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[Bug 1894781] Re: [Feature request] Provide a way to do TLS first in QEM

From: Eric Blake
Subject: [Bug 1894781] Re: [Feature request] Provide a way to do TLS first in QEMU/NBD connections (not after NBD negotiation)
Date: Tue, 08 Sep 2020 13:00:47 -0000

** Bug watch added: gitlab.com/libvirt/libvirt/-/issues #68

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  [Feature request] Provide a way to do TLS first in QEMU/NBD
  connections (not after NBD negotiation)

Status in QEMU:

Bug description:
  (following from

  As is very well explained in https://www.berrange.com/posts/2016/04/05
  and easily confirmed with captures, NBD stream starts in cleartext and
  upgrades to TLS inline (similar to STARTTLS mechanism). As a
  rationale, it is stated that this provides better error messages for
  the user of NBD.

  However, this approach has downsides:

  1) Clear indication to a network observer that NBD (and therefore likely 
qemu/libvirt) is used. In contrast, TLS1.3 hides even the SNI of the servers 
(ESNI, https://blog.cloudflare.com/encrypted-sni/).
  2) Potential for bugs in NBD protocol negotiation code. That code just 
statistically, likely less looked at code than gnutls. This is not a reflection 
on NBD code quality, just the fact that TLS code does receive a bit more 
  3) Inability to inspect TLS listener interface for compliance, e.g. with a 
security scanner. Making sure TLS listeners only select certain ciphersuits is 
a requirement of some compliance regimes. 

  I think it's fully possible to satisfy the original requirement of
  good error messages as well, detecting that the other end is
  initiating TLS connection.

  It's very unlikely that it's currently sae to recommend to run QEMU
  migration stream over a hostile network, but it should be possible to
  do with TLS only option.

  Solution to this, just like in the case of SMTP, is to provide TLS
  only option (no initial cleartext at all) for QEMU migration, which
  hopefully it not a large addition.

  Thank you for your consideration!

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