Re: [PATCH 0/7] Initial support for user namespace owned mounts

Amir Goldstein
2015-07-30T14:47:08Z

On Thu, Jul 30, 2015 at 4:55 PM, Seth Forshee
<seth.forshee@canonical.com> wrote:
>
> On Thu, Jul 30, 2015 at 07:24:11AM +0300, Amir Goldstein wrote:
> > On Tue, Jul 28, 2015 at 11:40 PM, Seth Forshee
> > <seth.forshee@canonical.com> wrote:
> > >
> > > On Wed, Jul 22, 2015 at 05:05:17PM -0700, Casey Schaufler wrote:
> > > > > This is what I currently think you want for user ns mounts:
> > > > >
> > > > > 1. smk_root and smk_default are assigned the label of the backing
> > > > > device.
> >
> > Seth,
> >
> > There were 2 main concerns discussed in this thread:
> > 1. trusting LSM labels outside the namespace
> > 2. trusting the content of the image file/loopdev
> >
> > While your approach addresses the first concern, I suspect it may be placing
> > an obstacle in a way for resolving the second concern.
> >
> > A viable security policy to mitigate the second concern could be:
> > - Allow only trusted programs (e.g. mkfs, fsck) to write to 'Loopback' images
> > - Allow mount only of 'Loopback' images
> >
> > This should allow the system as a whole to trust unprivileged mounts based on
> > the trust of the entities that had raw access the the fs layout.
>
> You don't really say what you mean by "trusted" programs. In a container
> context I'd have to assume that you mean suid-root or similar programs
> shared into the container by the host. In that case is any new kernel
> functionality even required?

Sorry I was not clear. I will try to explain better.
I meant that the programs are "trusted" by the LSM security policy.
I envisioned a system where unprivileged user is allowed to spawn
a container which contains "trusted" programs (e.g. mkfs) that are labeled
as 'FileSystemTools' by the admin of the host.
FileSystemTools are allowed to write into Loopback labeled files.

>
> That also doesn't work for some of our use cases, where we'd like to be
> able to do something like "mount -o loop foo.img /mnt/foo" in an
> unprivileged container where foo.img is not created on the local machine
> and not fully under control of the host environment.

That use case will not be addressed by the policy I suggested,
but the more common case of:
- create a loopback file
- mkfs
- mount
will be addressed.

So if the (host) admin of the system trusts that unprivileged user cannot create
a malicious fs layout using mkfs and fsck alone, then the system is
relatively safe
mounting (non fuse) file systems from loopback files.
IMHO, this statement is going to be easier for Ted to sign.

>
> Agreed though that the "attack from below" problem for untrusted
> filesystems is still an open question. At minimum we have fuse, which
> has been designed to protect against this threat. Others have mentioned
> on this thread that Ted had said something at kernel summit last year
> about being willing to support ext4 mounts from unprivileged user
> namespaces as well. I've added Ted to the Cc in case he wants to confirm
> or deny this rumor.
>
> > Alas, if you choose to propagate the backing dev label to contained files,
> > they would all share the designated 'Loopback' label and render the policy above
> > useless.
> >
> > Any thoughts on how to reconcile this conflict?
>
> I'm not seeing what the conflict is here - nothing you proposed says
> anything about security labels in the filesystem, and nothing would
> prevent a "trusted" program with CAP_MAC_ADMIN from setting whatever
> label was desired on the backing device. Care to elaborate?
>
> Seth