linux-kernel - Re: [PATCH 4/4] Allow unprivileged chroot when safe

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Date:	Mon, 16 Jan 2012 20:26:04 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Casey Schaufler <casey@...aufler-ca.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jamie Lokier <jamie@...reable.org>,
	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	keescook@...omium.org, john.johansen@...onical.com,
	serge.hallyn@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	segoon@...nwall.com, rostedt@...dmis.org, jmorris@...ei.org,
	scarybeasts@...il.com, avi@...hat.com, penberg@...helsinki.fi,
	mingo@...e.hu, akpm@...ux-foundation.org, khilman@...com,
	borislav.petkov@....com, amwang@...hat.com, oleg@...hat.com,
	ak@...ux.intel.com, eric.dumazet@...il.com, gregkh@...e.de,
	dhowells@...hat.com, daniel.lezcano@...e.fr,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com, corbet@....net,
	alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH 4/4] Allow unprivileged chroot when safe

On Mon, Jan 16, 2012 at 12:15:09PM -0800, Andy Lutomirski wrote:

> The first approach I tried was (from memory -- may not compile at all
> on any version) fs->root.mnt != fs->root.mnt->mnt_parent.  That didn't
> work.  The issue is that on dracut-based distros, AFAICT, the root (in
> the sense of the root of the tree of struct vfsmounts) is rootfs.  The
> apparent root (the filesystem containing /, /usr, etc) is mounted on
> top of (rootfs)/.  Dracut then does something with the effect of
> chroot("/").  So you end up with the vfsmount that contains "/" not
> being the actual root vfsmount.  But there's nothing hidden by the
> chroot -- even if fs->root.mnt pointed at rootfs, "/" would still
> follow the mountpoint into the actual filesystem.

That has nothing whatsoever to do with dracut.  _Everything_ ends up
that way; IOW, everything including init(8) runs chrooted into the
final userland root.  On any normal distro.  Your test is complete BS - e.g.
mount /dev/crap /mnt/blah
mount /dev/garbage /mnt/blah
chroot /mnt/blah
will *NOT* be chrooted per your definition.
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