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

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Date:	Mon, 22 Oct 2012 14:46:46 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	"H.K. Jerry Chu" <hkchu@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, ncardwell@...gle.com
Subject: Re: [PATCH] tcp: Reject invalid ack_seq to Fast Open sockets

On Mon, Oct 22, 2012 at 2:26 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: Yuchung Cheng <ycheng@...gle.com>
> ---
>  net/ipv4/tcp_input.c |   12 ++++++++++--
>  net/ipv4/tcp_timer.c |    2 +-
>  2 files changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 60cf836..ce3e3c7 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5970,7 +5970,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
>
>         req = tp->fastopen_rsk;
>         if (req != NULL) {
> -               BUG_ON(sk->sk_state != TCP_SYN_RECV &&
> +               WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
>                     sk->sk_state != TCP_FIN_WAIT1);
>
>                 if (tcp_check_req(sk, skb, req, NULL, true) == NULL)
> @@ -6059,7 +6059,15 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
>                          * ACK we have received, this would have acknowledged
>                          * our SYNACK so stop the SYNACK timer.
>                          */
> -                       if (acceptable && req != NULL) {
> +                       if (req != NULL) {
> +                               /* Return RST if ack_seq is invalid.
> +                                * Note that RFC793 only says to generate a
> +                                * DUPACK for it but for TCP Fast Open it seems
> +                                * better to treat this case like TCP_SYN_RECV
> +                                * above.
> +                                */
> +                               if (!acceptable)
> +                                       return 1;
>                                 /* We no longer need the request sock. */
>                                 reqsk_fastopen_remove(sk, req, false);
>                                 tcp_rearm_rto(sk);
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index fc04711..a24f4a4 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -347,7 +347,7 @@ void tcp_retransmit_timer(struct sock *sk)
>                 return;
>         }
>         if (tp->fastopen_rsk) {
> -               BUG_ON(sk->sk_state != TCP_SYN_RECV &&
> +               WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
>                     sk->sk_state != TCP_FIN_WAIT1);
>                 tcp_fastopen_synack_timer(sk);
>                 /* Before we receive ACK to our SYN-ACK don't retransmit
> --
> 1.7.7.3
>
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