Subject: Re: [PATCH bpf-next 4/8] bpf, sockmap: Don't let child socket inherit psock or its ops on copy

From: Jakub Sitnicki <[email-masked]>
Date: Tue, 26 Nov 2019 19:36:09 +0100

On Tue, Nov 26, 2019 at 06:16 PM CET, Martin Lau wrote:
> On Tue, Nov 26, 2019 at 04:54:33PM +0100, Jakub Sitnicki wrote:
>> On Mon, Nov 25, 2019 at 11:38 PM CET, Martin Lau wrote:
>> > On Sat, Nov 23, 2019 at 12:07:47PM +0100, Jakub Sitnicki wrote:
>> > [ ... ]
>> >
>> >> @@ -370,6 +378,11 @@ static inline void sk_psock_restore_proto(struct sock *sk,
>> >>  			sk->sk_prot = psock->sk_proto;
>> >>  		psock->sk_proto = NULL;
>> >>  	}
>> >> +
>> >> +	if (psock->icsk_af_ops) {
>> >> +		icsk->icsk_af_ops = psock->icsk_af_ops;
>> >> +		psock->icsk_af_ops = NULL;
>> >> +	}
>> >>  }
>> >
>> > [ ... ]
>> >
>> >> +static struct sock *tcp_bpf_syn_recv_sock(const struct sock *sk,
>> >> +					  struct sk_buff *skb,
>> >> +					  struct request_sock *req,
>> >> +					  struct dst_entry *dst,
>> >> +					  struct request_sock *req_unhash,
>> >> +					  bool *own_req)
>> >> +{
>> >> +	const struct inet_connection_sock_af_ops *ops;
>> >> +	void (*write_space)(struct sock *sk);
>> >> +	struct sk_psock *psock;
>> >> +	struct proto *proto;
>> >> +	struct sock *child;
>> >> +
>> >> +	rcu_read_lock();
>> >> +	psock = sk_psock(sk);
>> >> +	if (likely(psock)) {
>> >> +		proto = psock->sk_proto;
>> >> +		write_space = psock->saved_write_space;
>> >> +		ops = psock->icsk_af_ops;
>> > It is not immediately clear to me what ensure
>> > ops is not NULL here.
>> >
>> > It is likely I missed something.  A short comment would
>> > be very useful here.
>>
>> I can see the readability problem. Looking at it now, perhaps it should
>> be rewritten, to the same effect, as:
>>
>> static struct sock *tcp_bpf_syn_recv_sock(...)
>> {
>> 	const struct inet_connection_sock_af_ops *ops = NULL;
>>         ...
>>
>>         rcu_read_lock();
>> 	psock = sk_psock(sk);
>> 	if (likely(psock)) {
>> 		proto = psock->sk_proto;
>> 		write_space = psock->saved_write_space;
>> 		ops = psock->icsk_af_ops;
>> 	}
>> 	rcu_read_unlock();
>>
>>         if (!ops)
>> 		ops = inet_csk(sk)->icsk_af_ops;
>>         child = ops->syn_recv_sock(sk, skb, req, dst, req_unhash, own_req);
>>
>> If psock->icsk_af_ops were NULL, it would mean we haven't initialized it
>> properly. To double check what happens here:
> I did not mean the init path.  The init path is fine since it init
> eveything on psock before publishing the sk to the sock_map.
>
> I was thinking the delete path (e.g. sock_map_delete_elem).  It is not clear
> to me what prevent the earlier pasted sk_psock_restore_proto() which sets
> psock->icsk_af_ops to NULL from running in parallel with
> tcp_bpf_syn_recv_sock()?  An explanation would be useful.

Ah, I misunderstood. Nothing prevents the race, AFAIK.

Setting psock->icsk_af_ops to null on restore and not checking for it
here was a bad move on my side.  Also I need to revisit what to do about
psock->sk_proto so the child socket doesn't end up with null sk_proto.

This race should be easy enough to trigger. Will give it a shot.

Thank you for bringing this up,
Jakub

>
>>
>> In sock_map_link we do a setup dance where we first create the psock and
>> later initialize the socket callbacks (tcp_bpf_init).
>>
>> static int sock_map_link(struct bpf_map *map, struct sk_psock_progs *progs,
>> 			 struct sock *sk)
>> {
>>         ...
>> 	if (psock) {
>>                 ...
>> 	} else {
>> 		psock = sk_psock_init(sk, map->numa_node);
>> 		if (!psock) {
>> 			ret = -ENOMEM;
>> 			goto out_progs;
>> 		}
>> 		sk_psock_is_new = true;
>> 	}
>>         ...
>>         if (sk_psock_is_new) {
>> 		ret = tcp_bpf_init(sk);
>> 		if (ret < 0)
>> 			goto out_drop;
>> 	} else {
>> 		tcp_bpf_reinit(sk);
>> 	}
>>
>> The "if (sk_psock_new)" branch triggers the call chain that leads to
>> saving & overriding socket callbacks.
>>
>> tcp_bpf_init -> tcp_bpf_update_sk_prot -> sk_psock_update_proto
>>
>> Among them, icsk_af_ops.
>>
>> static inline void sk_psock_update_proto(...)
>> {
>>         ...
>> 	psock->icsk_af_ops = icsk->icsk_af_ops;
>> 	icsk->icsk_af_ops = af_ops;
>> }
>>
>> Goes without saying that a comment is needed.
>>
>> Thanks for the feedback,
>> Jakub