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    SubjectRe: [PATCH] User chroot
    Date
    H. Peter Anvin writes:
    > Albert D. Cahalan wrote:

    >> Normal users can use an environment provided for them.
    >>
    >> While trying to figure out why the "heyu" program would not
    >> work on a Red Hat box, I did just this. As root I set up all
    >> the device files needed, along Debian libraries and the heyu
    >> executable itself. It was annoying that I couldn't try out
    >> my chroot environment as a regular user.
    >>
    >> Creating the device files isn't a big deal. It wouldn't be
    >> hard to write a setuid app to make the few needed devices.
    >> If we had per-user limits, "mount --bind /dev/zero /foo/zero"
    >> could be allowed. One way or another, devices can be provided.
    >
    > Hell no! This would give the user a way to subvert root or other
    > system-provided things by having device nodes or such appear where
    > they aren't expected. NOT GOOD.

    On every normal (default Red Hat or Debian at least) system
    this is already trivial:

    ln /dev/zero /tmp/zero
    ln /dev/hda ~/hda
    ln /dev/mem /var/tmp/README

    So the user often can provide device nodes. The above is _worse_
    than allowing "mount --bind ..." because the admin has to search
    the whole filesystem to find such links.

    Never mind that though; it doesn't matter how the devices are
    created. Social engineering can work. Once the device problem
    is taken care of, chroot() becomes useful for normal users.

    BTW, it is way wrong that /dev/zero should be needed at all.
    Such use is undocumented ("man zero", "man mmap") anyway, and
    AFAIK one should use mmap() with MAP_ANON instead. Not that
    the documentation on MAP_ANON is any good either, but at least
    the mere existence of the flag is mentioned.


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