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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jul 1, 2020, at 9:39 PM, Eric Dumazet edumazet@google.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 ?

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@google.com>
> Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.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