“An unprivileged application can corrupt data in memory by accessing ‘hammering’ rows of DDR4 memory in certain patterns millions of times a second, giving those untrusted applications nearly unfettered system privileges,” writes long-time Slashdot reader shoor. Ars Technica reports: Rowhammer attacks work by accessing — or hammering — physical rows inside vulnerable chips millions of times per second in ways that cause bits in neighboring rows to flip, meaning 1s turn to 0s and vice versa. Researchers have shown the attacks can be used to give untrusted applications nearly unfettered system privileges, bypass security sandboxes designed to keep malicious code from accessing sensitive operating system resources, and root or infect Android devices, among other things. All previous Rowhammer attacks have hammered rows with uniform patterns, such as single-sided, double-sided, or n-sided. In all three cases, these “aggressor” rows — meaning those that cause bitflips in nearby “victim” rows — are accessed the same number of times.
Research published on Monday presented a new Rowhammer technique. It uses non-uniform patterns that access two or more aggressor rows with different frequencies. The result: all 40 of the randomly selected DIMMs in a test pool experienced bitflips, up from 13 out of 42 chips tested in previous work (PDF) from the same researchers. “We found that by creating special memory access patterns we can bypass all mitigations that are deployed inside DRAM,” Kaveh Razavi and Patrick Jattke, two of the research authors, wrote in an email. “This increases the number of devices that can potentially be hacked with known attacks to 80 percent, according to our analysis. These issues cannot be patched due to their hardware nature and will remain with us for many years to come.”
The non-uniform patterns work against Target Row Refresh. Abbreviated as TRR, the mitigation works differently from vendor to vendor but generally tracks the number of times a row is accessed and recharges neighboring victim rows when there are signs of abuse. The neutering of this defense puts further pressure on chipmakers to mitigate a class of attacks that many people thought more recent types of memory chips were resistant to. In Monday’s paper, the researchers wrote: “Proprietary, undocumented in-DRAM TRR is currently the only mitigation that stands between Rowhammer and attackers exploiting it in various scenarios such as browsers, mobile phones, the cloud, and even over the network. In this paper, we show how deviations from known uniform Rowhammer access patterns allow attackers to flip bits on all 40 recently-acquired DDR4 DIMMs, 2.6x more than the state of the art. The effectiveness of these new non-uniform patterns in bypassing TRR highlights the need for a more principled approach to address Rowhammer.” While PCs, laptops, and mobile phones are most affected by the new findings, the report notes that cloud services like AWS and Azure “remain largely safe from Rowhammer because they use higher-end chips that include a defense known as ECC, short for Error Correcting Code.”
“Concluding, our work confirms that the DRAM vendors’ claims about Rowhammer protections are false and lure you into a false sense of security,” the researchers wrote. “All currently deployed mitigations are insufficient to fully protect against Rowhammer. Our novel patterns show that attackers can more easily exploit systems than previously assumed.”
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