Half-Double: A New DRAM Rowhammer Vulnerability
Written by Michael Larabel in Hardware on 25 May 2021 at 12:32 PM EDT. 19 Comments
HARDWARE --
The Rowhammer security exploit affecting DRAM memory modules has a new chapter with Google now detailing "half-double" as a new technique for exploit of system memory.

Google security researchers discovered Half-Double as a new technique that "capitalizes on the worsening physics of some of the newer DRAM chips to alter the contents of memory."
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.

The Half-Double vulnerability affects current DDR4 system modules and Google has been working with JEDEC on new mitigation techniques.

More details on the Half-Double vulnerability via the Google Security Blog and the Half-Double whitepaper.
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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 20,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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