On 25.07.24 21:05, Steven Rostedt wrote:
> On Thu, 25 Jul 2024 20:12:33 +0200
> Mathias Krause <mini...@grsecurity.net> 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
