linux-kernel - Re: [PATCH v4 4/4] seccomp: add support for passing fds via USER

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Message-ID: <CAG48ez0d+tRsfvstU3Kd7T=kkdKbbfpNS+19NLTchhSL1fUyNg@mail.gmail.com>
Date:   Fri, 22 Jun 2018 01:34:18 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     Kees Cook <keescook@...omium.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        containers@...ts.linux-foundation.org,
        Linux API <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        suda.akihiro@....ntt.co.jp, "Tobin C. Harding" <me@...in.cc>
Subject: Re: [PATCH v4 4/4] seccomp: add support for passing fds via USER_NOTIF

On Fri, Jun 22, 2018 at 12:05 AM Tycho Andersen <tycho@...ho.ws> wrote:
>
> The idea here is that the userspace handler should be able to pass an fd
> back to the trapped task, for example so it can be returned from socket().
[...]
> +Userspace can also return file descriptors. For example, one may decide to
> +intercept ``socket()`` syscalls, and return some file descriptor from those
> +based on some policy. To return a file descriptor, the ``return_fd`` member
> +should be non-zero, the ``fd`` argument should be the fd in the listener's
> +table to send to the tracee (similar to how ``SCM_RIGHTS`` works), and
> +``fd_flags`` should be the flags that the fd in the tracee's table is opened
> +with (e.g. ``O_EXCL`` or similar).

fd_flags only contains file descriptor flags (meaning only O_CLOEXEC).
O_EXCL is a file creation flag, so setting it here wouldn't make sense.
Setting file status flags like O_APPEND does make sense, but those are
stored in the `struct file` and don't need to be passed separately;
the caller can e.g. set them via fcntl(fd, F_SETFD, flags) or on
open().
(The fcntl.2 manpage explains these.)