net/unix: support SCM_SECURITY for stream sockets

SCM_SECURITY was originally only implemented for datagram sockets,
not for stream sockets. However, SCM_CREDENTIALS is supported on
Unix stream sockets. For consistency, implement Unix stream support
for SCM_SECURITY as well. Also clean up the existing code and get
rid of the superfluous UNIXSID macro.

Motivated by https://bugzilla.redhat.com/show_bug.cgi?id=1224211,
where systemd was using SCM_CREDENTIALS and assumed wrongly that
SCM_SECURITY was also supported on Unix stream sockets.

Signed-off-by: Stephen Smalley <[e-mail]>
Acked-by: Paul Moore <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/af_unix.h

···
 };
 
 #define UNIXCB(skb) 	(*(struct unix_skb_parms *)&((skb)->cb))
-#define UNIXSID(skb)	(&UNIXCB((skb)).secid)
 
 #define unix_state_lock(s)	spin_lock(&unix_sk(s)->lock)
 #define unix_state_unlock(s)	spin_unlock(&unix_sk(s)->lock)

net/unix/af_unix.c

···
 #ifdef CONFIG_SECURITY_NETWORK
 static void unix_get_secdata(struct scm_cookie *scm, struct sk_buff *skb)
 {
-	memcpy(UNIXSID(skb), &scm->secid, sizeof(u32));
+	UNIXCB(skb).secid = scm->secid;
 }
 
 static inline void unix_set_secdata(struct scm_cookie *scm, struct sk_buff *skb)
 {
-	scm->secid = *UNIXSID(skb);
+	scm->secid = UNIXCB(skb).secid;
+}
+
+static inline bool unix_secdata_eq(struct scm_cookie *scm, struct sk_buff *skb)
+{
+	return (scm->secid == UNIXCB(skb).secid);
 }
 #else
 static inline void unix_get_secdata(struct scm_cookie *scm, struct sk_buff *skb)
···
 
 static inline void unix_set_secdata(struct scm_cookie *scm, struct sk_buff *skb)
 { }
+
+static inline bool unix_secdata_eq(struct scm_cookie *scm, struct sk_buff *skb)
+{
+	return true;
+}
 #endif /* CONFIG_SECURITY_NETWORK */
 
 /*
···
 	UNIXCB(skb).uid = scm->creds.uid;
 	UNIXCB(skb).gid = scm->creds.gid;
 	UNIXCB(skb).fp = NULL;
+	unix_get_secdata(scm, skb);
 	if (scm->fp && send_fds)
 		err = unix_attach_fds(scm, skb);
 
···
 	if (err < 0)
 		goto out_free;
 	max_level = err + 1;
-	unix_get_secdata(&scm, skb);
 
 	skb_put(skb, len - data_len);
 	skb->data_len = data_len;
···
 			/* Never glue messages from different writers */
 			if ((UNIXCB(skb).pid  != scm.pid) ||
 			    !uid_eq(UNIXCB(skb).uid, scm.creds.uid) ||
-			    !gid_eq(UNIXCB(skb).gid, scm.creds.gid))
+			    !gid_eq(UNIXCB(skb).gid, scm.creds.gid) ||
+			    !unix_secdata_eq(&scm, skb))
 				break;
 		} else if (test_bit(SOCK_PASSCRED, &sock->flags)) {
 			/* Copy credentials */
 			scm_set_cred(&scm, UNIXCB(skb).pid, UNIXCB(skb).uid, UNIXCB(skb).gid);
+			unix_set_secdata(&scm, skb);
 			check_creds = true;
 		}