[Devel] Re: mounts & namespaces

Serge E. Hallyn serue at us.ibm.com
Wed Nov 18 08:20:22 PST 2009

Quoting Michael Tokarev (mjt at tls.msk.ru):
> Hello.
> I asked similar question on lxc-devel@ but got no reply.  Since
> the issue is in kernel entirely, I'm asking in containers@ now.
> As a base, I used lxc utils.
> First of all, I'm concerned about file namespace security in a
> container.  It is, basically, just a chroot(2), or at least it
> looks like that to me.  But as we know, root can break out of
> chroot.  Are there any protection methods that prevents this
> break-out?

Not that I know of.  I haven't looked at the relevant source in
lxc/ in awhile, and haven't tested, but you should easily be
able to verify by finding the chroot escape code and running it
from inside a container...

Of course you can use Smack or SELinux (or probably even and
Apparmor profile) to prevent it.

> I see lxc-start performs one additional rbind-mount - whole root
> filesystem of a container to a temporary directory (mktemp), and
> uses that temp dir as root for the container.  I wonder what it
> is trying to achieve this way.  Is it related to the first
> question and prevents breaking out of chroot somehow?
> On a similar note, can pivot_root be used there instead of a chroot?
> But see below for this one.

That is what libvirt does, for that very reason.  However, that
can make startup a bit more fragile, due to requirements in
sys_pivot_root that mounts involved not be shared.  It can be
worked around, but it starts to feel kludgy.  In particular,
libvirt-lxc broke briefly because fedora was marking / as
MS_SHARED, while we were expecting / to be private (which is
the usual case).

So for the moment, I personally was quite happy that libvirt
and lxc were were each taking different approaches  :)

> Inside a container, /proc/mounts is a complete mess, since most
> paths are not accessible (out of chroot).  When using real
> /etc/mtab things become much nicer, but mtab has been made a
> link to /proc/mounts for a purpose.
> So I wonder if we can clean that stuff up for real.  I mean, not
> /proc/mounts by itself but the namespace.  For that, using
> pivot_root instead of a chroot to keep other filesystems
> available directly, and umount all of them which are not
> useful.  I understand it is not always possible (you can't
> umount /oldroot/usr as long as /oldroot/usr/containerroot
> is bind-mounted to /), but I think it's worth to try, at
> least in cases where it is doable (maybe not for general
> utils but in custom use-case).
> But before trying, I want to understand the security
> implicatins and the whole root barrier thing if any --
> see my first question.
> Thanks!
> /mjt
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