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

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Date: Tue, 20 Feb 2024 18:59:03 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Benjamin Tissoires <bentiss@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Jiri Kosina <jikos@...nel.org>, Benjamin Tissoires <benjamin.tissoires@...hat.com>, 
	Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-input@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC bpf-next v2 02/10] bpf/helpers: introduce sleepable
 timers

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