Patch "af-unix: passcred support for sendpage" has been added to the 4.3-stable tree

@@ -1550,6 +1550,14 @@ static int unix_scm_to_skb(struct scm_co
 	return err;
 }
 
+static bool unix_passcred_enabled(const struct socket *sock,
+				  const struct sock *other)
+{
+	return test_bit(SOCK_PASSCRED, &sock->flags) ||
+	       !other->sk_socket ||
+	       test_bit(SOCK_PASSCRED, &other->sk_socket->flags);
+}
+
 /*
  * Some apps rely on write() giving SCM_CREDENTIALS
  * We include credentials if source or destination socket

@@ -1560,14 +1568,41 @@ static void maybe_add_creds(struct sk_bu
 {
 	if (UNIXCB(skb).pid)
 		return;
-	if (test_bit(SOCK_PASSCRED, &sock->flags) ||
-	    !other->sk_socket ||
-	    test_bit(SOCK_PASSCRED, &other->sk_socket->flags)) {
+	if (unix_passcred_enabled(sock, other)) {
 		UNIXCB(skb).pid  = get_pid(task_tgid(current));
 		current_uid_gid(&UNIXCB(skb).uid, &UNIXCB(skb).gid);
 	}
 }
 
+static int maybe_init_creds(struct scm_cookie *scm,
+			    struct socket *socket,
+			    const struct sock *other)
+{
+	int err;
+	struct msghdr msg = { .msg_controllen = 0 };
+
+	err = scm_send(socket, &msg, scm, false);
+	if (err)
+		return err;
+
+	if (unix_passcred_enabled(socket, other)) {
+		scm->pid = get_pid(task_tgid(current));
+		current_uid_gid(&scm->creds.uid, &scm->creds.gid);
+	}
+	return err;
+}
+
+static bool unix_skb_scm_eq(struct sk_buff *skb,
+			    struct scm_cookie *scm)
+{
+	const struct unix_skb_parms *u = &UNIXCB(skb);
+
+	return u->pid == scm->pid &&
+	       uid_eq(u->uid, scm->creds.uid) &&
+	       gid_eq(u->gid, scm->creds.gid) &&
+	       unix_secdata_eq(scm, skb);
+}
+
 /*
  *	Send AF_UNIX data.
  */

@@ -1883,8 +1918,10 @@ out_err:
 static ssize_t unix_stream_sendpage(struct socket *socket, struct page *page,
 				    int offset, size_t size, int flags)
 {
-	int err = 0;
-	bool send_sigpipe = true;
+	int err;
+	bool send_sigpipe = false;
+	bool init_scm = true;
+	struct scm_cookie scm;
 	struct sock *other, *sk = socket->sk;
 	struct sk_buff *skb, *newskb = NULL, *tail = NULL;
 

@@ -1902,7 +1939,7 @@ alloc_skb:
 		newskb = sock_alloc_send_pskb(sk, 0, 0, flags & MSG_DONTWAIT,
 					      &err, 0);
 		if (!newskb)
-			return err;
+			goto err;
 	}
 
 	/* we must acquire readlock as we modify already present

@@ -1911,12 +1948,12 @@ alloc_skb:
 	err = mutex_lock_interruptible(&unix_sk(other)->readlock);
 	if (err) {
 		err = flags & MSG_DONTWAIT ? -EAGAIN : -ERESTARTSYS;
-		send_sigpipe = false;
 		goto err;
 	}
 
 	if (sk->sk_shutdown & SEND_SHUTDOWN) {
 		err = -EPIPE;
+		send_sigpipe = true;
 		goto err_unlock;
 	}
 

@@ -1925,17 +1962,27 @@ alloc_skb:
 	if (sock_flag(other, SOCK_DEAD) ||
 	    other->sk_shutdown & RCV_SHUTDOWN) {
 		err = -EPIPE;
+		send_sigpipe = true;
 		goto err_state_unlock;
 	}
 
+	if (init_scm) {
+		err = maybe_init_creds(&scm, socket, other);
+		if (err)
+			goto err_state_unlock;
+		init_scm = false;
+	}
+
 	skb = skb_peek_tail(&other->sk_receive_queue);
 	if (tail && tail == skb) {
 		skb = newskb;
-	} else if (!skb) {
-		if (newskb)
+	} else if (!skb || !unix_skb_scm_eq(skb, &scm)) {
+		if (newskb) {
 			skb = newskb;
-		else
+		} else {
+			tail = skb;
 			goto alloc_skb;
+		}
 	} else if (newskb) {
 		/* this is fast path, we don't necessarily need to
 		 * call to kfree_skb even though with newskb == NULL

@@ -1956,6 +2003,9 @@ alloc_skb:
 	atomic_add(size, &sk->sk_wmem_alloc);
 
 	if (newskb) {
+		err = unix_scm_to_skb(&scm, skb, false);
+		if (err)
+			goto err_state_unlock;
 		spin_lock(&other->sk_receive_queue.lock);
 		__skb_queue_tail(&other->sk_receive_queue, newskb);
 		spin_unlock(&other->sk_receive_queue.lock);

@@ -1965,7 +2015,7 @@ alloc_skb:
 	mutex_unlock(&unix_sk(other)->readlock);
 
 	other->sk_data_ready(other);
-
+	scm_destroy(&scm);
 	return size;
 
 err_state_unlock:

@@ -1976,6 +2026,8 @@ err:
 	kfree_skb(newskb);
 	if (send_sigpipe && !(flags & MSG_NOSIGNAL))
 		send_sig(SIGPIPE, current, 0);
+	if (!init_scm)
+		scm_destroy(&scm);
 	return err;
 }
 

@@ -2279,10 +2331,7 @@ unlock:
 
 		if (check_creds) {
 			/* 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) ||
-			    !unix_secdata_eq(&scm, skb))
+			if (!unix_skb_scm_eq(skb, &scm))
 				break;
 		} else if (test_bit(SOCK_PASSCRED, &sock->flags)) {
 			/* Copy credentials */