netdev - Re: [PATCH net] tcp: md5: allow changing MD5 keys in all socket states

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Date:   Thu, 2 Jul 2020 10:06:48 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net] tcp: md5: allow changing MD5 keys in all socket states

On Thu, Jul 2, 2020 at 7:29 AM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> ----- On Jul 1, 2020, at 9:39 PM, Eric Dumazet edumazet@...gle.com wrote:
>
> > This essentially reverts commit 721230326891 ("tcp: md5: reject TCP_MD5SIG
> > or TCP_MD5SIG_EXT on established sockets")
> >
> > Mathieu reported that many vendors BGP implementations can
> > actually switch TCP MD5 on established flows.
> >
> > Quoting Mathieu :
> >   Here is a list of a few network vendors along with their behavior
> >   with respect to TCP MD5:
> >
> >   - Cisco: Allows for password to be changed, but within the hold-down
> >     timer (~180 seconds).
> >   - Juniper: When password is initially set on active connection it will
> >     reset, but after that any subsequent password changes no network
> >     resets.
> >   - Nokia: No notes on if they flap the tcp connection or not.
> >   - Ericsson/RedBack: Allows for 2 password (old/new) to co-exist until
> >     both sides are ok with new passwords.
> >   - Meta-Switch: Expects the password to be set before a connection is
> >     attempted, but no further info on whether they reset the TCP
> >     connection on a change.
> >   - Avaya: Disable the neighbor, then set password, then re-enable.
> >   - Zebos: Would normally allow the change when socket connected.
> >
> > We can revert my prior change because commit 9424e2e7ad93 ("tcp: md5: fix
> > potential
> > overestimation of TCP option space") removed the leak of 4 kernel bytes to
> > the wire that was the main reason for my patch.
>
> Hi Eric,
>
> This is excellent news! Thanks for looking into it.
>
> As this revert re-enables all ABI scenarios previously supported, I suspect
> this means knowing whether transitions of live TCP sockets from no-md5 to
> enabled-md5 is often used in practice is now irrelevant ?

I can only answer this question for two linux peers. They should
behave correctly,
although lack of SACK might cause slowdown in lossy environments.

In any case, we revert to a prior situation, so if anything is broken,
we will need more details,
like a packet capture.


>
> Thanks,
>
> Mathieu
>
>
> >
> > While doing my investigations, I found a bug when a MD5 key is changed, leading
> > to these commits that stable teams want to consider before backporting this
> > revert :
> >
> > Commit 6a2febec338d ("tcp: md5: add missing memory barriers in
> > tcp_md5_do_add()/tcp_md5_hash_key()")
> > Commit e6ced831ef11 ("tcp: md5: refine tcp_md5_do_add()/tcp_md5_hash_key()
> > barriers")
> >
> > Fixes: 721230326891 "tcp: md5: reject TCP_MD5SIG or TCP_MD5SIG_EXT on
> > established sockets"
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Reported-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > ---
> > net/ipv4/tcp.c | 5 +----
> > 1 file changed, 1 insertion(+), 4 deletions(-)
> >
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index
> > c33f7c6aff8eea81d374644cd251bd2b96292651..861fbd84c9cf58af4126c80a27925cd6f70f300d
> > 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -3246,10 +3246,7 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
> > #ifdef CONFIG_TCP_MD5SIG
> >       case TCP_MD5SIG:
> >       case TCP_MD5SIG_EXT:
> > -             if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
> > -                     err = tp->af_specific->md5_parse(sk, optname, optval, optlen);
> > -             else
> > -                     err = -EINVAL;
> > +             err = tp->af_specific->md5_parse(sk, optname, optval, optlen);
> >               break;
> > #endif
> >       case TCP_USER_TIMEOUT:
> > --
> > 2.27.0.212.ge8ba1cc988-goog
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com