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

[Eric Dumazet <edumazet@google.com>]

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.

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