Lecture on Memory Robustness and Rowhammer

Introduction

• Lecturer: Giray Al
• Background: Researcher, lecturer, recently got a PhD in memory robustness
• Focus: Memory robustness, specifically Rowhammer

Rowhammer Vulnerability

• Rowhammer is a significant vulnerability in memory devices.
• Exploits include:
  • Gaining unauthorized access to data
  • Breaking out of virtual machine sandboxes
  • Corrupting data
  • Altering critical workload behavior
  • Stealing sensitive data (e.g., SSH keys)

Historical Context

• Key Paper: "Flipping Bits in Memory Without Accessing Them" (ISCA 2014, Safari group)
• Immediate impact demonstrated by Google’s Project Zero: Exploited Rowhammer to gain kernel privileges in Linux systems.

Technical Details of Rowhammer

• Rowhammer occurs when repeated access to a row of memory causes bit flips in adjacent rows.
• Exploits involve altering page table entries to gain access to larger memory areas.
• Memory spraying technique: filling physical memory with page tables to exploit bit flips.

Mitigation Techniques

• Error Correcting Codes (ECC): Found ineffective against Rowhammer.
• Solutions like increased refresh rates, physical isolation, and memory access throttling have been proposed.
• Probabilistic Adjacent Row Activation (PARA) is a low-cost, effective solution that refreshes neighboring rows with low probability to prevent bit flips.

Recent Developments

• Newer memory devices (DDR4, LPDDR4, HBM) are also vulnerable to Rowhammer.
• Techniques like four-sided Rowhammer attack exploit limitations in existing countermeasures.
• Rowhammer attacks have been demonstrated across multiple devices, including mobile phones.

Advanced Research and Future Directions

• Recent research indicates Rowhammer vulnerability is worsening with technology scaling.
• Strategies to mitigate Rowhammer include sophisticated memory refresh strategies and leveraging circuit-level insights.
• New attack patterns continue to be discovered, necessitating ongoing research into more robust solutions.

Conclusion

• Memory robustness, especially against Rowhammer, remains a critical area of research.
• Ongoing advancements in mitigation techniques are necessary as technology scales.
• Future memory systems need intelligent controllers for improved security, safety, and reliability.