[PATCH v6 5/7] fs: Expand __receive_fd() to accept existing fd

[Kees Cook]

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

Thanks to Colin Ian King <colin.k...@canonical.com> for pointing out an
uninitialized variable exposure in an earlier version of this patch.

Reviewed-by: Sargun Dhillon <sar...@sargun.me>
Signed-off-by: Kees Cook <keesc...@chromium.org>
---
 fs/file.c            | 24 ++++++++++++++++++------
 include/linux/file.h | 10 +++++++---
 2 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 0efdcf413210..11313ff36802 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -937,6 +937,7 @@ int replace_fd(unsigned fd, struct file *file, unsigned 
flags)
 /**
  * __receive_fd() - 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
@@ -950,7 +951,7 @@ int replace_fd(unsigned fd, struct file *file, unsigned 
flags)
  *
  * Returns newly install fd or -ve on error.
  */
-int __receive_fd(struct file *file, int __user *ufd, unsigned int o_flags)
+int __receive_fd(int fd, struct file *file, int __user *ufd, unsigned int 
o_flags)
 {
        struct socket *sock;
        int new_fd;
@@ -960,18 +961,30 @@ int __receive_fd(struct file *file, int __user *ufd, 
unsigned int o_flags)
        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;
+       } else
+               new_fd = fd;
 
        if (ufd) {
                error = put_user(new_fd, ufd);
                if (error) {
-                       put_unused_fd(new_fd);
+                       if (fd < 0)
+                               put_unused_fd(new_fd);
                        return error;
                }
        }
 
+       if (fd < 0)
+               fd_install(new_fd, get_file(file));
+       else {
+               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
@@ -982,7 +995,6 @@ int __receive_fd(struct file *file, int __user *ufd, 
unsigned int o_flags)
                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 d9fee9f5c8da..225982792fa2 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 __receive_fd(struct file *file, int __user *ufd,
+extern int __receive_fd(int 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)
 {
        if (ufd == NULL)
                return -EFAULT;
-       return __receive_fd(file, ufd, o_flags);
+       return __receive_fd(-1, file, ufd, o_flags);
 }
 static inline int receive_fd(struct file *file, unsigned int o_flags)
 {
-       return __receive_fd(file, NULL, o_flags);
+       return __receive_fd(-1, file, NULL, o_flags);
+}
+static inline int receive_fd_replace(int fd, struct file *file, unsigned int 
o_flags)
+{
+       return __receive_fd(fd, file, NULL, o_flags);
 }
 
 extern void flush_delayed_fput(void);
-- 
2.25.1