Re: tracing: user events UAF crash report

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 25 Jul 2024 21:42:41 +0200
Mathias Krause <minipli@grsecurity.net> wrote:

> Right. But the point is, that 'event_call' is really some '&user->call'.
> With 'user' being free'd memory, what gives? Dereferencing 'event_call'
> is UB, so this function is doomed to fail because it cannot know if its
> only argument points to still valid memory or not. And that's the core
> issue -- calling that function for an object that's long gone -- the
> missing refcounting I hinted at in my first Email.

Ah, I missed that the call was part of the user structure. But I think I
found the real fix.

> 
> > 
> > Where it calls the ->class->get_fields(event_call);
> > 
> > that calls this function. By setting:
> > 
> > 	user->call.get_fields = NULL;
> > 
> > this will never get called and no random data will be accessed.  
> 
> As 'user' is free'd or soon-to-be-free'd memory, that's a non-starter.
> 
> > 
> > That said, I was talking with Beau, we concluded that this shouldn't be the
> > responsibility of the user of event call, and should be cleaned up by the
> > event system.
> > 
> > Here's the proper fix:
> > 
> > diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> > index 6ef29eba90ce..3a2d2ff1625b 100644
> > --- a/kernel/trace/trace_events.c
> > +++ b/kernel/trace/trace_events.c
> > @@ -3140,8 +3140,10 @@ EXPORT_SYMBOL_GPL(trace_add_event_call);
> >   */
> >  static void __trace_remove_event_call(struct trace_event_call *call)
> >  {
> > +	lockdep_assert_held(&event_mutex);
> >  	event_remove(call);
> >  	trace_destroy_fields(call);
> > +	call->get_fields = NULL;

Actually, this is wrong as it would be: call->class->get_fields, and this
is not the right place to clear it, as class can be used by multiple calls.

> >  	free_event_filter(call->filter);
> >  	call->filter = NULL;
> >  }
> > 
> > Can you try it out?  
> 
> I can try but I don't think that's the proper fix for above reasons, I'm
> sorry.

I believe the issue is that f_start() needs to check if the event file has
been freed.

New patch:

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)


-- Steve