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

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Date:   Mon, 25 May 2020 01:05:37 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org,
        christian.brauner@...ntu.com, tycho@...ho.ws,
        keescook@...omium.org, cyphar@...har.com,
        Jeffrey Vander Stoep <jeffv@...gle.com>, jannh@...gle.com,
        rsesek@...gle.com, palmer@...gle.com,
        Matt Denton <mpdenton@...gle.com>,
        Kees Cook <keescook@...gle.com>
Subject: Re: [PATCH 2/5] seccomp: Introduce addfd ioctl to seccomp user
 notifier

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));
	}