Re: [Qemu-devel] spec, RFC: TLS support for NBD

[Markus Armbruster]

Stefan Hajnoczi <address@hidden> writes:

> On Mon, Oct 20, 2014 at 08:58:14AM +0100, Daniel P. Berrange wrote:
>> On Sat, Oct 18, 2014 at 07:33:22AM +0100, Richard W.M. Jones wrote:
>> > On Sat, Oct 18, 2014 at 12:03:23AM +0200, Wouter Verhelst wrote:
>> > > Hi all,
>> > > 
>> > > (added rjones from nbdkit fame -- hi there)
>> > 
>> > [I'm happy to implement whatever you come up with, but I've added
>> > Florian Weimer to CC who is part of Red Hat's product security group]
>> > 
>> > > So I think the following would make sense to allow TLS in NBD.
>> > > 
>> > > This would extend the newstyle negotiation by adding two options (i.e.,
>> > > client requests), one server reply, and one server error as well as
>> > > extend one existing reply, in the following manner:
>> > > 
>> > > - The two new commands are NBD_OPT_PEEK_EXPORT and NBD_OPT_STARTTLS. The
>> > >   former would be used to verify if the server will do TLS for a given
>> > >   export:
>> > > 
>> > >   C: NBD_OPT_PEEK_EXPORT
>> > >   S: NBD_REP_SERVER, with an extra field after the export name
>> > >      containing flags that describe the export (R/O vs R/W state,
>> > >      whether TLS is allowed and/or required).
>> 
>> IMHO the server should never provide *any* information about the exported
>> volume(s) until the TLS layer has been fully setup. ie we shouldn't only
>> think about the actual block data transfers, we should protect the entire
>> NBD protocol even metadata related operations.
>
> This makes sense.

Seconded.

> TLS is about the transport, not about a particular NBD export.  The only
> thing that should be communicated is STARTTLS.

Furthermore, STARTTLS is vulnerable to active attacks: if you can get
between the peers, you can make them fall back to unencrypted silently.
How do you plan to guard against that?

See also https://www.agwa.name/blog/post/starttls_considered_harmful