Right now if some LSM such as Smack denies an AF_UNIX socket peer to
receive an SCM_RIGHTS fd, the SCM_RIGHTS fd array will be cut short at
that point, and MSG_CTRUNC is set on return of recvmsg(). This is
highly problematic behaviour, because it leaves the receiver
wondering what happened. As per man page MSG_CTRUNC is supposed to
indicate that the control buffer was sized too short, but suddenly
a permission error might result in the exact same flag being set.
Moreover, the receiver has no chance to determine how many fds got
originally sent and how many were suppressed.[1]

Add a SO_RIGHTS_NOTRUNC option to UNIX sockets to enable more useful
handling of LSM denials when receiving SCM_RIGHTS messages: instead of
truncating the message at the first blocked fd, keep every fd slot
and store the LSM errno in the blocked slot.

[1]: https://github.com/uapi-group/kernel-features#useful-handling-of-lsm-denials-on-scm_rights

Signed-off-by: Jori Koolstra <redacted>

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 34f53dde65ce..bb1b3dee02e8 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h

@@ -49,6 +49,7 @@ struct unix_sock {
 	struct scm_stat		scm_stat;
 	int			inq_len;
 	bool			recvmsg_inq;
+	bool			scm_rights_notrunc;
 #if IS_ENABLED(CONFIG_AF_UNIX_OOB)
 	struct sk_buff		*oob_skb;
 #endif

diff --git a/include/net/scm.h b/include/net/scm.h
index c52519669349..761cda0803fb 100644
--- a/include/net/scm.h
+++ b/include/net/scm.h

@@ -50,8 +50,8 @@ struct scm_cookie {
 #endif
 };
 
-void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm);
-void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm);
+void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm, bool notrunc);
+void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm, bool notrunc);
 int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *scm);
 void __scm_destroy(struct scm_cookie *scm);
 struct scm_fp_list *scm_fp_dup(struct scm_fp_list *fpl);

@@ -108,11 +108,18 @@ void scm_recv_unix(struct socket *sock, struct msghdr *msg,
 		   struct scm_cookie *scm, int flags);
 
 static inline int scm_recv_one_fd(struct file *f, int __user *ufd,
-				  unsigned int flags)
+				  unsigned int flags, bool notrunc)
 {
+	bool filtered;
+	int error;
+
 	if (!ufd)
 		return -EFAULT;
-	return receive_fd(f, ufd, flags);
+
+	error = receive_fd_filtered(f, ufd, flags, &filtered);
+	if (filtered && notrunc)
+		return put_user(error, ufd);