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

[Wouter Verhelst <w@uter.be>]

[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).

> 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.

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