netdev - Re: [PATCH bpf-next v4 02/12] net, sk_msg: Annotate lockless access to sk

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Date:   Thu, 23 Jan 2020 09:18:39 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jakub Sitnicki <jakub@...udflare.com>, bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, kernel-team@...udflare.com,
        John Fastabend <john.fastabend@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>, Martin Lau <kafai@...com>
Subject: Re: [PATCH bpf-next v4 02/12] net, sk_msg: Annotate lockless access
 to sk_prot on clone



On 1/23/20 7:55 AM, 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, that is with neither sk_lock nor sk_callback_lock held.
> 
> Once we enable use of listening sockets with sockmap (and hence sk_msg),
> there will be shared access to sk->sk_prot if socket is getting cloned
> while being inserted/deleted to/from the sockmap from another CPU:
> 
> Read side:
> 
> 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)
> 
> Write side:
> 
> 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)
> 
> Mark the shared access with READ_ONCE/WRITE_ONCE annotations.
> 
> Acked-by: Martin KaFai Lau <kafai@...com>
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>  include/linux/skmsg.h | 3 ++-
>  net/core/sock.c       | 5 +++--
>  net/ipv4/tcp_bpf.c    | 4 +++-
>  net/ipv4/tcp_ulp.c    | 3 ++-
>  net/tls/tls_main.c    | 3 ++-
>  5 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> index 41ea1258d15e..55c834a5c25e 100644
> --- a/include/linux/skmsg.h
> +++ b/include/linux/skmsg.h
> @@ -352,7 +352,8 @@ static inline void sk_psock_update_proto(struct sock *sk,
>  	psock->saved_write_space = sk->sk_write_space;
>  
>  	psock->sk_proto = sk->sk_prot;
> -	sk->sk_prot = ops;
> +	/* Pairs with lockless read in sk_clone_lock() */
> +	WRITE_ONCE(sk->sk_prot, ops);


Note there are dozens of calls like

if (sk->sk_prot->handler)
    sk->sk_prot->handler(...);

Some of them being done lockless.

I know it is painful, but presumably we need

const struct proto *ops = READ_ONCE(sk->sk_prot);

if (ops->handler)
    ops->handler(....);