Re: [PATCH net] tcp: fix receive window update in tcp_add_backlog()

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Mon,  5 Oct 2020 06:48:13 -0700

> From: Eric Dumazet <edumazet@google.com>
> 
> We got reports from GKE customers flows being reset by netfilter
> conntrack unless nf_conntrack_tcp_be_liberal is set to 1.
> 
> Traces seemed to suggest ACK packet being dropped by the
> packet capture, or more likely that ACK were received in the
> wrong order.
> 
>  wscale=7, SYN and SYNACK not shown here.
> 
>  This ACK allows the sender to send 1871*128 bytes from seq 51359321 :
>  New right edge of the window -> 51359321+1871*128=51598809
 ...
>  Now imagine ACK were delivered out of order and tcp_add_backlog() sets window based on wrong packet.
>  New right edge of the window -> 51521241+859*128=51631193
> 
> Normally TCP stack handles OOO packets just fine, but it
> turns out tcp_add_backlog() does not. It can update the window
> field of the aggregated packet even if the ACK sequence
> of the last received packet is too old.
> 
> Many thanks to Alexandre Ferrieux for independently reporting the issue
> and suggesting a fix.
> 
> Fixes: 4f693b55c3d2 ("tcp: implement coalescing on backlog queue")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reported-by: Alexandre Ferrieux <alexandre.ferrieux@orange.com>

Applied and queued up for -stable, thank you.