netdev - Re: [PATCH v2] tcp: verify the checksum of the first data segment in a new connection

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Date:   Tue, 12 Jun 2018 21:53:17 +0000
From:   "van der Linden, Frank" <fllinden@...zon.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v2] tcp: verify the checksum of the first data segment in
 a new connection

The convention seems to be to call tcp_checksum_complete after tcp_filter has a chance to deal with the packet. I wanted to preserve that.

If that is not a concern, then I agree that this is a far better way to go.

Frank

On 6/12/18, 2:50 PM, "Eric Dumazet" <eric.dumazet@...il.com> wrote:

    
    
    On 06/12/2018 02:41 PM, Frank van der Linden wrote:
    > commit 079096f103fa ("tcp/dccp: install syn_recv requests into ehash
    > table") introduced an optimization for the handling of child sockets
    > created for a new TCP connection.
    > 
    > But this optimization passes any data associated with the last ACK of the
    > connection handshake up the stack without verifying its checksum, because it
    > calls tcp_child_process(), which in turn calls tcp_rcv_state_process()
    > directly.  These lower-level processing functions do not do any checksum
    > verification.
    > 
    > Insert a tcp_checksum_complete call in the TCP_NEW_SYN_RECEIVE path to
    > fix this.
    > 
    > Signed-off-by: Frank van der Linden <fllinden@...zon.com>
    
    
    This is way too complicated.
    
    You should call tcp_checksum_complete() earlier and avoid all this mess.
    
    
    IPV4 part shown here :
    
    diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
    index fed3f1c6616708997f621535efe9412e4afa0a50..7b5f32aa3835b0124b0a9bd342c371df7b46f471 100644
    --- a/net/ipv4/tcp_ipv4.c
    +++ b/net/ipv4/tcp_ipv4.c
    @@ -1730,6 +1730,10 @@ int tcp_v4_rcv(struct sk_buff *skb)
                            reqsk_put(req);
                            goto discard_it;
                    }
    +               if (unlikely(tcp_checksum_complete(skb))) {
    +                       reqsk_put(req);
    +                       goto csum_error;
    +               }
                    if (unlikely(sk->sk_state != TCP_LISTEN)) {
                            inet_csk_reqsk_queue_drop_and_put(sk, req);
                            goto lookup;