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

On Tue, Oct 7, 2014 at 2:42 PM, Eric W. Biederman [off-list ref] wrote:

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.

Yes.  rootfs might not be empty, it might have totally insane
permissions, and it's globally shared, which makes it into a wonderful
channel to pass things around that shouldn't be passed around.

Can non-root do this?  You'd need to be in a userns with a "/" that
isn't MNT_LOCKED.  Can this happen on any normal setup?

FWIW, I think we should unconditionally MNT_LOCKED the root on userns
unshare, even if it's the only mount.

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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

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Andy Lutomirski
AMA Capital Management, LLC