[PATCH net-next v4 2/3] net: af_unix: useful handling of LSM denials on SCM_RIGHTS

[Jori Koolstra <jkoolstra@xs4all.nl>]

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 <jkoolstra@xs4all.nl>
---
 arch/alpha/include/uapi/asm/socket.h  |  2 ++
 arch/mips/include/uapi/asm/socket.h   |  2 ++
 arch/parisc/include/uapi/asm/socket.h |  2 ++
 arch/sparc/include/uapi/asm/socket.h  |  2 ++
 include/net/af_unix.h                 |  1 +
 include/net/scm.h                     | 13 +++------
 include/uapi/asm-generic/socket.h     |  2 ++
 net/compat.c                          |  4 +--
 net/core/scm.c                        | 40 +++++++++++++++++++++++----
 net/unix/af_unix.c                    |  9 ++++++
 10 files changed, 61 insertions(+), 16 deletions(-)

diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
index 5ef57f88df6b..946a5fad2691 100644
--- a/arch/alpha/include/uapi/asm/socket.h
+++ b/arch/alpha/include/uapi/asm/socket.h
@@ -155,6 +155,8 @@
 #define SO_INQ			84
 #define SCM_INQ			SO_INQ
 
+#define SO_RIGHTS_NOTRUNC      85
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64
diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
index 72fb1b006da9..f1641dde135f 100644
--- a/arch/mips/include/uapi/asm/socket.h
+++ b/arch/mips/include/uapi/asm/socket.h
@@ -166,6 +166,8 @@
 #define SO_INQ			84
 #define SCM_INQ			SO_INQ
 
+#define SO_RIGHTS_NOTRUNC      85
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64
diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
index c16ec36dfee6..f3a3815c7dc2 100644
--- a/arch/parisc/include/uapi/asm/socket.h
+++ b/arch/parisc/include/uapi/asm/socket.h
@@ -147,6 +147,8 @@
 #define SO_INQ			0x4052
 #define SCM_INQ			SO_INQ
 
+#define SO_RIGHTS_NOTRUNC	0x4053
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64
diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
index 71befa109e1c..7907f3b1f0ee 100644
--- a/arch/sparc/include/uapi/asm/socket.h
+++ b/arch/sparc/include/uapi/asm/socket.h
@@ -148,6 +148,8 @@
 #define SO_INQ                   0x005d
 #define SCM_INQ                  SO_INQ
 
+#define SO_RIGHTS_NOTRUNC        0x005e
+
 #if !defined(__KERNEL__)
 
 
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..86ae6bc109ec 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);
@@ -107,13 +107,8 @@ void scm_recv(struct socket *sock, struct msghdr *msg,
 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)
-{
-	if (!ufd)
-		return -EFAULT;
-	return receive_fd(f, ufd, flags);
-}
+int scm_recv_one_fd(struct file *f, int __user *ufd, unsigned int flags,
+		    bool notrunc);
 
 #endif /* __LINUX_NET_SCM_H */
 
diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
index 53b5a8c002b1..84ea7b92936e 100644
--- a/include/uapi/asm-generic/socket.h
+++ b/include/uapi/asm-generic/socket.h
@@ -150,6 +150,8 @@
 #define SO_INQ			84
 #define SCM_INQ			SO_INQ
 
+#define SO_RIGHTS_NOTRUNC	85
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
diff --git a/net/compat.c b/net/compat.c
index d68cf9c3aad5..6bdf4a2c9077 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -286,7 +286,7 @@ static int scm_max_fds_compat(struct msghdr *msg)
 	return (msg->msg_controllen - sizeof(struct compat_cmsghdr)) / sizeof(int);
 }
 
-void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm)
+void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm, bool notrunc)
 {
 	struct compat_cmsghdr __user *cm =
 		(struct compat_cmsghdr __user *)msg->msg_control_user;
@@ -296,7 +296,7 @@ void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm)
 	int err = 0, i;
 
 	for (i = 0; i < fdmax; i++) {
-		err = scm_recv_one_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
+		err = scm_recv_one_fd(scm->fp->fp[i], cmsg_data + i, o_flags, notrunc);
 		if (err < 0)
 			break;
 	}
diff --git a/net/core/scm.c b/net/core/scm.c
index a73b1eb30fd2..9454d06ce5ed 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -351,7 +351,31 @@ static int scm_max_fds(struct msghdr *msg)
 	return (msg->msg_controllen - sizeof(struct cmsghdr)) / sizeof(int);
 }
 
-void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
+int scm_recv_one_fd(struct file *f, int __user *ufd, unsigned int flags,
+		    bool notrunc)
+{
+	int error;
+
+	if (!ufd)
+		return -EFAULT;
+
+	error = security_file_receive(f);
+	if (error)
+		return notrunc ? put_user(error, ufd) : error;
+
+	FD_PREPARE(fdf, flags, get_file(f));
+	if (fdf.err)
+		return fdf.err;
+
+	error = put_user(fd_prepare_fd(fdf), ufd);
+	if (error)
+		return error;
+
+	__receive_sock(fd_prepare_file(fdf));
+	return fd_publish(fdf);
+}
+
+void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm, bool notrunc)
 {
 	struct cmsghdr __user *cm =
 		(__force struct cmsghdr __user *)msg->msg_control_user;
@@ -365,12 +389,12 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
 		return;
 
 	if (msg->msg_flags & MSG_CMSG_COMPAT) {
-		scm_detach_fds_compat(msg, scm);
+		scm_detach_fds_compat(msg, scm, notrunc);
 		return;
 	}
 
 	for (i = 0; i < fdmax; i++) {
-		err = scm_recv_one_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
+		err = scm_recv_one_fd(scm->fp->fp[i], cmsg_data + i, o_flags, notrunc);
 		if (err < 0)
 			break;
 	}
@@ -542,8 +566,14 @@ void scm_recv_unix(struct socket *sock, struct msghdr *msg,
 	if (!__scm_recv_common(sock->sk, msg, scm, flags))
 		return;
 
-	if (scm->fp)
-		scm_detach_fds(msg, scm);
+	if (scm->fp) {
+		struct unix_sock *u;
+		bool notrunc;
+
+		u = unix_sk(sock->sk);
+		notrunc = READ_ONCE(u->scm_rights_notrunc);
+		scm_detach_fds(msg, scm, notrunc);
+	}
 
 	if (sock->sk->sk_scm_pidfd)
 		scm_pidfd_recv(msg, scm);
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index f7a9d55eee8a..83274ce18e06 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -921,6 +921,7 @@ static bool unix_custom_sockopt(int optname)
 {
 	switch (optname) {
 	case SO_INQ:
+	case SO_RIGHTS_NOTRUNC:
 		return true;
 	default:
 		return false;
@@ -956,6 +957,14 @@ static int unix_setsockopt(struct socket *sock, int level, int optname,
 
 		WRITE_ONCE(u->recvmsg_inq, val);
 		break;
+
+	case SO_RIGHTS_NOTRUNC:
+		if (val > 1 || val < 0)
+			return -EINVAL;
+
+		WRITE_ONCE(u->scm_rights_notrunc, val);
+		break;
+
 	default:
 		return -ENOPROTOOPT;
 	}
-- 
2.55.0