tirdad - TCP ISN CPU Information Leak Protection

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This page explains a Linux TCP Initial Sequence Number (ISN) side-channel information leak and introduces tirdad, a kernel module that helps mitigate it by randomizing TCP ISNs.

TCP ISN CPU Information Leaks

• Issue summary: The Linux kernel has a side-channel information leak bug that can leak information through outgoing TCP traffic. This can allow side-channel attacks because sensitive information about a system's CPU activity may be inferred remotely.

• Why it matters: This may be particularly dangerous for long-running cryptographic operations.
• Real-world impact: Research has demonstrated that related techniques can be used for de-anonymization of location-hidden services.
• Clock skew notes:
  • Leak source: Clock skew is leaked through TCP ISNs (Initial Sequence Numbers) by the Linux kernel.
  • Remote detection: Clock skew can be remotely detected by observing ISNs.
  • Attack amplification: Clock skew can be induced by an attacker by producing load on the victim machine.

What the Linux ISN leaks is the difference between two timestamps, not the timestamp itself. A difference lets you work out drift and skew, which can help someone fingerprint the computer hardware, its environment and load. Of course that only works if you can probe a computer, and maintain the same source/destination port and IP address.Add research idea for Linux TCP Initial Sequence Numbers may aid correlation: comment by Steven J. Murdoch

... it is worth complaining to the kernel developers for the simple reason that adding the 64ns timer post-hash probably *does* leak side channels about CPU activity, and that may prove very dangerous for long-running cryptographic operations (along the lines of the hot-or-not issue). Unfortunately, someone probably needs to produce more research papers before they will listen.Add research idea for Linux TCP Initial Sequence Numbers may aid correlation: comment by Mike Perry

• Further reading:
  • Introduction: An analysis of TCP secure sequence number generation in Linux and its privacy issues.
  • Background discussion: Initial sequence number generation in Linux TCP stack
  • Linux source reference: Linux source: secure_seq.c (v3.16-rc1)
  • Forum discussion: TCP ISNs and temperature induced clock skews
  • Forum discussion: TCP ISN CPU information leak protection (tirdad)
  • IETF tcpm thread: IETF: [tcpm Security concerns with relative timestamp exposure through TCP ISNs - mailing list thread]

Mitigation

TCP Initial Sequence Numbers (ISNs) randomization to help prevent TCP ISN based CPU information leaks.

tirdad is a kernel module to hot-patch the Linux kernel to generate random TCP Initial Sequence Numbers for IPv4 and IPv6 TCP connections.

Platform specific.

• Kicksecure: tirdad is installed by default.
• Kicksecure for Qubes: Requires In-VM Kernel.

Development

• source code: https://github.com/Kicksecure/tirdad

Footnotes

Kicksecure A secure by default operating system with the latest security research in place.

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