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References: <20180906152859.7810-1-tycho@tycho.ws> <20180906152859.7810-4-tycho@tycho.ws>
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From:   Jann Horn <jannh@google.com>
Date:   Thu, 6 Sep 2018 17:45:25 +0200
Message-ID: <CAG48ez3gRpGZT4Vk7ZZXk5dkYeVfnGsaqeFZeOio0GpqFd=Vow@mail.gmail.com>
Subject: Re: [PATCH v6 3/5] seccomp: add a way to get a listener fd from ptrace
To:     Tycho Andersen <tycho@tycho.ws>
Cc:     Kees Cook <keescook@chromium.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        containers@lists.linux-foundation.org,
        Linux API <linux-api@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Oleg Nesterov <oleg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Tyler Hicks <tyhicks@canonical.com>, suda.akihiro@lab.ntt.co.jp
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On Thu, Sep 6, 2018 at 5:29 PM Tycho Andersen <tycho@tycho.ws> wrote:
>
> As an alternative to SECCOMP_FILTER_FLAG_GET_LISTENER, perhaps a ptrace()
> version which can acquire filters is useful. There are at least two reasons
> this is preferable, even though it uses ptrace:
>
> 1. You can control tasks that aren't cooperating with you
> 2. You can control tasks whose filters block sendmsg() and socket(); if the
>    task installs a filter which blocks these calls, there's no way with
>    SECCOMP_FILTER_FLAG_GET_LISTENER to get the fd out to the privileged task.
[...]
> +long seccomp_new_listener(struct task_struct *task,
> +                         unsigned long filter_off)
> +{
> +       struct seccomp_filter *filter;
> +       struct file *listener;
> +       int fd;
> +
> +       if (!capable(CAP_SYS_ADMIN))
> +               return -EACCES;
> +
> +       filter = get_nth_filter(task, filter_off);
> +       if (IS_ERR(filter))
> +               return PTR_ERR(filter);
> +
> +       fd = get_unused_fd_flags(0);
> +       if (fd < 0) {
> +               __put_seccomp_filter(filter);
> +               return fd;
> +       }
> +
> +       listener = init_listener(task, task->seccomp.filter);

Did you mean to write something like `init_listener(task, filter)` here?