Frank Fricke reported that hostfs does not verify that a chmod +s, for
instance, is done by a sufficiently privileged user, as long as the UML
kernel itself can complete the operation on the host.

So, for instance, if UML is run as root and under /mnt/host we have a hostfs
mount, this works successfully:

paolo@zion:~ (0)$ chmod 4755 /mnt/host/bin/bash
paolo@zion:~ (0)$ ll /mnt/host/bin/bash

 -rwsr-xr-x  1 root root 662724 2004-10-20 02:15 /mnt/host/bin/bash*

(bash refuses running as setuid, but you could have another shell on the
host, as dash or whatever).

In general, if UML is run as uid 500 on the host, a hostfs mount is done
and under the hostfs mount there is a file with uid 500 on the host, I can
freely make it setuid (if it's executable).

This is especially bad when UML is run as root (which you should not do),
but is a problem in general, since it allows any user to create setuid 500
(in this example) executables on the host filesystem.

Finally, while I was looking at the chmod() implementation, I spotted a
kludge in the code and explained it with a comment.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Frank 'xraz' Fricke <xraz@rwxr-xr-x.de>
Cc: Alexander Viro <viro@parcelfarce.linux.theplanet.co.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>