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

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Date:   Wed, 13 Jun 2018 15:08:51 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Frank van der Linden <fllinden@...zon.com>, edumazet@...gle.com,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v2] tcp: verify the checksum of the first data segment in
 a new connection

On Wed, Jun 13, 2018 at 7:50 AM, 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;
> +               }

I like this variant, it is not under the sock_lock, but it skips
tcp_filter() as Frank pointed out.

I tested a variant of this patch with an increment of MIB/CSUM errors
and jump to discard_and_relse and it seemed to pass my testing


Balbir Singh.