tracing: Add down_write(trace_event_sem) when adding trace event

BugLink: https://bugs.launchpad.net/bugs/2123805

commit b5e8acc14dcb314a9b61ff19dcd9fdd0d88f70df upstream.

When a module is loaded, it adds trace events defined by the module. It
may also need to modify the modules trace printk formats to replace enum
names with their values.

If two modules are loaded at the same time, the adding of the event to the
ftrace_events list can corrupt the walking of the list in the code that is
modifying the printk format strings and crash the kernel.

The addition of the event should take the trace_event_sem for write while
it adds the new event.

Also add a lockdep_assert_held() on that semaphore in
__trace_add_event_dirs() as it iterates the list.

Cc: [email protected]
Cc: Mathieu Desnoyers <[email protected]>
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Link: https://lore.kernel.org/[email protected]
Reported-by: Fusheng Huang(黄富生)  <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Fixes: 110bf2b ("tracing: add protection around module events unload")
Signed-off-by: Steven Rostedt (Google) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Noah Wager <[email protected]>
Signed-off-by: Edoardo Canepa <[email protected]>

Author: rostedt

kernel/trace/trace_events.c

@@ -3109,7 +3109,10 @@ __register_event(struct trace_event_call *call, struct module *mod)
 	if (ret < 0)
 		return ret;
 
+	down_write(&trace_event_sem);
 	list_add(&call->list, &ftrace_events);
+	up_write(&trace_event_sem);
+
 	if (call->flags & TRACE_EVENT_FL_DYNAMIC)
 		atomic_set(&call->refcnt, 0);
 	else
@@ -3723,6 +3726,8 @@ __trace_add_event_dirs(struct trace_array *tr)
 	struct trace_event_call *call;
 	int ret;
 
+	lockdep_assert_held(&trace_event_sem);
+
 	list_for_each_entry(call, &ftrace_events, list) {
 		ret = __trace_add_new_event(call, tr);
 		if (ret < 0)