linux-kernel - Re: [PATCH v4 1/4] seccomp: add a return code to trap to userspace

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Message-ID: <CAGXu5jKWQCDzXPdTz=+bxT9+4enaBvgYfMseUiGti+t1KwZT8g@mail.gmail.com>
Date:   Fri, 22 Jun 2018 14:51:23 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Tycho Andersen <tycho@...ho.ws>, Jann Horn <jannh@...gle.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>,
        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>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>,
        "Tobin C. Harding" <me@...in.cc>
Subject: Re: [PATCH v4 1/4] seccomp: add a return code to trap to userspace

On Fri, Jun 22, 2018 at 11:09 AM, Andy Lutomirski <luto@...capital.net> wrote:
> One possible extra issue: IIRC /proc/.../mem uses FOLL_FORCE, which is not what we want here.
>
> How about just adding an explicit “read/write the seccomp-trapped task’s memory” primitive?  That should be easier than a “open mem fd” primitive.

Uuugh. Can we avoid adding another "read/write remote process memory"
interface? The point of this series was to provide a lightweight
approach to what should normally be possible via the existing
seccomp+ptrace interface. I do like Jann's context idea, but I agree
with Andy: it can't be a handle to /proc/$pid/mem, since it's
FOLL_FORCE. Is there any other kind of process context id we can use
for this instead of pid? There was once an idea of pid-fd but it never
landed... This would let us get rid of the "id" in the structure too.
And if that existed, we could make process_vm_*v() safer too (taking a
pid-fd instead of a pid).

-Kees

-- 
Kees Cook
Pixel Security