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Re: Hurdish applications for persistence

From: Alfred M. Szmidt
Subject: Re: Hurdish applications for persistence
Date: Tue, 11 Oct 2005 12:29:35 +0200 (CEST)

   But let's take a closer look.  Take for example a "firmlink"
   translator, which is a bit of a cross between a soft link and a
   hard link.  Furthermore, consider a user running a shell in a
   chrooted environment.  The user is malicious and wants to escape
   the chroot.  If the user installs an active firmlink to "/"
   somewhere in its chrooted filesystem tree, this fails: The firmlink
   translator will see the same chrooted root directory as the user
   themselves.  But what if the user installs a passive firmlink
   translator and then accesses it (for example via an ls -l)?  Then
   the filesystem will do the startup _and give the firmlink
   translator a port to the real, unchrooted directory port_.  The
   user can escape the chroot following the firmlink.  This is an
   obvious security exploit.

A obvious security exploit in the chroot() implementation (or really,
file_reparent) and not in how passive translators work.  If you want a
secure chroot enviroment (right now atleast) then you should run a
sub-hurd.  Where this isn't possible (atleast, I have never been able
to break out of a sub-hurd, and I have tried).  So instead of using
broken UNIXoid ideas like chroot, it would make far more sense to
implement a light-weight sub-hurd which can be used like chroot.

   Now the question is, can this be fixed?  Can for example the
   filesystem realize that the user installing the passive translator
   is chrooted, and thus store the chroot information in the passive
   translator setting?

There is no reason to do that.

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