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    SubjectRe: [PATCH 00/12] Add kdbus implementation
    On Wed, Oct 29, 2014 at 3:27 PM, Greg Kroah-Hartman
    <gregkh@linuxfoundation.org> wrote:
    > On Wed, Oct 29, 2014 at 03:15:51PM -0700, Andy Lutomirski wrote:
    >> (reply 1/2 -- I'm replying twice to keep the threading sane)
    >>
    >> On Wed, Oct 29, 2014 at 3:00 PM, Greg Kroah-Hartman
    >> <gregkh@linuxfoundation.org> wrote:
    >> > kdbus is a kernel-level IPC implementation that aims for resemblance to
    >> > the the protocol layer with the existing userspace D-Bus daemon while
    >> > enabling some features that couldn't be implemented before in userspace.
    >> >
    >>
    >> > * Support for multiple domains, completely separated from each other,
    >> > allowing multiple virtualized instances to be used at the same time.
    >>
    >> Given that there is no such thing as a device namespace, how does this work?
    >
    > See the document for the details.

    They seem insufficient to me, so I tried to dig in to the code. My
    understanding is:

    The parent container has /dev mounted. It sends an IOCTL (which
    requires global capabilities). In response, kdbus creates a whole
    bunch of devices that get put (by udev or devtmpfs, I presume) in a
    subdirectory. Then the parent container mounts that subdirectory in
    the new container.

    This is IMO rather problematic.

    First, it enforces the existence of a kdbus domain hierarchy where
    none should be needed.

    Second, it's incompatible with nested user namespaces. The middle
    namespace can't issue the ioctl.

    Third, it requires a devtmpfs submount in the child container. This
    scares me, especially since there are no device namespaces. Also, the
    child container appears to be dependent on the host udev to arbitrate
    everything, which seems totally wrong to me. (Also, now we're exposed
    to attacks where the child container creates busses or endpoints or
    whatever with malicious names to try to trick the host into screwing
    up.)

    ISTM this should be solved either with device namespaces (which is
    well known to be a giant can of worms) or by abandoning the concept of
    kdbus using device nodes entirely.

    What if kdbus were kdbusfs? If you want to use it in a container, you
    mount a brand-new kdbusfs there. No weird domain hierarchy, no global
    privilege, no need to name containers, obvious migration semantics, no
    dependence on udev/devtmpfs at all, etc.

    Eric, any thoughts here?

    --Andy


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