fs: Move __scm_install_fd() to __receive_fd()

In preparation for users of the "install a received file" logic outside
of net/ (pidfd and seccomp), relocate and rename __scm_install_fd() from
net/core/scm.c to __receive_fd() in fs/file.c, and provide a wrapper
named receive_fd_user(), as future patches will change the interface
to __receive_fd().

Additionally add a comment to fd_install() as a counterpoint to how
__receive_fd() interacts with fput().

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Dmitry Kadashev <dkadashev@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Sargun Dhillon <sargun@sargun.me>
Cc: Ido Schimmel <idosch@idosch.org>
Cc: Ioana Ciornei <ioana.ciornei@nxp.com>
Cc: linux-fsdevel@vger.kernel.org
Cc: netdev@vger.kernel.org
Reviewed-by: Sargun Dhillon <sargun@sargun.me>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

fs/file.c

@@ -18,6 +18,7 @@
 #include <linux/bitops.h>
 #include <linux/spinlock.h>
 #include <linux/rcupdate.h>
+#include <net/sock.h>
 
 unsigned int sysctl_nr_open __read_mostly = 1024*1024;
 unsigned int sysctl_nr_open_min = BITS_PER_LONG;
@@ -613,6 +614,10 @@ void __fd_install(struct files_struct *files, unsigned int fd,
 	rcu_read_unlock_sched();
 }
 
+/*
+ * This consumes the "file" refcount, so callers should treat it
+ * as if they had called fput(file).
+ */
 void fd_install(unsigned int fd, struct file *file)
 {
 	__fd_install(current->files, fd, file);
@@ -931,6 +936,46 @@ out_unlock:
 	return err;
 }
 
+/**
+ * __receive_fd() - Install received file into file descriptor table
+ *
+ * @file: struct file that was received from another process
+ * @ufd: __user pointer to write new fd number to
+ * @o_flags: the O_* flags to apply to the new fd entry
+ *
+ * Installs a received file into the file descriptor table, with appropriate
+ * checks and count updates. Writes the fd number to userspace.
+ *
+ * This helper handles its own reference counting of the incoming
+ * struct file.
+ *
+ * Returns -ve on error.
+ */
+int __receive_fd(struct file *file, int __user *ufd, unsigned int o_flags)
+{
+	int new_fd;
+	int error;
+
+	error = security_file_receive(file);
+	if (error)
+		return error;
+
+	new_fd = get_unused_fd_flags(o_flags);
+	if (new_fd < 0)
+		return new_fd;
+
+	error = put_user(new_fd, ufd);
+	if (error) {
+		put_unused_fd(new_fd);
+		return error;
+	}
+
+	/* Bump the sock usage counts, if any. */
+	__receive_sock(file);
+	fd_install(new_fd, get_file(file));
+	return 0;
+}
+
 static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
 {
 	int err = -EBADF;

include/linux/file.h

@@ -91,6 +91,14 @@ extern void put_unused_fd(unsigned int fd);
 
 extern void fd_install(unsigned int fd, struct file *file);
 
+extern int __receive_fd(struct file *file, int __user *ufd,
+			unsigned int o_flags);
+static inline int receive_fd_user(struct file *file, int __user *ufd,
+				  unsigned int o_flags)
+{
+	return __receive_fd(file, ufd, o_flags);
+}
+
 extern void flush_delayed_fput(void);
 extern void __fput_sync(struct file *);
 

include/net/scm.h

@@ -37,7 +37,6 @@ struct scm_cookie {
 #endif
 };
 
-int __scm_install_fd(struct file *file, int __user *ufd, unsigned int o_flags);
 void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm);
 void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm);
 int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *scm);

net/compat.c

@@ -298,7 +298,7 @@ void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm)
 	int err = 0, i;
 
 	for (i = 0; i < fdmax; i++) {
-		err = __scm_install_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
+		err = receive_fd_user(scm->fp->fp[i], cmsg_data + i, o_flags);
 		if (err)
 			break;
 	}

net/core/scm.c

@@ -280,31 +280,6 @@ void put_cmsg_scm_timestamping(struct msghdr *msg, struct scm_timestamping_inter
 }
 EXPORT_SYMBOL(put_cmsg_scm_timestamping);
 
-int __scm_install_fd(struct file *file, int __user *ufd, unsigned int o_flags)
-{
-	int new_fd;
-	int error;
-
-	error = security_file_receive(file);
-	if (error)
-		return error;
-
-	new_fd = get_unused_fd_flags(o_flags);
-	if (new_fd < 0)
-		return new_fd;
-
-	error = put_user(new_fd, ufd);
-	if (error) {
-		put_unused_fd(new_fd);
-		return error;
-	}
-
-	/* Bump the sock usage counts, if any. */
-	__receive_sock(file);
-	fd_install(new_fd, get_file(file));
-	return 0;
-}
-
 static int scm_max_fds(struct msghdr *msg)
 {
 	if (msg->msg_controllen <= sizeof(struct cmsghdr))
@@ -331,7 +306,7 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
 	}
 
 	for (i = 0; i < fdmax; i++) {
-		err = __scm_install_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
+		err = receive_fd_user(scm->fp->fp[i], cmsg_data + i, o_flags);
 		if (err)
 			break;
 	}