Re: tracing: user events UAF crash report

[Steven Rostedt]

On Thu, 25 Jul 2024 22:41:23 +0200
Mathias Krause <minipli@xxxxxxxxxxxxxx> wrote:

> > diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> > index 6ef29eba90ce..5fbfa1c885de 100644
> > --- a/kernel/trace/trace_events.c
> > +++ b/kernel/trace/trace_events.c
> > @@ -1627,12 +1627,14 @@ static int f_show(struct seq_file *m, void *v)
> >  
> >  static void *f_start(struct seq_file *m, loff_t *pos)
> >  {
> > +	struct trace_event_file *file;
> >  	void *p = (void *)FORMAT_HEADER;
> >  	loff_t l = 0;
> >  
> >  	/* ->stop() is called even if ->start() fails */
> >  	mutex_lock(&event_mutex);
> > -	if (!event_file_data(m->private))
> > +	file = event_file_data(m->private);
> > +	if (!file || (file->flags & EVENT_FILE_FL_FREED))
> >  		return ERR_PTR(-ENODEV);
> >  
> >  	while (l < *pos && p)
> > 
> >   
> 
> Nope, still the same splats.

Can you reshow the splats. Because I'm now confused.

destroy_user_event() which is under event_mutex calls
user_event_set_call_visible() with false, that will then call:

trace_remove_event_call() -> probe_remove_event_call() ->
 __trace_remove_event_call() -> event_remove() ->
 remove_event_from_tracers()

Where remove_event_from_tracers() loops over all the instances and will set
each of the file pointers flags associated to the event: EVENT_FILE_FL_FREED

Then it returns back to destroy_user_event() that would free the event.

The f_start() that was in your crash, with the new patch, should take the
event_mutex before referencing the event that was freed. And with that flag
being set, it should exit out.

Did you remove all the other patches before applying this one?

-- Steve