Re: [PATCH RFC bpf-next v2 02/10] bpf/helpers: introduce sleepable timers

[Benjamin Tissoires]

[replying to both of your messages here]

On Wed, Feb 21, 2024 at 3:59 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Fri, Feb 16, 2024 at 10:50:10AM +0100, Benjamin Tissoires wrote:
> >  static bool is_rbtree_lock_required_kfunc(u32 btf_id)
> >  {
> >       return is_bpf_rbtree_api_kfunc(btf_id);
> > @@ -12140,6 +12143,16 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> >               }
> >       }
> >
> > +     if (is_bpf_timer_set_sleepable_cb_kfunc(meta.func_id)) {
> > +             err = push_callback_call(env, insn, insn_idx, meta.subprogno,
> > +                                      set_timer_callback_state);
> > +             if (err) {
> > +                     verbose(env, "kfunc %s#%d failed callback verification\n",
> > +                             func_name, meta.func_id);
> > +                     return err;
> > +             }
> > +     }
>
> All makes sense so far.
> Please squash all the fix and repost.
> It's hard to do a proper review in this shape of the patch.

Yeah, I was expecting a very quick "I know why you are crashing", not
a full review here.

> As far as rcu_read_lock/unlock that is done in callback...
> it feels buggy and unnecessary.

This rcu approach is indeed wrong, but there still needs to be some
locking if bpf_timer_set_callback() or bpf_timer_set_sleepable_cb() is
called while the work just started. I went with a semaphore in v3 as
it seemed lightweight enough there. Please shout if you disagree :)

Anyway, I've also dropped the flags in bpf_timer_init() in v3 to only
add BPF_F_TIMER_SLEEPABLE in bpf_timer_start().

V3 (not RFC) is coming.

Cheers,
Benjamin

> bpf prog and timer won't disappear while work is queued.
> array and hash map will call bpf_obj_free_timer() before going away.
>
> And things like:
> +       rcu_read_lock();
> +       callback_fn = rcu_dereference(t->sleepable_cb_fn);
> +       rcu_read_unlock();
> +       if (!callback_fn)
> +               return;
>
> is 99% broken. if (!callback_fn) line is UAF.
>