Re: [PATCH nf] netfilter: nf_conntrack_tcp: re-init for syn packets only

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Mon, Apr 25, 2022 at 11:47:11AM +0200, 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).

Applied, thanks