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

Rich Felker
2020-10-29T02:07:39Z

On Wed, Oct 28, 2020 at 06:34:56PM +0100, Jann Horn wrote:
> On Wed, Oct 28, 2020 at 5:49 PM Rich Felker <dalias@libc.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@gmail.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.)

EPERM is a major bug in current sandbox implementations, so ENOSYS is
at least mildly better, but indeed it should be controllable, probably
by allowing a code path for the BPF to continue with a jump to a
different logic path if the notify listener is missing.

Rich