Re: [Qemu-devel] NBD TLS support in QEMU

[Wouter Verhelst]

On Fri, Sep 05, 2014 at 09:46:18AM +0100, Hani Benhabiles wrote:
> 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 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.
> 
> Not sold on this because:
> - It requires (unnecessary) changes to the NBD specification.

They may not be necessary from a libvirt/qemu point of view, but I've
had requests to implement encryption from other people, too. So while
they're not very high on my priority list, I do think the changes are
necessary.

> - It would still be vulnerable to a protocol downgrade attack: ie. An attacker
>   could strip the TLS flag from the server's response resulting in either:
>   * Connection fallback to cleartext, if both ends don't force TLS.
>   * Loss of backward compatibility, if one of the ends forces TLS, making the
>     reason for which such a flag is added moot IIUC.

Tunneling the entire protocol inside an SSL connection doesn't fix that;
if an attacker is able to hijack your TCP connections and change flags,
then this attacker is also able to hijack your TCP connection and
redirect it to a decrypting/encrypting proxy.

I agree that preventing a possible SSL downgrade attack (and other forms
of MITM) should be high on the priority list, but "tunnel the whole
thing in SSL" doesn't do that.

[...]

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