Disabling SMT on Flatcar Container Linux

Recent Intel CPU vulnerabilities ( L1TF and MDS ) cannot be fully mitigated in software without disabling Simultaneous Multi-Threading. This can have a substantial performance impact and is only necessary for certain workloads, so for compatibility reasons, SMT is enabled by default.

In addition, the Intel TAA vulnerability cannot be fully mitigated without disabling either of SMT or the Transactional Synchronization Extensions (TSX). Disabling TSX generally has less performance impact, so is the preferred approach on systems that don’t otherwise need to disable SMT. For compatibility reasons, TSX is enabled by default.

SMT and TSX should be disabled on affected Intel processors under the following circumstances:

1. A bare-metal host runs untrusted virtual machines, and other arrangements have not been made for mitigation.
2. A bare-metal host runs untrusted code outside a virtual machine.

SMT can be conditionally disabled by passing mitigations=auto,nosmt on the kernel command line. This will disable SMT only if required for mitigating a vulnerability. This approach has two caveats:

1. It does not protect against unknown vulnerabilities in SMT.
2. It allows future Flatcar Container Linux updates to disable SMT if needed to mitigate new vulnerabilities.

Alternatively, SMT can be unconditionally disabled by passing nosmt on the kernel command line. This provides the most protection and avoids possible behavior changes on upgrades, at the cost of a potentially unnecessary reduction in performance.

TSX can be conditionally disabled on vulnerable CPUs by passing tsx=auto on the kernel command line, or unconditionally disabled by passing tsx=off. However, neither setting takes effect on systems affected by MDS, since MDS mitigation automatically protects against TAA as well.

For typical use cases, we recommend enabling the mitigations=auto,nosmt and tsx=auto command-line options.

Configuring new machines

The following Butane Config performs two tasks:

1. Adds mitigations=auto,nosmt tsx=auto to the kernel command line. This affects the second and subsequent boots of the machine, but not the first boot.
2. On the first boot, disables SMT at runtime if the system has an Intel processor. This is sufficient to protect against currently-known SMT vulnerabilities until the system is rebooted. After reboot, SMT will be re-enabled if the processor is not actually vulnerable.

# Add kernel command-line arguments to automatically disable SMT or TSX
# on CPUs where they are vulnerable.
# Disable SMT on CPUs affected by MDS or similar vulnerabilities.
# Disable TSX on CPUs affected by TAA but not by MDS.
variant: flatcar
version: 1.0.0
kernel_arguments:
  should_exist:
    - mitigations=auto,nosmt
    - tsx=auto

Configuring existing machines

To add mitigations=auto,nosmt tsx=auto to the kernel command line on an existing system, add the following line to /usr/share/oem/grub.cfg:

set linux_append="$linux_append mitigations=auto,nosmt tsx=auto"

For example, using SSH:

ssh core@node01 'sudo sh -c "echo \"set linux_append=\\\"\\\$linux_append mitigations=auto,nosmt tsx=auto\\\"\" >> /usr/share/oem/grub.cfg && systemctl reboot"'

If you use locksmith for reboot coordination, replace systemctl reboot with locksmithctl send-need-reboot.