Re: [PATCH v7 4/6] files: add a replace_fd_files() function

[Kees Cook]

On Thu, Sep 27, 2018 at 8:11 AM, Tycho Andersen <tycho@xxxxxxxx> wrote:
> Similar to fd_install/__fd_install, we want to be able to replace an fd of
> an arbitrary struct files_struct, not just current's. We'll use this in the
> next patch to implement the seccomp ioctl that allows inserting fds into a
> stopped process' context.
>
> v7: new in v7
>
> Signed-off-by: Tycho Andersen <tycho@xxxxxxxx>
> CC: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> CC: Kees Cook <keescook@xxxxxxxxxxxx>
> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> CC: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> CC: "Serge E. Hallyn" <serge@xxxxxxxxxx>
> CC: Christian Brauner <christian.brauner@xxxxxxxxxx>
> CC: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
> CC: Akihiro Suda <suda.akihiro@xxxxxxxxxxxxx>
> ---
>  fs/file.c            | 22 +++++++++++++++-------
>  include/linux/file.h |  8 ++++++++
>  2 files changed, 23 insertions(+), 7 deletions(-)
>
> diff --git a/fs/file.c b/fs/file.c
> index 7ffd6e9d103d..3b3c5aadaadb 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -850,24 +850,32 @@ __releases(&files->file_lock)
>  }
>
>  int replace_fd(unsigned fd, struct file *file, unsigned flags)
> +{
> +       return replace_fd_task(current, fd, file, flags);
> +}
> +
> +/*
> + * Same warning as __alloc_fd()/__fd_install() here.
> + */
> +int replace_fd_task(struct task_struct *task, unsigned fd,
> +                   struct file *file, unsigned flags)
>  {
>         int err;
> -       struct files_struct *files = current->files;

Same feedback as Jann: on a purely "smaller diff" note, this could
just be s/current/task/ here and all the other s/files/task->files/
would go away...

>
>         if (!file)
> -               return __close_fd(files, fd);
> +               return __close_fd(task->files, fd);
>
> -       if (fd >= rlimit(RLIMIT_NOFILE))
> +       if (fd >= task_rlimit(task, RLIMIT_NOFILE))
>                 return -EBADF;
>
> -       spin_lock(&files->file_lock);
> -       err = expand_files(files, fd);
> +       spin_lock(&task->files->file_lock);
> +       err = expand_files(task->files, fd);
>         if (unlikely(err < 0))
>                 goto out_unlock;
> -       return do_dup2(files, file, fd, flags);
> +       return do_dup2(task->files, file, fd, flags);
>
>  out_unlock:
> -       spin_unlock(&files->file_lock);
> +       spin_unlock(&task->files->file_lock);
>         return err;
>  }
>
> diff --git a/include/linux/file.h b/include/linux/file.h
> index 6b2fb032416c..f94277fee038 100644
> --- a/include/linux/file.h
> +++ b/include/linux/file.h
> @@ -11,6 +11,7 @@
>  #include <linux/posix_types.h>
>
>  struct file;
> +struct task_struct;
>
>  extern void fput(struct file *);
>
> @@ -79,6 +80,13 @@ static inline void fdput_pos(struct fd f)
>
>  extern int f_dupfd(unsigned int from, struct file *file, unsigned flags);
>  extern int replace_fd(unsigned fd, struct file *file, unsigned flags);
> +/*
> + * Warning! This is only safe if you know the owner of the files_struct is
> + * stopped outside syscall context. It's a very bad idea to use this unless you
> + * have similar guarantees in your code.
> + */
> +extern int replace_fd_task(struct task_struct *task, unsigned fd,
> +                          struct file *file, unsigned flags);

Perhaps call this __replace_fd() to indicate the "please don't use
this unless you're very sure"ness of it?

>  extern void set_close_on_exec(unsigned int fd, int flag);
>  extern bool get_close_on_exec(unsigned int fd);
>  extern int get_unused_fd_flags(unsigned flags);
> --
> 2.17.1
>

If I can get an Ack from Al, that would be very nice. :)

-Kees

-- 
Kees Cook
Pixel Security