On Fri, 12 Jul 2019, Florian Westphal wrote:
> Jakub Jankowski reported following oddity:
>
> After 3 way handshake completes, timeout of new connection is set to
> max_retrans (300s) instead of established (5 days).
>
> shortened excerpt from pcap provided:
> 25.070622 IP (flags [DF], proto TCP (6), length 52)
> 10.8.5.4.1025 > 10.8.1.2.80: Flags [S], seq 11, win 64240, [wscale 8]
> 26.070462 IP (flags [DF], proto TCP (6), length 48)
> 10.8.1.2.80 > 10.8.5.4.1025: Flags [S.], seq 82, ack 12, win 65535, [wscale 3]
> 27.070449 IP (flags [DF], proto TCP (6), length 40)
> 10.8.5.4.1025 > 10.8.1.2.80: Flags [.], ack 83, win 512, length 0
>
> Turns out the last_win is of u16 type, but we store the scaled value:
> 512 << 8 (== 0x20000) becomes 0 window.
>
> The Fixes tag is not correct, as the bug has existed forever, but
> without that change all that this causes might cause is to mistake a
> window update (to-nonzero-from-zero) for a retransmit.
>
> Fixes: fbcd253d2448b8 ("netfilter: conntrack: lower timeout to RETRANS
> seconds if window is 0")
> Reported-by: Jakub Jankowski <sha...@toxcorp.com>
> Tested-by: Jakub Jankowski <sha...@toxcorp.com>
> Signed-off-by: Florian Westphal <f...@strlen.de>
Acked-by: Jozsef Kadlecsik <kad...@blackhole.kfki.hu>
It was a nice report and catch!
Best regards,
Jozsef
> ---
> net/netfilter/nf_conntrack_proto_tcp.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/net/netfilter/nf_conntrack_proto_tcp.c
> b/net/netfilter/nf_conntrack_proto_tcp.c
> index 7ba01d8ee165..9fe1d5e46249 100644
> --- a/net/netfilter/nf_conntrack_proto_tcp.c
> +++ b/net/netfilter/nf_conntrack_proto_tcp.c
> @@ -475,6 +475,7 @@ static bool tcp_in_window(const struct nf_conn *ct,
> struct ip_ct_tcp_state *receiver = &state->seen[!dir];
> const struct nf_conntrack_tuple *tuple = &ct->tuplehash[dir].tuple;
> __u32 seq, ack, sack, end, win, swin;
> + u16 win_raw;
> s32 receiver_offset;
> bool res, in_recv_win;
>
> @@ -483,7 +484,8 @@ static bool tcp_in_window(const struct nf_conn *ct,
> */
> seq = ntohl(tcph->seq);
> ack = sack = ntohl(tcph->ack_seq);
> - win = ntohs(tcph->window);
> + win_raw = ntohs(tcph->window);
> + win = win_raw;
> end = segment_seq_plus_len(seq, skb->len, dataoff, tcph);
>
> if (receiver->flags & IP_CT_TCP_FLAG_SACK_PERM)
> @@ -658,14 +660,14 @@ static bool tcp_in_window(const struct nf_conn *ct,
> && state->last_seq == seq
> && state->last_ack == ack
> && state->last_end == end
> - && state->last_win == win)
> + && state->last_win == win_raw)
> state->retrans++;
> else {
> state->last_dir = dir;
> state->last_seq = seq;
> state->last_ack = ack;
> state->last_end = end;
> - state->last_win = win;
> + state->last_win = win_raw;
> state->retrans = 0;
> }
> }
> --
> 2.21.0
>
>
-
E-mail : kad...@blackhole.kfki.hu, kadlecsik.joz...@wigner.mta.hu
PGP key : http://www.kfki.hu/~kadlec/pgp_public_key.txt
Address : Wigner Research Centre for Physics, Hungarian Academy of Sciences
H-1525 Budapest 114, POB. 49, Hungary
