tracing: Save the event file instead of the compatible histogram

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@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://lore.kernel.org/all/20260914110753.221E61F000FF@smtp.kernel.org/
Link: https://patch.msgid.link/20260914234754.3676905-1-donggeunyoo.kernel@gmail.com
Fixes: 02205a6752f2 ("tracing: Add support for 'field variables'")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Assisted-by: Claude:claude-fable-5
Acked-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

0 files changed, 0 insertions, 0 deletions