On 04/14/2016 03:08 PM, Max Reitz wrote:
> On 07.04.2016 22:29, Eric Blake wrote:
>> Upstream NBD is documenting that servers MAY choose to operate
>> in a conditional mode, where it is up to the client whether to
>> use TLS.  For qemu's case, we want to always be in FORCEDTLS
>> mode, because of the risk of man-in-the-middle attacks, and since
>> we never export more than one device; likewise, the qemu client
>> will ALWAYS send NBD_OPT_STARTTLS as its first option.  But now
>> that SELECTIVETLS servers exist, it is feasible to encounter a
>> (non-qemu) client that does not do NBD_OPT_STARTTLS first, but
>> rather wants to take advantage of the conditional modes it might
>> find elsewhere.
>>
>> Since we require TLS, we are within our rights to drop connections
>> on any client that doesn't negotiate it right away, or which
>> attempts to negotiate it incorrectly, without violating the intent
>> of the NBD Protocol.  However, it's better to allow the client to
>> continue trying, on the grounds that maybe the client will get the
>> hint to send NBD_OPT_STARTTLS.
>>
>> Signed-off-by: Eric Blake <eblake@redhat.com>
>> ---

>> +++ b/nbd/server.c
>> @@ -450,9 +450,12 @@ static int nbd_negotiate_options(NBDClient *client)
>>
>>              default:
>>                  TRACE("Option 0x%x not permitted before TLS", clientflags);
>> +                if (nbd_negotiate_drop_sync(client->ioc, length) != length) {
>> +                    return -EIO;
>> +                }
>>                  nbd_negotiate_send_rep(client->ioc, NBD_REP_ERR_TLS_REQD,
>>                                         clientflags);
>> -                return -EINVAL;
>> +                break;
>>              }
> 
> What about NBD_OPT_EXPORTNAME? The specification says that this option
> does not allow for errors, and so the session must be terminated if this
> option is sent in FORCEDTLS mode without TLS having been negotiated.

Oh, good catch. v2 coming up.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org