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

Andy Lutomirski [off-list ref] writes:

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

quoted

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.

But if only root with proc mounted can reach it...  I don't know.
There might be a case for setting MNT_LOCKED when we overmount "/"
as root but I don't yet see it.

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.

To the best of my knowledge MNT_LOCKED is set uncondintially on userns
unshare.

Eric