netdev - Re: [PATCH] tcp: Reject invalid ack

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Date:	Mon, 22 Oct 2012 20:56:18 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	"H.K. Jerry Chu" <hkchu@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH] tcp: Reject invalid ack_seq to Fast Open sockets

On Mon, Oct 22, 2012 at 5:26 PM, H.K. Jerry Chu <hkchu@...gle.com> wrote:
> From: Jerry Chu <hkchu@...gle.com>
>
> A packet with an invalid ack_seq may cause a TCP Fast Open socket to switch
> to the unexpected TCP_CLOSING state, triggering a BUG_ON kernel panic.
>
> When a FIN packet with an invalid ack_seq# arrives at a socket in
> the TCP_FIN_WAIT1 state, rather than discarding the packet, the current
> code will accept the FIN, causing state transition to TCP_CLOSING.
>
> This may be a small deviation from RFC793, which seems to say that the
> packet should be dropped. Unfortunately I did not expect this case for
> Fast Open hence it will trigger a BUG_ON panic.
>
> It turns out there is really nothing bad about a TFO socket going into
> TCP_CLOSING state so I could just remove the BUG_ON statements. But after
> some thought I think it's better to treat this case like TCP_SYN_RECV
> and return a RST to the confused peer who caused the unacceptable ack_seq
> to be generated in the first place.
>
> Signed-off-by: H.K. Jerry Chu <hkchu@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>

Acked-by: Neal Cardwell <ncardwell@...gle.com>
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