Re: tracing: user events UAF crash report

[Steven Rostedt]

On Thu, 25 Jul 2024 22:00:14 +0530
Ajay Kaher <ajay.kaher@xxxxxxxxxxxx> wrote:

> Thread A (read event)                      Thread B (remove event)
> 
> .                                          worker_thread()
> .                                          delayed_destroy_user_event()
> .                                                   -> acquire event_mutex
> .                                          destroy_user_event()
> vfs_read()                                 .
> seq_read()                                .
> f_start() -> acquire event_mutex           eventfs_remove_dir()
> . (waiting)                                kfree(user)
> . (waiting)                                 -> released event_mutex
> acquired event_mutex
> f_next()
> trace_get_fields():

Hmm, I wonder if this would fix it?

-- Steve

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 3a2b46847c8b..e9ed2826ff46 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -321,6 +321,8 @@ static void user_event_put(struct user_event *user, bool locked)
 	 */
 	refcount_set(&user->refcnt, 1);
 
+	user->call.data = NULL;
+
 	if (WARN_ON_ONCE(!schedule_work(&user->put_work))) {
 		/*
 		 * If we fail we must wait for an admin to attempt delete or
@@ -973,6 +975,11 @@ size_t copy_nofault(void *addr, size_t bytes, struct iov_iter *i)
 static struct list_head *user_event_get_fields(struct trace_event_call *call)
 {
 	struct user_event *user = (struct user_event *)call->data;
+	static LIST_HEAD(head);
+
+	/* If the user event is about to be deleted, return no fields */
+	if (!user)
+		return &head;
 
 	return &user->fields;
 }