af_unix: Allow passing cred for embryo without SO_PASSCRED/SO_PASSPIDFD.

Before the cited commit, the kernel unconditionally embedded SCM
credentials to skb for embryo sockets even when both the sender
and listener disabled SO_PASSCRED and SO_PASSPIDFD.

Now, the credentials are added to skb only when configured by the
sender or the listener.

However, as reported in the link below, it caused a regression for
some programs that assume credentials are included in every skb,
but sometimes not now.

The only problematic scenario would be that a socket starts listening
before setting the option.  Then, there will be 2 types of non-small
race window, where a client can send skb without credentials, which
the peer receives as an "invalid" message (and aborts the connection
it seems ?):

  Client                    Server
  ------                    ------
                            s1.listen()  <-- No SO_PASS{CRED,PIDFD}
  s2.connect()
  s2.send()  <-- w/o cred
                            s1.setsockopt(SO_PASS{CRED,PIDFD})
  s2.send()  <-- w/  cred

or

  Client                    Server
  ------                    ------
                            s1.listen()  <-- No SO_PASS{CRED,PIDFD}
  s2.connect()
  s2.send()  <-- w/o cred
                            s3, _ = s1.accept()  <-- Inherit cred options
  s2.send()  <-- w/o cred                            but not set yet

                            s3.setsockopt(SO_PASS{CRED,PIDFD})
  s2.send()  <-- w/  cred

It's unfortunate that buggy programs depend on the behaviour,
but let's restore the previous behaviour.

Fixes: 3f84d57 ("af_unix: Inherit sk_flags at connect().")
Reported-by: Jacek Łuczak <difrost.kernel@gmail.com>
Closes: https://lore.kernel.org/all/68d38b0b-1666-4974-85d4-15575789c8d4@gmail.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Tested-by: Christian Heusel <christian@heusel.eu>
Tested-by: André Almeida <andrealmeid@igalia.com>
Tested-by: Jacek Łuczak <difrost.kernel@gmail.com>
Link: https://patch.msgid.link/20250611202758.3075858-1-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: q2ven

net/unix/af_unix.c

@@ -1971,7 +1971,8 @@ static void unix_maybe_add_creds(struct sk_buff *skb, const struct sock *sk,
 	if (UNIXCB(skb).pid)
 		return;
 
-	if (unix_may_passcred(sk) || unix_may_passcred(other)) {
+	if (unix_may_passcred(sk) || unix_may_passcred(other) ||
+	    !other->sk_socket) {
 		UNIXCB(skb).pid = get_pid(task_tgid(current));
 		current_uid_gid(&UNIXCB(skb).uid, &UNIXCB(skb).gid);
 	}