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
• Email: fkrenzel@redhat.com

• SIG: crypto-team

Current status

• Targeted release: Fedora Linux 38
• Last updated: 2022-08-08
• 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

This change will affect broad spectrum of tools/packages that use GnuTLS. 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: 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

This feature is completely optional. Nor developers or users will be required to do anything regarding this change unless they would like to use the feature, in which case they would follow instructions in How to test section below.

How To Test

To enable this feature, user has to load TLS kernel module (modprobe tls) and enable ktls with crypto policies

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

$ update-crypto-policies

The following API is added

To check if KTLS was properly initialized on the interfaces, see: (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);

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);

User Experience

This change might interest package maintainers i.e. developers that picked GnuTLS as their go to solution when it come to transferring data securely, and wish to accelerate it. The acceleration is most noticeable on large data transfer such that of files.

Dependencies

KTLS will not initialize if app uses custom push/pull callback for GnuTLS.

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

N/A (not a System Wide Change)