mptcp: fully established after ADD_ADDR echo on MPJ

Before this patch, receiving an ADD_ADDR echo on the just connected
MP_JOIN subflow -- initiator side, after the MP_JOIN 3WHS -- was
resulting in an MP_RESET. That's because only ACKs with a DSS or
ADD_ADDRs without the echo bit were allowed.

Not allowing the ADD_ADDR echo after an MP_CAPABLE 3WHS makes sense, as
we are not supposed to send an ADD_ADDR before because it requires to be
in full established mode first. For the MP_JOIN 3WHS, that's different:
the ADD_ADDR can be sent on a previous subflow, and the ADD_ADDR echo
can be received on the recently created one. The other peer will already
be in fully established, so it is allowed to send that.

We can then relax the conditions here to accept the ADD_ADDR echo for
MPJ subflows.

Fixes: 67b12f79 ("mptcp: full fully established support after ADD_ADDR")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240731-upstream-net-20240731-mptcp-endp-subflow-signal-v1-1-c8a9b036493b@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/options.c

@@ -958,7 +958,8 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *ssk,
 
 	if (subflow->remote_key_valid &&
 	    (((mp_opt->suboptions & OPTION_MPTCP_DSS) && mp_opt->use_ack) ||
-	     ((mp_opt->suboptions & OPTION_MPTCP_ADD_ADDR) && !mp_opt->echo))) {
+	     ((mp_opt->suboptions & OPTION_MPTCP_ADD_ADDR) &&
+	      (!mp_opt->echo || subflow->mp_join)))) {
 		/* subflows are fully established as soon as we get any
 		 * additional ack, including ADD_ADDR.
 		 */