[libvirt] NBD TLS support in QEMU

[Daniel P. Berrange]

On Fri, Sep 05, 2014 at 12:02:18AM +0200, Wouter Verhelst wrote:
> [Cc: to nbd-general list added]
> 
> On Wed, Sep 03, 2014 at 05:44:17PM +0100, Stefan Hajnoczi wrote:
> > Hi,
> > QEMU offers both NBD client and server functionality.  The NBD protocol
> > runs unencrypted, which is a problem when the client and server
> > communicate over an untrusted network.
> > 
> > The particular use case that prompted this mail is storage migration in
> > OpenStack.  The goal is to encrypt the NBD connection between source and
> > destination hosts during storage migration.
> 
> I've never given encrypted NBD high priority, since I don't think
> encryption without authentication serves much purpose -- and I haven't
> gotten around to adding authentication yet (for which I have plans; but
> other things have priority).

While have an authentication layer like SASL wired into the NBD protocol
would be nice, it shouldn't be considered a blocker / pre-requisite. It
is pretty straightforward for the server to require x509 certificates
from the client and validate those as a means of authentication. We've
used that as an authentication mechanism in libvirt and VNC with success,
though we did later add SASL integration as an option too.

> > I think we can integrate TLS into the NBD protocol as an optional flag.
> > A quick web search does not reveal existing open source SSL/TLS NBD
> > implementations.  I do see a VMware NBDSSL protocol but there is no
> > specification so I guess it is proprietary.
> > 
> > The NBD protocol starts with a negotiation phase.  This would be the
> > appropriate place to indicate that TLS will be used.  After client and
> > server complete TLS setup the connection can continue as normal.
> > 
> > Besides QEMU, the userspace NBD tools (http://nbd.sf.net/) can also be
> > extended to support TLS.  In this case the kernel needs a localhost
> > socket and userspace handles TLS.
> 
> That introduces a possibility for a deadlock, since now your network
> socket isn't on the PF_MEMALLOC-protected socket anymore, which will
> cause the kernel to throw away packets which are needed for your nbd
> connection, in hopes of clearing some memory.
> 
> I suppose you could theoretically do the encryption in kernel space.
> Not convinced that trying TLS in kernel space is a good idea, though.
> 
> I have heard of people using stunnel or the likes to pipe the NBD
> protocol over a secure channel, with various levels of success.

Regards,
Daniel
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