[lxc-devel] [RFC PATCH 00/11] Add support for devtmpfs in user namespaces

[James Bottomley]

On Fri, 2014-05-16 at 11:57 -0700, Greg Kroah-Hartman wrote:
> On Fri, May 16, 2014 at 09:06:07AM -0500, Seth Forshee wrote:
> > On Thu, May 15, 2014 at 09:35:32PM -0700, Greg Kroah-Hartman wrote:
> > > On Fri, May 16, 2014 at 01:49:59AM +0000, Serge Hallyn wrote:
> > > > > I think having to pick and choose what device nodes you want in a
> > > > > container is a good thing.  Becides, you would have to do the same thing
> > > > > in the kernel anyway, what's wrong with userspace making the decision
> > > > > here, especially as it knows exactly what it wants to do much more so
> > > > > than the kernel ever can.
> > > > 
> > > > For 'real' devices that sounds sensible.  The thing about loop devices
> > > > is that we simply want to allow a container to say "give me a loop
> > > > device to use" and have it receive a unique loop device (or 3), without
> > > > having to pre-assign them.  I think that would be cleaner to do using
> > > > a pseudofs and loop-control device, rather than having to have a
> > > > daemon in userspace on the host farming those out in response to
> > > > some, I don't know, dbus request?
> > > 
> > > I agree that loop devices would be nice to have in a container, and that
> > > the existing loop interface doesn't really lend itself to that.  So
> > > create a new type of thing that acts like a loop device in a container.
> > > But don't try to mess with the whole driver core just for a single type
> > > of device.
> > 
> > No matter what I don't think we get out of this without driver core
> > changes, whether this was done in loop or by creating something new.
> > Not unless the whole thing is punted to userspace, anyway.
> > 
> > The first problem is that many block device ioctls check for
> > CAP_SYS_ADMIN. Most of these might not ever be used on loop devices, I'm
> > not really sure. But loop does at minimum support partitions, and to get
> > that functionality in an unprivileged container at least the block layer
> > needs to know the namespace which has privileges for that device.
> 
> That's fine, you should have those permissions in a container if you
> want to do something like that on a loop device, right?

Really, no.  CAP_SYS_ADMIN is effectively a pseudo root security hole.
Any user possessing CAP_SYS_ADMIN can do about as much damage as real
root can, whether or not you use user namespaces, so it would compromise
a lot of the security we're just bringing to containers.

> > The second is that all block devices automatically appear in devtmpfs.
> > The scenario I'm concerned about is that the host could unknowingly use
> > a loop device exposed to a container, then the container could see data
> > from the host.
> 
> I don't think that's a real issue, the host should know not to do that.
> 
> > So we either need a flag to tell the driver core not to create a node
> > in devtmpfs, or we need a privileged manager in userspace to remove
> > them (which kind of defeats the purpose). And it gets more complicated
> > when partition block devs are mixed in, because they can be created
> > without involvement from the driver - they would need to inherit the
> > "no devtmpfs node" property from their parent, and if the driver uses
> > a psuedo fs to create device nodes for userspace then it needs to be
> > informed about the partitions too so it can create those nodes.
> 
> I don't think that will be needed.  Root in a host can do whatever it
> wants in the containers, so mixing up block devices is the least of the
> issues involved :)
> 
> > So maybe we could get by without the privileged ioctls, as long as it
> > was understood that unprivileged containers can't do partitioning. But I
> > do think the devtmpfs problem would need to be addressed.
> 
> I don't think unpriviliged containers should be able to do partitioning.
> An unpriviliged user can't do that, so why should a container be any
> different?

To make sure we're on the same page with terminology, there's an
unprivileged container and a secure container.  In the former, there's
no root user (all the processes run as non-root), so the container isn't
expected to perform any actions root would ... that's easy.  In a secure
container, root is mapped to a nobody user in the host, so is
effectively unprivileged, but root in the container expects to look like
a real root within the VPS (and thus may expect to partition things,
depending on how they've been given access to the block device).  The
big problem is giving back capabilities to the container root such that
a) it loses them if it escapes the container and b) it doesn't get
sufficient capabilities to damage the system.

James