Supporting Rowhammer analysis to guard the DRAM ecosystem

To handle this hole and assist the ecosystem with deploying strong defenses, Google has supported educational analysis and developed check platforms to research DDR5 reminiscence. Our effort has led to the invention of latest assaults and a deeper understanding of Rowhammer on the present DRAM modules, serving to to forge the way in which for additional, stronger mitigations.

What’s Rowhammer? 

Rowhammer exploits a vulnerability in DRAM. DRAM cells retailer knowledge as electrical expenses, however these electrical expenses leak over time, inflicting knowledge corruption. To stop knowledge loss, the reminiscence controller periodically refreshes the cells. Nonetheless, if a cell discharges earlier than the refresh cycle, its saved bit might corrupt. Initially thought of a reliability situation, it has been leveraged by safety researchers to exhibit privilege escalation assaults. By repeatedly accessing a reminiscence row, an attacker may cause bit flips in neighboring rows. An adversary can exploit Rowhammer by way of:

1. Reliably trigger bit flips by repeatedly accessing adjoining DRAM rows.

2. Coerce different purposes or the OS into utilizing these susceptible reminiscence pages.

3. Goal security-sensitive code or knowledge to realize privilege escalation.

4. Or just corrupt system’s reminiscence to trigger denial of service. 

Earlier work has repeatedly demonstrated the potential for such assaults from software program [Revisiting rowhammer, Are we susceptible to rowhammer?, Drammer,  Flip feng shui, Jolt]. Consequently, defending towards Rowhammer is required for safe isolation in multi-tenant environments just like the cloud. 

Rowhammer Mitigations 

The first method to mitigate Rowhammer is to detect which reminiscence rows are being aggressively accessed and refreshing close by rows earlier than a bit flip happens. TRR is a typical instance, which makes use of a variety of counters to trace accesses to a small variety of rows adjoining to a possible sufferer row. If the entry rely for these aggressor rows reaches a sure threshold, the system points a refresh to the sufferer row. TRR could be integrated inside the DRAM or within the host CPU.

Nonetheless, this mitigation will not be foolproof. For instance, the TRRespass assault confirmed that by concurrently hammering a number of, non-adjacent rows, TRR could be bypassed. Over the previous couple of years, extra subtle assaults [Half-Double, Blacksmith] have emerged, introducing extra environment friendly assault patterns. 

In response, one in every of our efforts was to collaborate with JEDEC, exterior researchers, and consultants to outline the PRAC as a brand new mitigation that deterministically detects Rowhammer by monitoring all reminiscence rows. 

Nonetheless, present techniques geared up with DDR5 lack help for PRAC or different strong mitigations. Consequently, they depend on probabilistic approaches equivalent to ECC and enhanced TRR to scale back the danger. Whereas these measures have mitigated older assaults, their general effectiveness towards new methods was not absolutely understood till our latest findings.

Challenges with Rowhammer Evaluation 

Mitigating Rowhammer assaults includes making it troublesome for an attacker to reliably trigger bit flips from software program. Due to this fact, for an efficient mitigation, we have now to grasp how a decided adversary introduces reminiscence accesses that bypass present mitigations. Three key info parts might help with such an evaluation:

1. How the improved TRR and in-DRAM ECC work.

2. How reminiscence entry patterns from software program translate into low-level DDR instructions.

3. (Optionally) How any mitigations (e.g., ECC or TRR) within the host processor work.

Step one is especially difficult and includes reverse-engineering the proprietary in-DRAM TRR mechanism, which varies considerably between completely different producers and machine fashions. This course of requires the flexibility to situation exact DDR instructions to DRAM and analyze its responses, which is troublesome on an off-the-shelf system. Due to this fact, specialised check platforms are important.

The second and third steps contain analyzing the DDR visitors between the host processor and the DRAM. This may be executed utilizing an off-the-shelf interposer, a device that sits between the processor and DRAM. A vital a part of this evaluation is knowing how a dwell system interprets software-level reminiscence accesses into the DDR protocol.

The third step, which includes analyzing host-side mitigations, is usually optionally available. For instance, host-side ECC (Error Correcting Code) is enabled by default on servers, whereas host-side TRR has solely been carried out in some CPUs. 

Rowhammer testing platforms

For the primary problem, we partnered with Antmicro to develop two specialised, open-source FPGA-based Rowhammer check platforms. These platforms permit us to conduct in-depth testing on various kinds of DDR5 modules.

• DDR5 RDIMM Platform: A brand new DDR5 Tester board to fulfill the {hardware} necessities of Registered DIMM (RDIMM) reminiscence, frequent in server computer systems.

• SO-DIMM Platform: A model that helps the usual SO-DIMM pinout appropriate with off-the-shelf DDR5 SO-DIMM reminiscence sticks, frequent in workstations and end-user gadgets.

Antmicro designed and manufactured these open-source platforms and we labored intently with them, and researchers from ETH Zurich, to check the applicability of those platforms for analyzing off-the-shelf reminiscence modules in RDIMM and SO-DIMM kinds.

Antmicro DDR5 RDIMM FPGA check platform in motion.

Phoenix Assaults on DDR5

In collaboration with researchers from ETH, we utilized the brand new Rowhammer check platforms to judge the effectiveness of present in-DRAM DDR5 mitigations. Our findings, detailed within the not too long ago co-authored “Phoenix” analysis paper, reveal that we efficiently developed customized assault patterns able to bypassing enhanced TRR (Goal Row Refresh) protection on DDR5 reminiscence. We had been in a position to create a novel self-correcting refresh synchronization assault approach, which allowed us to carry out the first-ever Rowhammer privilege escalation exploit on an ordinary, production-grade desktop system geared up with DDR5 reminiscence. Whereas this experiment was carried out on an off-the-shelf workstation geared up with latest AMD Zen processors and SK Hynix DDR5 reminiscence, we proceed to research the applicability of our findings to different {hardware} configurations.

Classes discovered 

We confirmed that present mitigations for Rowhammer assaults aren’t adequate, and the difficulty stays a widespread downside throughout the trade. They do make it harder “however not unattainable” to hold out assaults, since an attacker wants an in-depth understanding of the particular reminiscence subsystem structure they want to goal.

Present mitigations based mostly on TRR and ECC depend on probabilistic countermeasures which have inadequate entropy. As soon as an analyst understands how TRR operates, they’ll craft particular reminiscence entry patterns to bypass it. Moreover, present ECC schemes weren’t designed as a safety measure and are due to this fact incapable of reliably detecting errors.

Reminiscence encryption is another countermeasure for Rowhammer. Nonetheless, our present evaluation is that with out cryptographic integrity, it affords no worthwhile protection towards Rowhammer. Extra analysis is required to develop viable, sensible encryption and integrity options.

Path ahead

Google has been a frontrunner in JEDEC standardization efforts, as an illustration with PRAC, a completely permitted commonplace to be supported in upcoming variations of DDR5/LPDDR6. It really works by precisely counting the variety of occasions a DRAM wordline is activated and alerts the system if an extreme variety of activations is detected. This shut coordination between the DRAM and the system provides PRAC a dependable option to tackle Rowhammer. 

Within the meantime, we proceed to judge and enhance different countermeasures to make sure our workloads are resilient towards Rowhammer. We collaborate with our educational and trade companions to enhance evaluation methods and check platforms, and to share our findings with the broader ecosystem.

Need to be taught extra?

“Phoenix: Rowhammer Assaults on DDR5 with Self-Correcting Synchronization” will likely be introduced at IEEE Safety & Privateness 2026 in San Francisco, CA (MAY 18-21, 2026).