[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.
>
