Changes/KTLSSupportForGnuTLS

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KTLS implementation for GnuTLS

This is a proposed Change for Fedora Linux.
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: František Krenželok, Daiki Ueno
• Email: fkrenzel@redhat.com, dueno@redhat.com

Current status

• Targeted release: Fedora Linux 38
• Last updated: 2022-08-10
• FESCo issue: <will be assigned by the Wrangler>
• Tracker bug: <will be assigned by the Wrangler>
• Release notes tracker: <will be assigned by the Wrangler>

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). This approach saves us from frequent context switching as well as data copies in userspace when using send_file() function.

Benefit to Fedora

The improvement lies in acceleration of large data transfer 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 buffers. This is especially useful for NBD

One of the benefits might be acceleration of live VM migration, which should mitigate the downtime for various services used by both the users and the developers.

packages that might benefit: nbd nbdkit qemu

Scope

• Proposal owners:

• Other developers:

• Release engineering: #Releng issue number

• Policies and guidelines: N/A (not needed for this Change)

• Trademark approval: N/A (not needed for this Change)

• Alignment with Objectives:

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:

1. load TLS kernel module (modprobe tls)
2. enable ktls with crypto policies

Once proposal accepted
KTLS will be enabled by default and this step will not be needed.

$ cat > /etc/crypto-policies/local.d/gnutls-ktls.config <<EOF
[global]
ktls = true
EOF

$ update-crypto-policies

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.

Contingency Plan

• Contingency mechanism: (What to do? Who will do it?) N/A (not a System Wide Change)
• Contingency deadline: N/A (not a System Wide Change)
• Blocks release? N/A (not a System Wide Change), Yes/No

Documentation

API

gnutls_transport_is_ktls_enabled() To check if KTLS was properly initialized on the interfaces:

it has to be invoked no earlier that after a TLS-handshake

 gnutls_transport_ktls_enable_flags_t gnutls_transport_is_ktls_enabled(gnutls_session_t session);

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:

 ssize_t gnutls_record_send_file(gnutls_session_t session, int fd, off_t *offset, size_t count);

N/A (not a System Wide Change)