5
Acked-by: Tom Zanussi <zanussi@xxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/trace/trace_events_hist.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 5527605758cb..42b89171134d 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -557,7 +557,7 @@ struct field_var {
 };
 
 struct field_var_hist {
-	struct hist_trigger_data	*hist_data;
+	struct trace_event_file		*file;
 	char				*cmd;
 };
 
@@ -3129,8 +3129,8 @@ create_field_var_hist(struct hist_trigger_data *target_hist_data,
 		return ERR_PTR(-ENOMEM);
 	}
 
-	/* Save the compatible histogram information */
-	var_hist->hist_data = hist_data;
+	/* Needed to remove the histogram when the target goes away */
+	var_hist->file = file;
 
 	/* Create the new histogram with our variable */
 	ret = event_hist_trigger_parse(&trigger_hist_cmd, file,
@@ -6362,7 +6362,7 @@ static void unregister_field_var_hists(struct hist_trigger_data *hist_data)
 	int ret;
 
 	for (i = 0; i < hist_data->n_field_var_hists; i++) {
-		file = hist_data->field_var_hists[i]->hist_data->event_file;
+		file = hist_data->field_var_hists[i]->file;
 		cmd = hist_data->field_var_hists[i]->cmd;
 		ret = event_hist_trigger_parse(&trigger_hist_cmd, file,
 					       "!hist", "hist", cmd);
-- 
2.53.0