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

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Date:	Sat, 17 May 2014 11:01:45 -0500
From:	Seth Forshee <seth.forshee@...onical.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	"Michael H. Warfield" <mhw@...tsEnd.com>,
	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Arnd Bergmann <arnd@...db.de>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	lxc-devel@...ts.linuxcontainers.org,
	James Bottomley <James.Bottomley@...senPartnership.com>
Subject: Re: [lxc-devel] [RFC PATCH 00/11] Add support for devtmpfs in user
 namespaces

On Fri, May 16, 2014 at 09:31:37PM -0700, Eric W. Biederman wrote:
> Greg Kroah-Hartman <gregkh@...uxfoundation.org> writes:
> 
> > 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.
> 
> Yes. Something like devpts (without the newinstance option).  Built to
> allow unprivileged users to create loopback devices.

That's where I started, and I've got code, so I guess I'll clean it up
and send patches. If the stance is that only system-wide CAP_SYS_ADMIN
gets to do privileged block device ioctls, including reading partitions
on a block device which has been assigned to a contiainer, then I guess
that approach works well enough.

> There is still a huge kettle of fish in with verifying a filesystem is
> safe from a hostile user that has acess to the block device while the
> filesystem is mounted.
> 
> Having a few filesystems that are robust enough to trust with arbitrary
> filesystem corruption would be very interesting.
> 
> I assume unprivileged and hostile users because if you trusted the real
> root inside of your container this would not be an issue.
> 
> Eric
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