netdev - Re: [PATCH bpf-next v2 02/11] net, sk_msg: Annotate lockless access to sk

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Date:   Mon, 13 Jan 2020 19:14:32 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Jakub Sitnicki <jakub@...udflare.com>,
        John Fastabend <john.fastabend@...il.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        kernel-team@...udflare.com, Eric Dumazet <edumazet@...gle.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH bpf-next v2 02/11] net, sk_msg: Annotate lockless access
 to sk_prot on clone

Jakub Sitnicki wrote:
> On Sun, Jan 12, 2020 at 12:14 AM CET, John Fastabend wrote:
> > Jakub Sitnicki wrote:
> >> sk_msg and ULP frameworks override protocol callbacks pointer in
> >> sk->sk_prot, while TCP accesses it locklessly when cloning the listening
> >> socket.
> >>
> >> Once we enable use of listening sockets with sockmap (and hence sk_msg),
> >> there can be shared access to sk->sk_prot if socket is getting cloned while
> >> being inserted/deleted to/from the sockmap from another CPU. Mark the
> >> shared access with READ_ONCE/WRITE_ONCE annotations.
> >>
> >> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> >
> > In sockmap side I fixed this by wrapping the access in a lock_sock[0]. So
> > Do you think this is still needed with that in mind? The bpf_clone call
> > is using sk_prot_creater and also setting the newsk's proto field. Even
> > if the listening parent sock was being deleted in parallel would that be
> > a problem? We don't touch sk_prot_creator from the tear down path. I've
> > only scanned the 3..11 patches so maybe the answer is below. If that is
> > the case probably an improved commit message would be helpful.
> 
> I think it is needed. Not because of tcp_bpf_clone or that we access
> listener's sk_prot_creator from there, if I'm grasping your question.
> 
> Either way I'm glad this came up. Let's go though my reasoning and
> verify it. tcp stack accesses the listener sk_prot while cloning it:
> 
> tcp_v4_rcv
>   sk = __inet_lookup_skb(...)
>   tcp_check_req(sk)
>     inet_csk(sk)->icsk_af_ops->syn_recv_sock
>       tcp_v4_syn_recv_sock
>         tcp_create_openreq_child
>           inet_csk_clone_lock
>             sk_clone_lock
>               READ_ONCE(sk->sk_prot)
> 
> It grabs a reference to the listener, but doesn't grab the sk_lock.
> 
> On another CPU we can be inserting/removing the listener socket from the
> sockmap and writing to its sk_prot. We have the update and the remove
> path:
> 
> sock_map_ops->map_update_elem
>   sock_map_update_elem
>     sock_map_update_common
>       sock_map_link_no_progs
>         tcp_bpf_init
>           tcp_bpf_update_sk_prot
>             sk_psock_update_proto
>               WRITE_ONCE(sk->sk_prot, ops)
> 
> sock_map_ops->map_delete_elem
>   sock_map_delete_elem
>     __sock_map_delete
>      sock_map_unref
>        sk_psock_put
>          sk_psock_drop
>            sk_psock_restore_proto
>              tcp_update_ulp
>                WRITE_ONCE(sk->sk_prot, proto)
> 
> Following the guidelines from KTSAN project [0], sk_prot looks like a
> candidate for annotating it. At least on these 3 call paths.
> 
> If that sounds correct, I can add it to the patch description.
> 

Logic looks correct to me thanks for the details, please put those in
the commit so we don't lose them. Can you also add a comment where it
makes most sense in the code? This is a bit subtle and we don't want
to miss it later. Probably in tcp_update_ulp near that WRITE_ONCE would
do. It doesn't need to be too verbose but something as simple as,

"{WRITE|READ}_ONCE wrappers needed around sk_prot to protect unlocked
 reads in sk_clone_lock"

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