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

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Date:	Fri, 16 May 2014 21:31:37 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	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

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.

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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