[PATCH v5 5/7] fs: Expand __fd_install_received() to accept fd

[Kees Cook <keescook@xxxxxxxxxxxx>]

Expand __fd_install_received() with support for replace_fd() for the
coming seccomp "addfd" ioctl(). Add new wrapper fd_replace_received()
for the new mode and update existing wrappers to retain old mode.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 fs/file.c            | 22 +++++++++++++++++-----
 include/linux/file.h | 10 +++++++---
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index de85a42defe2..9568bcfd1f44 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -937,6 +937,7 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
 /**
  * __fd_install_received() - Install received file into file descriptor table
  *
+ * @fd: fd to install into (if negative, a new fd will be allocated)
  * @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
@@ -947,7 +948,8 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
  *
  * Returns newly install fd or -ve on error.
  */
-int __fd_install_received(struct file *file, int __user *ufd, unsigned int o_flags)
+int __fd_install_received(int fd, struct file *file, int __user *ufd,
+			  unsigned int o_flags)
 {
 	struct socket *sock;
 	int new_fd;
@@ -957,9 +959,11 @@ int __fd_install_received(struct file *file, int __user *ufd, unsigned int o_fla
 	if (error)
 		return error;
 
-	new_fd = get_unused_fd_flags(o_flags);
-	if (new_fd < 0)
-		return new_fd;
+	if (fd < 0) {
+		new_fd = get_unused_fd_flags(o_flags);
+		if (new_fd < 0)
+			return new_fd;
+	}
 
 	if (ufd) {
 		error = put_user(new_fd, ufd);
@@ -969,6 +973,15 @@ int __fd_install_received(struct file *file, int __user *ufd, unsigned int o_fla
 		}
 	}
 
+	if (fd < 0)
+		fd_install(new_fd, get_file(file));
+	else {
+		new_fd = fd;
+		error = replace_fd(new_fd, file, o_flags);
+		if (error)
+			return error;
+	}
+
 	/* Bump the usage count and install the file. The resulting value of
 	 * "error" is ignored here since we only need to take action when
 	 * the file is a socket and testing "sock" for NULL is sufficient.
@@ -978,7 +991,6 @@ int __fd_install_received(struct file *file, int __user *ufd, unsigned int o_fla
 		sock_update_netprioidx(&sock->sk->sk_cgrp_data);
 		sock_update_classid(&sock->sk->sk_cgrp_data);
 	}
-	fd_install(new_fd, get_file(file));
 	return new_fd;
 }
 
diff --git a/include/linux/file.h b/include/linux/file.h
index e19974ed9322..04389b0da11b 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -92,18 +92,22 @@ extern void put_unused_fd(unsigned int fd);
 
 extern void fd_install(unsigned int fd, struct file *file);
 
-extern int __fd_install_received(struct file *file, int __user *ufd,
+extern int __fd_install_received(int fd, struct file *file, int __user *ufd,
 				 unsigned int o_flags);
 static inline int fd_install_received_user(struct file *file, int __user *ufd,
 					   unsigned int o_flags)
 {
 	if (ufd == NULL)
 		return -EFAULT;
-	return __fd_install_received(file, ufd, o_flags);
+	return __fd_install_received(-1, file, ufd, o_flags);
 }
 static inline int fd_install_received(struct file *file, unsigned int o_flags)
 {
-	return __fd_install_received(file, NULL, o_flags);
+	return __fd_install_received(-1, file, NULL, o_flags);
+}
+static inline int fd_replace_received(int fd, struct file *file, unsigned int o_flags)
+{
+	return __fd_install_received(fd, file, NULL, o_flags);
 }
 
 extern void flush_delayed_fput(void);
-- 
2.25.1