[PATCH v2] tracing/user_events: Don't free an event that a dynamic event still references

[David Carlier <devnexen@gmail.com>]

An eprobe takes a reference on the call of the event it attaches to,
and kprobe, uprobe, fprobe and synth events all refuse to go away while
trace_event_dyn_busy() says such a reference exists. user_events never
checks it, so a user event can be freed under an eprobe:

  echo 'u:test u32 a' >> dynamic_events
  echo 'e:ep user_events/test a=$a' >> dynamic_events
  echo '-:user_events/test' >> dynamic_events
  echo '-:eprobes/ep' >> dynamic_events

The last write drops the reference on the freed call.

If the last fd of a non-persistent event closes while such a reference
is held, the delayed destroy now fails. Mark the event persistent then,
so it can still be deleted once the reference is gone instead of being
leaked.

Return -EBUSY from destroy_user_event() in that case.

Fixes: 7f5a08c79df3 ("user_events: Add minimal support for trace_event into ftrace")
Cc: stable@vger.kernel.org
Signed-off-by: David Carlier <devnexen@gmail.com>
---
v2: keep an event whose delayed destroy fails deletable by marking it
persistent (Sashiko). Deleting it then needs CAP_PERFMON, like any
persistent event.
v1: https://lore.kernel.org/all/20261031210420.501187-1-devnexen@gmail.com/
 kernel/trace/trace_events_user.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index f658c3a77aa7..ea6a20a30cf5 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -256,6 +256,11 @@ static void delayed_destroy_user_event(struct work_struct *work)
 		 * attempt to delete it later.
 		 */
 		pr_warn("user_events: Unable to delete event\n");
+		/*
+		 * Keep it as persistent so it can be still deleted
+		 * since nobody own this event
+		 */
+		user->reg_flags |= USER_EVENT_REG_PERSIST;
 		refcount_set(&user->refcnt, 1);
 	}
 out:
@@ -1506,6 +1511,10 @@ static int destroy_user_event(struct user_event *user)
 
 	lockdep_assert_held(&event_mutex);
 
+	/* If there is a reference to a dynamic event */
+	if (trace_event_dyn_busy(&user->call))
+		return -EBUSY;
+
 	/*
 	 * Detach the fields before removing the call. Removing the event
 	 * frees the field list memory (trace_destroy_fields() is run on
-- 
2.55.0