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Date: Mon, 16 Jan 2012 20:26:04 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Casey Schaufler <casey@schaufler-ca.com>,
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Subject: Re: [PATCH 4/4] Allow unprivileged chroot when safe
Message-ID: <20120116202604.GO23916@ZenIV.linux.org.uk>
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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.