Re: [PATCH 12/18] shared mount handling: bind and rbind

[Shaya Potter <spotter@cs.columbia.edu>]

On Wed, 2005-11-16 at 09:19 +0100, Miklos Szeredi wrote:
> > > This is why we have "pivot_root()" and "chroot()", which can both be used
> > > to do what you want to do. You mount the new root somewhere else, and then
> > > you chroot (or pivot-root) to it. And THEN you do 'chdir("/")' to move the
> > > cwd into the new root too (and only at that point have you "lost" the old
> > > root - although you can actually get it back if you have some file
> > > descriptor open to it).
> > 
> > Wouldn't this constitute a security flaw?
> > 
> > Shouldn't chroot jail you?
> 
> No, chroot should just change the root.
> 
> If you don't want to be able to get back the old root, just close all
> file descriptors _in addition_ to chroot() and chdir().

hah.  As long as you're running as root, chroot() again to a directory
below you, and you effectively broken the chroot and can make a relative
path to the old root. :)

I created a patch years ago that creates a chain of "chroot" points, and
any past chroot point would be considered a place that follow_dotdot
would consider a root.  There didn't seem much interest in the patch
though.