Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root

Serge Hallyn [off-list ref] writes:

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

quoted

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.

quoted

quoted

quoted

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