Re: F38 proposal: KTLS implementation for GnuTLS

[Richard W.M. Jones]

On Wed, Sep 07, 2022 at 11:53:03AM -0400, Ben Cotton wrote:
>  https://fedoraproject.org/wiki/Changes/KTLSSupportForGnuTLS
>  
>  This document represents a proposed Change. As part of the Changes
>  process, proposals are publicly announced in order to receive
>  community feedback. This proposal will only be implemented if approved
>  by the Fedora Engineering Steering Committee.
>  
>  == Summary ==
>  Acceleration of GnuTLS with software ''Kernel TLS'' (KTLS)
>  
>  == Owner ==
>  * Name: [[User:Fkrenzel| František Krenželok]], [[User:Ueno| Daiki Ueno]]
>  * Email: fkrenzel(a)redhat.com, dueno(a)redhat.com
>  
>  
>  == Detailed Description ==
>  The goal of this change is to provide GnuTLS users with a high
>  throughput data transfer mechanism on encrypted channels, with
>  emphasis on ''network block devices'' (NBD).
>  
>  We accomplish this with KTLS which offloads enc/decryption (TLS
>  record) to the kernel, while GnuTLS handles initial connection (TLS
>  handshake).
>  
>  GNUTLS will detect whether the kernel supports kTLS and will
>  automatically enable its usage when compatible. Any package built
>  against GNUTLS, is likely to see some performance benefit from kTLS,
>  provided it has not installed custom push/pull I/O function callbacks.
>  
>  kTLS enables a reduction in context switching and reduced data copies
>  when using send_file(). With suitable NIC hardware the encryption
>  operations can be offloaded, freeing time on the main CPUs for
>  application usage. Without offload hardware, kTLS may still improve
>  parallelism for applications as the kernel can perform encryption
>  operations on a differen host CPU to that running the application
>  threads
>  
>  == Feedback ==
>  <!-- Summarize the feedback from the community and address why you
>  chose not to accept proposed alternatives. This section is optional
>  for all change proposals but is strongly suggested. Incorporating
>  feedback here as it is raised gives FESCo a clearer view of your
>  proposal and leaves a good record for the future. If you get no
>  feedback, that is useful to note in this section as well. For
>  innovative or possibly controversial ideas, consider collecting
>  feedback before you file the change proposal. -->
>  
>  == Benefit to Fedora ==
>  The improvement lies in acceleration of large data transfers trough
>  encrypted channels.
>  The send_file function enables us to send data directly trough socket
>  without entering user space, saving us from 2 context switches and 2
>  additional user space buffers. This is especially useful for NBD
>  
>  '''Benefits'''
>  * Acceleration of ''live VM migration'', which should mitigate the
>  downtime for various services used by both the users and the
>  developers.
>  
>  * Increased speed at which files can be retrieved from NBD via
>  encrypted channel and less CPU and memory strain on NBD server.
>  
>  packages that might benefit: {{package|nbd}} {{package|nbdkit}} {{package|qemu}} 
Also: {{package|libnbd}}

From the point of the packages above, kTLS has been enabled already
for a while, but it does require the gnutls default to change.

>  == Scope ==
>  * Proposal owners: Support for KTLS key update in GnuTLS ''track'':
>  [https://gitlab.com/gnutls/gnutls/-/merge_requests/1625 gitlab]
>  * Other developers: Support for TLS1.3 key update in KTLS (tls kernel module)
>  * Release engineering: not needed for this Change
>  * Policies and guidelines: not needed for this Change
>  * Trademark approval: not needed for this Change
>  * Alignment with Objectives: None
>  
>  == Upgrade/compatibility impact ==
>  Although this feature will be enabled by default, users will not
>  notice any change, as in case of failure to initialize KTLS, GnuTLS
>  will fallback to the currently used mode of operation.
>  
>  Users will be also provided with means to disable this feature trough
>  crypto-policies
>  
>  == How To Test ==
>  '''To enable this feature user has to:'''
>  # load TLS kernel module (`modprobe tls`) 
It would be nice if manually loading the module wasn't necessary.

>  # enable ktls with crypto policies
>  {{admon/note|Once proposal accepted|KTLS will be enabled by default
>  and this step will not be needed.}}
>  <pre>$ cat > /etc/crypto-policies/local.d/gnutls-ktls.config <<EOF
>  [global]
>  ktls = true
>  EOF
>  </pre>
>  <pre>$ update-crypto-policies</pre>
>  {{admon/important||KTLS will not initialize if app uses custom
>  push/pull callback for GnuTLS.}}
>  
>  == User Experience ==
>  This change should accelerate large data transfers especially that of files.
>  This will affect users that use applications which utilize GnuTLS for
>  encrypting communication channels.
>  
>  == Dependencies ==
>  
>  Currently '''KTLS doesn't support key_update''' (The keys
> delivered to
>  the kernel can’t be set more than once per session) so a kernel module
>  patch would be needed for this functionality.
>  This not only impacts key_update invokation by user of either side,
>  but also by [https://datatracker.ietf.org/doc/html/rfc8446#section-5.5
>  AES-GCM key usage limit]. As this could weaken the security of TLS
>  protocol, GnuTLS will disable KTLS for rest of the session and
>  fallback to default mode of operation i.e. move encryption and
>  decryption back to usersace after the key_update is called.
>  
>  {{admon/note|GnuTLS supports KTLS key_update| This feature is
>  supported if apropriate kernel patch is applied}}
>  
>  == Contingency Plan ==
>  * Contingency mechanism: Feature will be disabled by default in crypto-policies.
>  * Contingency deadline: 2023-02-21
>  * Blocks release? No
>  
>  
>  == Documentation ==
>  === API ===
>
> '''[https://gnutls.org/manual/gnutls.html#index-gnutls_005ftransport_005fis_005fktls_005fenabled
>  gnutls_transport_is_ktls_enabled()]'''
>  To check if KTLS was properly initialized on the interfaces:
>  {{admon/important||it has to be invoked no earlier that after a TLS-handshake}}
>  <pre> gnutls_transport_ktls_enable_flags_t
>  gnutls_transport_is_ktls_enabled(gnutls_session_t session);</pre>
>  
>  
>
> '''[https://gnutls.org/manual/gnutls.html#index-gnutls_005frecord_005fsend_005ffile
>  gnutls_record_send_file()]'''
>  To send data directly from a file descriptor in a zero-copy manner if
>  KTLS is enabled; otherwise it will just iteratively read from the file
>  descriptor:
>  <pre> ssize_t gnutls_record_send_file(gnutls_session_t session, int
>  fd, off_t *offset, size_t count);</pre>
>  
>  == Release Notes ==
>  
>  
>  
>  -- 
>  Ben Cotton
>  He / Him / His
>  Fedora Program Manager
>  Red Hat
>  TZ=America/Indiana/Indianapolis 
Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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