linux-kernel - Re: [PATCH] tcp: Add listening address to SYN flood message

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Message-ID: <CAMuHMdU4_BACY_7M8rQxPt+_A64Oz8O9PoAkFQfsP2yaWFUM6g@mail.gmail.com>
Date:   Tue, 15 Nov 2022 11:29:12 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Jamie Bainbridge <jamie.bainbridge@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tcp: Add listening address to SYN flood message

Hi Andrew,

On Thu, Nov 10, 2022 at 11:42 PM Andrew Lunn <andrew@...n.ch> wrote:
> On Fri, Nov 11, 2022 at 08:20:18AM +1100, Jamie Bainbridge wrote:
> > On Fri, 11 Nov 2022 at 00:51, Andrew Lunn <andrew@...n.ch> wrote:
> > > On Thu, Nov 10, 2022 at 09:21:06PM +1100, Jamie Bainbridge wrote:
> > > > The SYN flood message prints the listening port number, but on a system
> > > > with many processes bound to the same port on different IPs, it's
> > > > impossible to tell which socket is the problem.
> > > >
> > > > Add the listen IP address to the SYN flood message. It might have been
> > > > nicer to print the address first, but decades of monitoring tools are
> > > > watching for the string "SYN flooding on port" so don't break that.
> > > >
> > > > Tested with each protcol's "any" address and a host address:
> > > >
> > > >  Possible SYN flooding on port 9001. IP 0.0.0.0.
> > > >  Possible SYN flooding on port 9001. IP 127.0.0.1.
> > > >  Possible SYN flooding on port 9001. IP ::.
> > > >  Possible SYN flooding on port 9001. IP fc00::1.
> > > >
> > > > Signed-off-by: Jamie Bainbridge <jamie.bainbridge@...il.com>
> > > > ---
> > > >  net/ipv4/tcp_input.c | 16 +++++++++++++---
> > > >  1 file changed, 13 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > > > index 0640453fce54b6daae0861d948f3db075830daf6..fb86056732266fedc8ad574bbf799dbdd7a425a3 100644
> > > > --- a/net/ipv4/tcp_input.c
> > > > +++ b/net/ipv4/tcp_input.c
> > > > @@ -6831,9 +6831,19 @@ static bool tcp_syn_flood_action(const struct sock *sk, const char *proto)
> > > >               __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDROP);
> > > >
> > > >       if (!queue->synflood_warned && syncookies != 2 &&
> > > > -         xchg(&queue->synflood_warned, 1) == 0)
> > > > -             net_info_ratelimited("%s: Possible SYN flooding on port %d. %s.  Check SNMP counters.\n",
> > > > -                                  proto, sk->sk_num, msg);
> > > > +         xchg(&queue->synflood_warned, 1) == 0) {
> > > > +#if IS_ENABLED(CONFIG_IPV6)
> > > > +             if (sk->sk_family == AF_INET6) {
> > >
> > > Can the IS_ENABLED() go inside the if? You get better build testing
> > > that way.
> > >
> > >      Andrew
> >
> > Are you sure? Why would the IS_ENABLED() be inside of a condition
> > which isn't compiled in? If IPv6 isn't compiled in then the condition
> > would never evaluate as true, so seems pointless a pointless
> > comparison to make? People not compiling in IPv6 have explicitly asked
> > *not* to have their kernel filled with a bunch of "if (family ==
> > AF_INET6)" haven't they?
> >
> > There are many other examples of this pattern of "IS_ENABLED()" first
> > and "if (family == AF_INET6)" inside it, but I can't see any of the
> > inverse which I think you're suggesting, see:
> >
> >  grep -C1 -ERHn "IS_ENABLED\(CONFIG_IPV6\)" net | grep -C1 "family == AF_INET6"
> >
> > Please let me know if I've misunderstood?
>
> So what i'm suggesting is
>
>                if (IS_ENABLED(CONFIG_IPV6) && sk->sk_family == AF_INET6) {
>                        net_info_ratelimited("%s: Possible SYN flooding on port %d. IP %pI6c. %s.  Check SNMP counters.\n",
>                                        proto, sk->sk_num,
>                                        &sk->sk_v6_rcv_saddr, msg);
>                 }

Unfortunately the IPv6-specific members are not defined if
CONFIG_IPV6=n. Patch sent.

https://lore.kernel.org/netdev/d1ecf500f07e063d4e8e34f4045ddca55416c686.1668507036.git.geert+renesas@glider.be

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds