Re: [PATCH] tcp: Reject invalid ack_seq to Fast Open sockets

[David Miller <davem@davemloft.net>]

From: "H.K. Jerry Chu" <hkchu@google.com>
Date: Mon, 22 Oct 2012 14:26:36 -0700

> From: Jerry Chu <hkchu@google.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@google.com>

Applied, thanks.

> -		BUG_ON(sk->sk_state != TCP_SYN_RECV &&
> +		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
>  		    sk->sk_state != TCP_FIN_WAIT1);

I fixed up the indentation of the second line of the test when I
applied this.