From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH] tcp: should drop incoming frames without ACK flag set
Date: Wed, 26 Dec 2012 14:11:54 -0800 (PST)
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From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Wed, 26 Dec 2012 09:10:01 -0800

> @@ -5540,6 +5540,9 @@ no_ack:
>  	}
>  
>  slow_path:
> +	if (!th->ack)
> +			goto discard;

One too many tabs there on that last line :-)

> +
>  	if (len < (th->doff << 2) || tcp_checksum_complete_user(sk, skb))
>  		goto csum_error;
>  
> @@ -5551,7 +5554,7 @@ slow_path:


Also, I would say that this checksum test should come first, because
that takes priority since you could be testing the ACK bit of a
corrupted packet.

Better to get the statistic bump on the bad checksum then a silent
drop on the ACK being cleared.

> @@ -5984,11 +5987,15 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
>  		if (tcp_check_req(sk, skb, req, NULL, true) == NULL)
>  			goto discard;
>  	}
> +
> +	if (!th->ack)
> +		goto discard;
> +

And that is effectively what is going to happen in this case since
the caller has already done the checksum checks.

Thanks.