Re: [PATCH 2/5] seccomp: Introduce addfd ioctl to seccomp user notifier

[Al Viro]

On Sun, May 24, 2020 at 04:39:39PM -0700, Sargun Dhillon wrote:
> +static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd)
> +{
> +     int ret;
> +
> +     /*
> +      * Remove the notification, and reset the list pointers, indicating
> +      * that it has been handled.
> +      */
> +     list_del_init(&addfd->list);
> +
> +     ret = security_file_receive(addfd->file);
> +     if (ret)
> +             goto out;
> +
> +     if (addfd->fd >= 0) {
> +             ret = replace_fd(addfd->fd, addfd->file, addfd->flags);
> +             if (ret >= 0)
> +                     fput(addfd->file);
> +     } else {
> +             ret = get_unused_fd_flags(addfd->flags);
> +             if (ret >= 0)
> +                     fd_install(ret, addfd->file);

Bad refcounting rules.  *IF* we go with anything of that sort (and I'm not
convinced that the entire series makes sense), it's better to have more
uniform rules re reference consumption/disposal.

Make the destructor of addfd *ALWAYS* drop its reference.  And have this
function go

        if (addfd->fd >= 0) {
                ret = replace_fd(addfd->fd, addfd->file, addfd->flags);
        } else {
                ret = get_unused_fd_flags(addfd->flags);
                if (ret >= 0)
                        fd_install(ret, get_file(addfd->file));
        }