net: scm: introduce and use scm_recv_unix helper

Recently, our friends from bluetooth subsystem reported [1] that after
commit 5e2ff67 ("scm: add SO_PASSPIDFD and SCM_PIDFD") scm_recv()
helper become unusable in kernel modules (because it uses unexported
pidfd_prepare() API).

We were aware of this issue and workarounded it in a hard way
by commit 97154bc ("af_unix: Kconfig: make CONFIG_UNIX bool").

But recently a new functionality was added in the scope of commit
817efd3 ("Bluetooth: hci_sock: Forward credentials to monitor")
and after that bluetooth can't be compiled as a kernel module.

After some discussion in [1] we decided to split scm_recv() into
two helpers, one won't support SCM_PIDFD (used for unix sockets),
and another one will be completely the same as it was before commit
5e2ff67 ("scm: add SO_PASSPIDFD and SCM_PIDFD").

Link: https://lore.kernel.org/lkml/CAJqdLrpFcga4n7wxBhsFqPQiN8PKFVr6U10fKcJ9W7AcZn+o6Q@mail.gmail.com/ [1]
Fixes: 5e2ff67 ("scm: add SO_PASSPIDFD and SCM_PIDFD")
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20230627174314.67688-3-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: mihalicyn

include/net/scm.h

@@ -153,16 +153,16 @@ static __inline__ void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm
 		fd_install(pidfd, pidfd_file);
 }
 
-static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
-				struct scm_cookie *scm, int flags)
+static inline bool __scm_recv_common(struct socket *sock, struct msghdr *msg,
+				     struct scm_cookie *scm, int flags)
 {
 	if (!msg->msg_control) {
 		if (test_bit(SOCK_PASSCRED, &sock->flags) ||
 		    test_bit(SOCK_PASSPIDFD, &sock->flags) ||
 		    scm->fp || scm_has_secdata(sock))
 			msg->msg_flags |= MSG_CTRUNC;
 		scm_destroy(scm);
-		return;
+		return false;
 	}
 
 	if (test_bit(SOCK_PASSCRED, &sock->flags)) {
@@ -175,19 +175,34 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
 		put_cmsg(msg, SOL_SOCKET, SCM_CREDENTIALS, sizeof(ucreds), &ucreds);
 	}
 
-	if (test_bit(SOCK_PASSPIDFD, &sock->flags))
-		scm_pidfd_recv(msg, scm);
+	scm_passec(sock, msg, scm);
 
-	scm_destroy_cred(scm);
+	if (scm->fp)
+		scm_detach_fds(msg, scm);
 
-	scm_passec(sock, msg, scm);
+	return true;
+}
 
-	if (!scm->fp)
+static inline void scm_recv(struct socket *sock, struct msghdr *msg,
+			    struct scm_cookie *scm, int flags)
+{
+	if (!__scm_recv_common(sock, msg, scm, flags))
 		return;
-	
-	scm_detach_fds(msg, scm);
+
+	scm_destroy_cred(scm);
 }
 
+static inline void scm_recv_unix(struct socket *sock, struct msghdr *msg,
+				 struct scm_cookie *scm, int flags)
+{
+	if (!__scm_recv_common(sock, msg, scm, flags))
+		return;
+
+	if (test_bit(SOCK_PASSPIDFD, &sock->flags))
+		scm_pidfd_recv(msg, scm);
+
+	scm_destroy_cred(scm);
+}
 
 #endif /* __LINUX_NET_SCM_H */
 

net/unix/af_unix.c

@@ -2427,7 +2427,7 @@ int __unix_dgram_recvmsg(struct sock *sk, struct msghdr *msg, size_t size,
 	}
 	err = (flags & MSG_TRUNC) ? skb->len - skip : size;
 
-	scm_recv(sock, msg, &scm, flags);
+	scm_recv_unix(sock, msg, &scm, flags);
 
 out_free:
 	skb_free_datagram(sk, skb);
@@ -2808,7 +2808,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 
 	mutex_unlock(&u->iolock);
 	if (state->msg)
-		scm_recv(sock, state->msg, &scm, flags);
+		scm_recv_unix(sock, state->msg, &scm, flags);
 	else
 		scm_destroy(&scm);
 out: