Re: [PATCH nf] netfilter: conntrack: always store window size un-scaled

[Reindl Harald <h.reindl@thelounge.net>]

this seemed to be fixed in 5.1.19 but not announced in
https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.1.19 and after
update to 5.1.20 last night the freezes of a ssh-tunneled vnc session
are starting again without exclude "lo" while the session was stable 3 days

see topic "Connection timeouts due to INVALID state rule"

Am 12.07.19 um 00:29 schrieb Florian Westphal:
> 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 <shasta@toxcorp.com>
> Tested-by: Jakub Jankowski <shasta@toxcorp.com>
> Signed-off-by: Florian Westphal <fw@strlen.de>
> ---
>  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;
>  			}
>  		}
>