Re: [PATCH v3 2/5] seccomp: add the concept of a seccomp filter FD

[Tycho Andersen <tycho.andersen@canonical.com>]

On Wed, Sep 30, 2015 at 11:27:34AM -0700, Andy Lutomirski wrote:
> On Wed, Sep 30, 2015 at 11:13 AM, Tycho Andersen
> <tycho.andersen@canonical.com> wrote:
> > This patch introduces the concept of a seccomp fd, with a similar interface
> > and usage to ebpf fds. Initially, one is allowed to create, install, and
> > dump these fds. Any manipulation of seccomp fds requires users to be root
> > in their own user namespace, matching the checks done for
> > SECCOMP_SET_MODE_FILTER.
> >
> > Installing a filterfd has some gotchas, though. Andy mentioned previously
> > that we should restrict installation to filter fds whose parent is already
> > in the filter tree. This doesn't quite work in the case of created seccomp
> > fds, since once you install a filter fd, you can't install any other filter
> > fd since it has no parent and there is no way to "pre-chain" filters before
> > installing them.
> 
> ISTM, if we like the seccomp fd approach, we should have them be
> created with a parent already set.  IOW the default should be that
> their parent is the creator's seccomp fd and, if needed, creators
> could specify a different parent.

Allowing people doing SECCOMP_FD_NEW to specify a parent fd would
work. Then we can disallow installing a seccomp fd if its parent is
not the current filter, and get rid of the whole mess with prev
locking and all that.

Tycho