fs: Expand __receive_fd() to accept existing fd

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.king@canonical.com> for pointing out an
uninitialized variable exposure in an earlier version of this patch.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Dmitry Kadashev <dkadashev@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-fsdevel@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>

diff --git a/fs/file.c b/fs/file.c
index 56d96d5..4fb1117 100644 (file)
--- a/fs/file.c
+++ b/fs/file.c
@@ -939,6 +939,7 @@ out_unlock:
 /**
  * __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
@@ -952,7 +953,7 @@ out_unlock:
  *
  * 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)
 {
        int new_fd;
        int error;
@@ -961,21 +962,33 @@ 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 sock usage counts, if any. */
        __receive_sock(file);
-       fd_install(new_fd, get_file(file));
        return new_fd;
 }
 

diff --git a/include/linux/file.h b/include/linux/file.h
index d9fee9f..2259827 100644 (file)
--- 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);