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Date:   Fri, 23 Oct 2020 17:06:28 +0100
From:   Ricardo Dias <rdias@memsql.com>
To:     Eric Dumazet <edumazet@google.com>
Cc:     David Miller <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] tcp: fix race condition when creating child sockets from
 syncookies
Message-ID: <20201023160628.GA316690@rdias-suse-pc.lan>
References: <20201023111352.GA289522@rdias-suse-pc.lan>
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 <20201023155145.GA316015@rdias-suse-pc.lan>
 <CANn89iL2VOH+Mg9-U7pkpMkKykDfhoX-GMRnF-oBmZmCGohDtA@mail.gmail.com>
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On Fri, Oct 23, 2020 at 05:56:07PM +0200, Eric Dumazet wrote:
> On Fri, Oct 23, 2020 at 5:51 PM Ricardo Dias <rdias@memsql.com> wrote:
> >
> > On Fri, Oct 23, 2020 at 04:03:27PM +0200, Eric Dumazet wrote:
> > > On Fri, Oct 23, 2020 at 1:14 PM Ricardo Dias <rdias@memsql.com> wrote:
> > >
> > > ...
> > >
> > > Note that normally, all packets for the same 4-tuple should be handled
> > > by the same cpu,
> > > so this race is quite unlikely to happen in standard setups.
> >
> > I was able to write a small client/server program that used the
> > loopback interface to create connections, which could hit the race
> > condition in 1/200 runs.
> >
> > The server when accepts a connection sends an 8 byte identifier to
> > the client, and then waits for the client to echo the same identifier.
> > The client creates hundreds of simultaneous connections to the server,
> > and in each connection it sends one byte as soon as the connection is
> > established, then reads the 8 byte identifier from the server and sends
> > it back to the server.
> >
> > When we hit the race condition, one of the server connections gets an 8
> > byte identifier different from its own identifier.
> 
> That is on loopback, right ?

Yes it's on loopback.

> 
> A server under syn flood is usually hit on a physical NIC, and a NIC
> will always put all packets of a TCP flow in a single RX queue.
> The cpu associated with this single RX queue won't process two packets
> in parallel.

And what about the loopback interface? Why couldn't the loopback
interface also use a single RX queue?

> 
> Note this issue is known, someone tried to fix it in the past but the
> attempt went nowhere.