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    SubjectRe: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root
    Serge Hallyn <serge.hallyn@ubuntu.com> writes:

    > Quoting Eric W. Biederman (ebiederm@xmission.com):

    >> What I meant is that it isn't about containers. It is about something
    >> root can do. So this is not a "container" problem.
    >
    > Oh, ok.
    >
    > Sorry, I'm getting the two thread confused anyway. I'm going to bow out
    > here until I can pay proper attention.

    I think there is half a point here that may be legit, when you are using
    mount namespaces to jail applications, there may be a problem
    with umounting / and making it to the underlying rootfs filesystem.

    I am squinting and looking this way and that but while I can imagine
    someone more clever than I can think up some unique property of rootfs
    that makes it a little more exploitable than just mounting a ramfs,
    but since you have to be root to exploit those properties I think the
    game is pretty much lost.

    >> >> So it is only root (and not root in a container) who can get to the
    >> >> exposed rootfs.
    >> >>
    >> >> I have a vague memory someone actually had a real use in miminal systems
    >> >> for being able to get back to the rootfs and being able to use rootfs as
    >> >> the rootfs. There was even a patch at that time that Andrew Morton was
    >> >> carrying for a time to allow unmounting root and get at rootfs, and to
    >> >> prevent the oops on rootfs unmount in some way.
    >> >>
    >> >> So not only do I not think it is a bug to get back too rootfs, I think
    >> >> it is a feature that some people have expressed at least half-way sane
    >> >> uses for.
    >> >
    >> > They can still do that if they want, using chroot :)
    >>
    >> It would take fchdir or fchroot and a directory file descriptor open on
    >> rootfs. Frequently there is no appropriate directory file descriptor.
    >
    > ? you can always escape if you're simply chrooted. waterbuffalo :)

    filesystem type rootfs.

    Eric



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