On Mon, 25 Apr 2022, Florian Westphal wrote:
> Jaco Kroon reported tcp problems that Eric Dumazet and Neal Cardwell
> pinpointed to nf_conntrack tcp_in_window() bug.
>
> tcp trace shows following sequence:
>
> I > R Flags [S], seq 3451342529, win 62580, options [.. tfo [|tcp]>
> R > I Flags [S.], seq 2699962254, ack 3451342530, win 65535, options [..]
> R > I Flags [P.], seq 1:89, ack 1, [..]
>
> Note 3rd ACK is from responder to initiator so following branch is taken:
> } else if (((state->state == TCP_CONNTRACK_SYN_SENT
> && dir == IP_CT_DIR_ORIGINAL)
> || (state->state == TCP_CONNTRACK_SYN_RECV
> && dir == IP_CT_DIR_REPLY))
> && after(end, sender->td_end)) {
>
> ... because state == TCP_CONNTRACK_SYN_RECV and dir is REPLY.
> This causes the scaling factor to be reset to 0: window scale option
> is only present in syn(ack) packets. This in turn makes nf_conntrack
> mark valid packets as out-of-window.
>
> This was always broken, it exists even in original commit where
> window tracking was added to ip_conntrack (nf_conntrack predecessor)
> in 2.6.9-rc1 kernel.
>
> Restrict to 'tcph->syn', just like the 3rd condtional added in
> commit 82b72cb94666 ("netfilter: conntrack: re-init state for retransmitted syn-ack").
>
> Upon closer look, those conditionals/branches can be merged:
>
> Because earlier checks prevent syn-ack from showing up in
> original direction, the 'dir' checks in the conditional quoted above are
> redundant, remove them. Return early for pure syn retransmitted in reply
> direction (simultaneous open).
>
> Fixes: 9fb9cbb1082d ("[NETFILTER]: Add nf_conntrack subsystem.")
> Reported-by: Jaco Kroon <jaco@xxxxxxxxx>
> Cc: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxx>
> Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
Acked-by: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxx>
[Sorry, I was away whole last week as well.]
Best regards,
Jozsef
> ---
> net/netfilter/nf_conntrack_proto_tcp.c | 21 ++++++---------------
> 1 file changed, 6 insertions(+), 15 deletions(-)
>
> diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
> index 8ec55cd72572..204a5cdff5b1 100644
> --- a/net/netfilter/nf_conntrack_proto_tcp.c
> +++ b/net/netfilter/nf_conntrack_proto_tcp.c
> @@ -556,24 +556,14 @@ static bool tcp_in_window(struct nf_conn *ct,
> }
>
> }
> - } else if (((state->state == TCP_CONNTRACK_SYN_SENT
> - && dir == IP_CT_DIR_ORIGINAL)
> - || (state->state == TCP_CONNTRACK_SYN_RECV
> - && dir == IP_CT_DIR_REPLY))
> - && after(end, sender->td_end)) {
> + } else if (tcph->syn &&
> + after(end, sender->td_end) &&
> + (state->state == TCP_CONNTRACK_SYN_SENT ||
> + state->state == TCP_CONNTRACK_SYN_RECV)) {
> /*
> * RFC 793: "if a TCP is reinitialized ... then it need
> * not wait at all; it must only be sure to use sequence
> * numbers larger than those recently used."
> - */
> - sender->td_end =
> - sender->td_maxend = end;
> - sender->td_maxwin = (win == 0 ? 1 : win);
> -
> - tcp_options(skb, dataoff, tcph, sender);
> - } else if (tcph->syn && dir == IP_CT_DIR_REPLY &&
> - state->state == TCP_CONNTRACK_SYN_SENT) {
> - /* Retransmitted syn-ack, or syn (simultaneous open).
> *
> * Re-init state for this direction, just like for the first
> * syn(-ack) reply, it might differ in seq, ack or tcp options.
> @@ -581,7 +571,8 @@ static bool tcp_in_window(struct nf_conn *ct,
> tcp_init_sender(sender, receiver,
> skb, dataoff, tcph,
> end, win);
> - if (!tcph->ack)
> +
> + if (dir == IP_CT_DIR_REPLY && !tcph->ack)
> return true;
> }
>
> --
> 2.35.1
>
>
-
E-mail : kadlec@xxxxxxxxxxxxxxxxx, kadlecsik.jozsef@xxxxxxxxx
PGP key : https://wigner.hu/~kadlec/pgp_public_key.txt
Address : Wigner Research Centre for Physics
H-1525 Budapest 114, POB. 49, Hungary
