May 25, 2021

Introducing Half-Double: New hammering technique for DRAM Rowhammer bug

Today, we are sharing details around our discovery of Half-Double, a new Rowhammer technique that capitalizes on the worsening physics of some of the newer DRAM chips to alter the contents of memory.

Rowhammer is a DRAM vulnerability whereby repeated accesses to one address can tamper with the data stored at other addresses. Much like speculative execution vulnerabilities in CPUs, Rowhammer is a breach of the security guarantees made by the underlying hardware. As an electrical coupling phenomenon within the silicon itself, Rowhammer allows the potential bypass of hardware and software memory protection policies. This can allow untrusted code to break out of its sandbox and take full control of the system.

Rowhammer was first discussed in a paper in 2014 for what was then the mainstream generation of DRAM: DDR3. The following year, Google’s Project Zero released a working privilege-escalation exploit. In response, DRAM manufacturers implemented proprietary logic inside their chips that attempted to track frequently accessed addresses and reactively mitigate when necessary.

As DDR4 became widely adopted, it appeared as though Rowhammer had faded away thanks in part to these built-in defense mechanisms. However, in 2020, the TRRespass paper showed how to reverse-engineer and neutralize the defense by distributing accesses, demonstrating that Rowhammer techniques are still viable. Earlier this year, the SMASH research went one step further and demonstrated exploitation from JavaScript, without invoking cache-management primitives or system calls.

Traditionally, Rowhammer was understood to operate at a distance of one row: when a DRAM row is accessed repeatedly (the “aggressor”), bit flips were found only in the two adjacent rows (the “victims”). However, with Half-Double, we have observed Rowhammer effects propagating to rows beyond adjacent neighbors, albeit at a reduced strength. Given three consecutive rows A, B, and C, we were able to attack C by directing a very large number of accesses to A, along with just a handful (~dozens) to B. Based on our experiments, accesses to B have a non-linear gating effect, in which they appear to “transport” the Rowhammer effect of A onto C. Unlike TRRespass, which exploits the blind spots of manufacturer-dependent defenses, Half-Double is an intrinsic property of the underlying silicon substrate. This is likely an indication that the electrical coupling responsible for Rowhammer is a property of distance, effectively becoming stronger and longer-ranged as cell geometries shrink down. Distances greater than two are conceivable.

Google has been working with JEDEC, an independent semiconductor engineering trade organization, along with other industry partners, in search of possible solutions for the Rowhammer phenomenon. JEDEC has published two documents about DRAM and system-level mitigation techniques (JEP 300-1 and JEP301-1).

We are disclosing this work because we believe that it significantly advances the understanding of the Rowhammer phenomenon, and that it will help both researchers and industry partners to work together, to develop lasting solutions. The challenge is substantial and the ramifications are industry-wide. We encourage all stakeholders (server, client, mobile, automotive, IoT) to join the effort to develop a practical and effective solution that benefits all of our users.

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