linux-kernel - Re: [seccomp] Request for a "enable on execve" mode for Seccomp filters

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Date:   Wed, 28 Oct 2020 18:34:56 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Rich Felker <dalias@...c.org>
Cc:     Camille Mougey <commial@...il.com>,
        Kees Cook <keescook@...omium.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Tycho Andersen <tycho@...ho.pizza>,
        Sargun Dhillon <sargun@...gun.me>,
        Christian Brauner <christian.brauner@...ntu.com>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        Denis Efremov <efremov@...ux.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [seccomp] Request for a "enable on execve" mode for Seccomp filters

On Wed, Oct 28, 2020 at 5:49 PM Rich Felker <dalias@...c.org> wrote:
> On Wed, Oct 28, 2020 at 01:42:13PM +0100, Jann Horn wrote:
> > On Wed, Oct 28, 2020 at 12:18 PM Camille Mougey <commial@...il.com> wrote:
> > 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.

Ah, yeah, good point.

> If you really just want the "activate at exec" behavior, it might be
> possible (depending on how SECCOMP_RET_USER_NOTIF behaves when there's
> no notify fd open; I forget)

syscall returns -ENOSYS. Yeah, that'd probably do the job. (Even
though it might be a bit nicer if userspace had control over the errno
there, such that it could be EPERM instead... oh well.)