linux-kernel - Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace

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Date:   Thu, 27 Sep 2018 19:35:06 +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, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace

On Thu, Sep 27, 2018 at 5:11 PM Tycho Andersen <tycho@...ho.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.
[...]
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 21fec73d45d4..289960ac181b 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -1096,6 +1096,10 @@ int ptrace_request(struct task_struct *child, long request,
>                 ret = seccomp_get_metadata(child, addr, datavp);
>                 break;
>
> +       case PTRACE_SECCOMP_NEW_LISTENER:
> +               ret = seccomp_new_listener(child, addr);
> +               break;

Actually, could you amend this to also ensure that `data == 0` and
return -EINVAL otherwise? Then if we want to abuse `data` for passing
flags in the future, we don't have to worry about what happens if
someone passes in garbage as `data`.