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Date:   Wed, 28 Oct 2020 15:03:14 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Rich Felker <dalias@libc.org>
Cc:     Jann Horn <jannh@google.com>, Camille Mougey <commial@gmail.com>,
        lkml <linux-kernel@vger.kernel.org>,
        Tycho Andersen <tycho@tycho.pizza>,
        Sargun Dhillon <sargun@sargun.me>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
        Denis Efremov <efremov@linux.com>,
        Andy Lutomirski <luto@kernel.org>
Subject: Re: [seccomp] Request for a "enable on execve" mode for Seccomp
 filters
Message-ID: <202010281500.855B950FE@keescook>
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On Wed, Oct 28, 2020 at 12:49:36PM -0400, Rich Felker wrote:
> On Wed, Oct 28, 2020 at 01:42:13PM +0100, Jann Horn wrote:
> > +luto just in case he has opinions on this
> > 
> > On Wed, Oct 28, 2020 at 12:18 PM Camille Mougey <commial@gmail.com> wrote:
> > > From my understanding, there is no way to delay the activation of
> > > seccomp filters, for instance "until an _execve_ call".
> > > [...]
> > > I only see hackish ways to restrict the use of _execve_ in a
> > > non-cooperative executable. These methods seem globally bypassables
> > > and not satisfactory from a security point of view.
> > 
> > You're just focusing on execve() - I think it's important to keep in
> > mind what happens after execve() for normal, dynamically-linked
> > binaries: The next step is that the dynamic linker runs, and it will
> > poke around in the file system with access() and openat() and fstat(),
> > it will mmap() executable libraries into memory, it will mprotect()
> > some memory regions, it will set up thread-local storage (e.g. using
> > arch_prctl(); even if the process is single-threaded), and so on.
> > 
> > The earlier you install the seccomp filter, the more of these steps
> > you have to permit in the filter. And if you want the filter to take
> > effect directly after execve(), the syscalls you'll be forced to
> > permit are sufficient to cobble something together in userspace that
> > effectively does almost the same thing as execve().
> 
> I would assume you use SECCOMP_RET_USER_NOTIF to implement policy for
> controlling these operations and allowing only the ones that are valid
> during dynamic linking. This also allows you to defer application of
> the filter until after execve. So unless I'm missing some reason why
> this doesn't work, I think the requested functionality is already
> available.

Oof. Yeah, that's possible, but I view it as kind of not the point of
USER_NOTIF -- I'd rather design a workable solution for the
delayed-apply case.

-- 
Kees Cook