Hi,
On Thu, 7 May 2015, Jesper Dangaard Brouer wrote:
> In compliance with RFC5961, the network stack send challenge ACK in
> response to spurious SYN packets, since commit 0c228e833c88 ("tcp:
> Restore RFC5961-compliant behavior for SYN packets").
>
> This pose a problem for netfilter conntrack in state LAST_ACK, because
> this challenge ACK is (falsely) seen as ACKing last FIN, causing a
> false state transition (into TIME_WAIT).
>
> The challenge ACK is hard to distinguish from real last ACK. Thus,
> solution introduce a flag that tracks the potential for seeing a
> challenge ACK, in case a SYN packet is let through and current state
> is LAST_ACK.
>
> When conntrack transition LAST_ACK to TIME_WAIT happens, this flag is
> used for determining if we are expecting a challenge ACK.
>
> Scapy based reproducer script avail here:
> https://github.com/netoptimizer/network-testing/blob/master/scapy/tcp_hacks_3WHS_LAST_ACK.py
>
> Fixes: 0c228e833c88 ("tcp: Restore RFC5961-compliant behavior for SYN packets")
> Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
The patch looks OK to me, thanks Jesper.
Acked-by: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxxxxxx>
Best regards,
Jozsef
> ---
>
> include/uapi/linux/netfilter/nf_conntrack_tcp.h | 3 ++
> net/netfilter/nf_conntrack_proto_tcp.c | 35 +++++++++++++++++++++--
> 2 files changed, 35 insertions(+), 3 deletions(-)
>
> diff --git a/include/uapi/linux/netfilter/nf_conntrack_tcp.h b/include/uapi/linux/netfilter/nf_conntrack_tcp.h
> index 9993a42..ef9f80f 100644
> --- a/include/uapi/linux/netfilter/nf_conntrack_tcp.h
> +++ b/include/uapi/linux/netfilter/nf_conntrack_tcp.h
> @@ -42,6 +42,9 @@ enum tcp_conntrack {
> /* The field td_maxack has been set */
> #define IP_CT_TCP_FLAG_MAXACK_SET 0x20
>
> +/* Marks possibility for expected RFC5961 challenge ACK */
> +#define IP_CT_EXP_CHALLENGE_ACK 0x40
> +
> struct nf_ct_tcp_flags {
> __u8 flags;
> __u8 mask;
> diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
> index 5caa0c4..ad0db66 100644
> --- a/net/netfilter/nf_conntrack_proto_tcp.c
> +++ b/net/netfilter/nf_conntrack_proto_tcp.c
> @@ -202,7 +202,7 @@ static const u8 tcp_conntracks[2][6][TCP_CONNTRACK_MAX] = {
> * sES -> sES :-)
> * sFW -> sCW Normal close request answered by ACK.
> * sCW -> sCW
> - * sLA -> sTW Last ACK detected.
> + * sLA -> sTW Last ACK detected (RFC5961 challenged)
> * sTW -> sTW Retransmitted last ACK. Remain in the same state.
> * sCL -> sCL
> */
> @@ -261,7 +261,7 @@ static const u8 tcp_conntracks[2][6][TCP_CONNTRACK_MAX] = {
> * sES -> sES :-)
> * sFW -> sCW Normal close request answered by ACK.
> * sCW -> sCW
> - * sLA -> sTW Last ACK detected.
> + * sLA -> sTW Last ACK detected (RFC5961 challenged)
> * sTW -> sTW Retransmitted last ACK.
> * sCL -> sCL
> */
> @@ -906,6 +906,7 @@ static int tcp_packet(struct nf_conn *ct,
> 1 : ct->proto.tcp.last_win;
> ct->proto.tcp.seen[ct->proto.tcp.last_dir].td_scale =
> ct->proto.tcp.last_wscale;
> + ct->proto.tcp.last_flags &= ~IP_CT_EXP_CHALLENGE_ACK;
> ct->proto.tcp.seen[ct->proto.tcp.last_dir].flags =
> ct->proto.tcp.last_flags;
> memset(&ct->proto.tcp.seen[dir], 0,
> @@ -923,7 +924,9 @@ static int tcp_packet(struct nf_conn *ct,
> * may be in sync but we are not. In that case, we annotate
> * the TCP options and let the packet go through. If it is a
> * valid SYN packet, the server will reply with a SYN/ACK, and
> - * then we'll get in sync. Otherwise, the server ignores it. */
> + * then we'll get in sync. Otherwise, the server potentially
> + * respons with a challenge ACK if implementing RFC5961.
> + */
> if (index == TCP_SYN_SET && dir == IP_CT_DIR_ORIGINAL) {
> struct ip_ct_tcp_state seen = {};
>
> @@ -939,6 +942,13 @@ static int tcp_packet(struct nf_conn *ct,
> ct->proto.tcp.last_flags |=
> IP_CT_TCP_FLAG_SACK_PERM;
> }
> + /* Mark the potential for RFC5961 challenge ACK,
> + * this pose a special problem for LAST_ACK state
> + * as ACK is intrepretated as ACKing last FIN.
> + */
> + if (old_state == TCP_CONNTRACK_LAST_ACK)
> + ct->proto.tcp.last_flags |=
> + IP_CT_EXP_CHALLENGE_ACK;
> }
> spin_unlock_bh(&ct->lock);
> if (LOG_INVALID(net, IPPROTO_TCP))
> @@ -970,6 +980,25 @@ static int tcp_packet(struct nf_conn *ct,
> nf_log_packet(net, pf, 0, skb, NULL, NULL, NULL,
> "nf_ct_tcp: invalid state ");
> return -NF_ACCEPT;
> + case TCP_CONNTRACK_TIME_WAIT:
> + /* RFC5961 compliance cause stack to send "challenge-ACK"
> + * e.g. in response to spurious SYNs. Conntrack MUST
> + * not believe this ACK is acking last FIN.
> + */
> + if (old_state == TCP_CONNTRACK_LAST_ACK
> + && index == TCP_ACK_SET
> + && ct->proto.tcp.last_dir != dir
> + && ct->proto.tcp.last_index == TCP_SYN_SET
> + && (ct->proto.tcp.last_flags & IP_CT_EXP_CHALLENGE_ACK)) {
> + /* Detected RFC5961 challenge ACK */
> + ct->proto.tcp.last_flags &= ~IP_CT_EXP_CHALLENGE_ACK;
> + spin_unlock_bh(&ct->lock);
> + if (LOG_INVALID(net, IPPROTO_TCP))
> + nf_log_packet(net, pf, 0, skb, NULL, NULL, NULL,
> + "nf_ct_tcp: challenge-ACK ignored ");
> + return NF_ACCEPT; /* Don't change state */
> + }
> + break;
> case TCP_CONNTRACK_CLOSE:
> if (index == TCP_RST_SET
> && (ct->proto.tcp.seen[!dir].flags & IP_CT_TCP_FLAG_MAXACK_SET)
>
> --
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-
E-mail : kadlec@xxxxxxxxxxxxxxxxx, kadlecsik.jozsef@xxxxxxxxxxxxx
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Address : Wigner Research Centre for Physics, Hungarian Academy of Sciences
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