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: Wed, 27 Nov 2019 23:18:44 +0100

On Tue, Nov 26, 2019 at 07:43 PM CET, John Fastabend wrote:
> 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.
>>
>
> I'll answer. Updates are protected via sk_callback_lock so we don't have
> parrallel updates in-flight causing write_space and sk_proto to be out
> of sync. However access should be OK because its a pointer write we
> never update the pointer in place, e.g.
>
> static inline void sk_psock_restore_proto(struct sock *sk,
> 					  struct sk_psock *psock)
> {
> +       struct inet_connection_sock *icsk = inet_csk(sk);
> +
> 	sk->sk_write_space = psock->saved_write_space;
>
> 	if (psock->sk_proto) {
> 		struct inet_connection_sock *icsk = inet_csk(sk);
> 		bool has_ulp = !!icsk->icsk_ulp_data;
>
> 		if (has_ulp)
> 			tcp_update_ulp(sk, psock->sk_proto);
> 		else
> 			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;
> +       }
> }
>
> In restore case either psock->icsk_af_ops is null or not. If its
> null below code catches it. If its not null (from init path) then
> we have a valid pointer.
>
>         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);
>
>
> We should do this with proper READ_ONCE/WRITE_ONCE to make it clear
> what is going on and to stop compiler from breaking these assumptions. I
> was going to generate that patch after this series but can do it before
> as well. I didn't mention it here because it seems a bit out of scope
> for this series because its mostly a fix to older code.

+1, looking forward to your patch. Also, as I've recently learned, that
should enable KTSAN to reason about the psock code [0].

> Also I started to think that write_space might be out of sync with ops but
> it seems we never actually remove psock_write_space until after
> rcu grace period so that should be OK as well and always point to the
> previous write_space.
>
> Finally I wondered if we could remove the ops and then add it back
> quickly which seems at least in theory possible, but that would get
> hit with a grace period because we can't have conflicting psock
> definitions on the same sock. So expanding the rcu block to include
> the ops = inet_csk(sk)->icsk_af_ops would fix that case.

I see, ops = inet_csk(sk)->icsk_af_ops might read out a re-overwritten
ops after sock_map_unlink, followed by sock_map_link. Ouch.

> So in summary I think we should expand the rcu lock here to include the
> ops = inet_csk(sk)->icsk_af_ops to ensure we dont race with tear
> down and create. I'll push the necessary update with WRITE_ONCE and
> READ_ONCE to fix that up. Seeing we have to wait until the merge
> window opens most likely anyways I'll send those out sooner rather
> then later and this series can add the proper annotations as well.

Or I could leave psock->icsk_af_ops set in restore_proto, like we do for
write_space as you noted. Restoring it twice doesn't seem harmful, it
has no side-effects. Less state changes to think about?

I'll still have to apply what you suggest for saving psock->sk_proto,
though.

Thanks,
Jakub

[0] https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE