Re: tracing: user events UAF crash report

[Mathias Krause]

On 25.07.24 21:05, Steven Rostedt wrote:
> On Thu, 25 Jul 2024 20:12:33 +0200
> Mathias Krause <minipli@xxxxxxxxxxxxxx> wrote:
>>>> @@ -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;  
>>
>> Dereferencing a potentially free'd object, so 'user' is now "random" data.
> 
> This is the callback function of user->call.get_fields.
> 
> That is, we have:
> 
> 	user->call.get_fields = user_event_get_fields;
> 
> And the f_start() code eventually calls trace_get_fields() that has (from a
> previous email in this thread).
> 
>     trace_get_fields(struct trace_event_call *event_call)
>     {
> 	if (!event_call->class->get_fields)
>         	return &event_call->class->fields;
>         return event_call->class->get_fields(event_call);
>     }

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.

> 
> 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;
>  	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.

Thanks,
Mathias