af_unix: Inherit sk_flags at connect().

af_unix: Inherit sk_flags at connect().

For SOCK_STREAM embryo sockets, the SO_PASS{CRED,PIDFD,SEC} options
are inherited from the parent listen()ing socket.

Currently, this inheritance happens at accept(), because these
attributes were stored in sk->sk_socket->flags and the struct socket
is not allocated until accept().

This leads to unintentional behaviour.

When a peer sends data to an embryo socket in the accept() queue,
unix_maybe_add_creds() embeds credentials into the skb, even if
neither the peer nor the listener has enabled these options.

If the option is enabled, the embryo socket receives the ancillary
data after accept().  If not, the data is silently discarded.

This conservative approach works for SO_PASS{CRED,PIDFD,SEC}, but
would not for SO_PASSRIGHTS; once an SCM_RIGHTS with a hung file
descriptor was sent, it'd be game over.

To avoid this, we will need to preserve SOCK_PASSRIGHTS even on embryo
sockets.

Commit aed6ecef55d7 ("af_unix: Save listener for embryo socket.")
made it possible to access the parent's flags in sendmsg() via
unix_sk(other)->listener->sk->sk_socket->flags, but this introduces
an unnecessary condition that is irrelevant for most sockets,
accept()ed sockets and clients.

Therefore, we moved SOCK_PASSXXX into struct sock.

Let’s inherit sk->sk_scm_recv_flags at connect() to avoid receiving
SCM_RIGHTS on embryo sockets created from a parent with SO_PASSRIGHTS=0.

Note that the parent socket is locked in connect() so we don't need
READ_ONCE() for sk_scm_recv_flags.

Now, we can remove !other->sk_socket check in unix_maybe_add_creds()
to avoid slow SOCK_PASS{CRED,PIDFD} handling for embryo sockets
created from a parent with SO_PASS{CRED,PIDFD}=0.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>