There’s a new way to flip bits in DRAM, and it works against the latest defenses


In 2015, researchers reported a stunning discovery that stoked industry-wide safety considerations—an assault referred to as RowHammer that might corrupt, modify, or steal delicate knowledge when a easy user-level software repeatedly accessed sure areas of DDR reminiscence chips. Within the coming years, reminiscence chipmakers scrambled to develop defenses that prevented the assault, primarily by limiting the variety of occasions packages may open and shut the focused chip areas in a given time.

Not too long ago, researchers devised a brand new technique for creating the identical forms of RowHammer-induced bitflips even on the most recent technology of chips, often called DDR4, which have the RowHammer mitigations constructed into them. Often known as RowPress, the brand new assault works not by “hammering” fastidiously chosen areas repeatedly, however as a substitute by leaving them open for longer durations than regular. Bitflips confer with the phenomenon of bits represented as ones change to zeros and vice versa.

Additional amplifying the vulnerability of DDR4 chips to read-disturbance assaults—the generic time period for inducing bitflips via irregular accesses to reminiscence chips—RowPress bitflips will be enhanced by combining them with RowHammer accesses. Curiously, elevating the temperature of the chip additionally intensifies the impact.

“We exhibit a proof of idea RowPress program that may trigger bitflips in an actual system that already employs protections towards RowHammer,” Onur Mutlu, a professor at ETH Zürich and a co-author of a not too long ago revealed paper titled RowPress: Amplifying Read Disturbance in Modern DRAM Chips, wrote in an e mail. “Observe that this isn’t in itself an assault. It merely exhibits that bitflips are potential and lots, which might simply type the idea of an assault. As many prior works in safety have proven, as soon as you may induce a bitflip, you need to use that bitflip for varied assaults.”

DIMMs, bits, and cosmic rays

The fundamental constructing block of DRAM (dynamic random entry reminiscence) is a storage cell. Every cell includes a capacitor and a transistor and shops a single bit of knowledge. When a bit is put right into a reminiscence cell, the transistor is used to cost or discharge the capacitor. A charged capacitor is represented with a 1, whereas a discharged capacitor is represented with a 0. Most frequently, DRAM cells are organized into an oblong array of rows and columns.

Diagram illustrating the heirarchical organization of a DRAM chip.
Enlarge / Diagram illustrating the heirarchical group of a DRAM chip.

These rows and columns are organized into ranks and banks to type what’s often called the twin in-line reminiscence module. Higher often called a DIMM, this rectangular stick plugs into the pc motherboard. When a pc accesses a bit of reminiscence, it opens the rows to the cells storing the specified knowledge and transfers it to the CPU.

Photo of a DIMM.
Enlarge / Picture of a DIMM.

Bitflips have lengthy been often called a uncommon, naturally occurring phenomenon. They happen when an sudden circulation of electrons inside a transistor’s gates modifications the voltage between the supply, or the a part of the circuit the place circulation originates, and the sink, the place the circulation is obtained. Throughout a 2003 election in Belgium, cosmic rays from outer house are extensively believed to have brought on bitflips that gave a politician an additional 4,096 votes earlier than being corrected. (The quantity 4,096, which can be expressed as 212, suggests there was a bitflip within the twelfth bit.)

Then, in 2015, researchers on Google’s Mission Zero vulnerability crew found a beforehand unknown approach to artificially induce bitflips at will. When a pc repeatedly hammered a DRAM row, they discovered they may cause bitflips in adjacent rows. In RowHammer parlance, the row that will get hammered is called the aggressor row. The row the place bits are flipped is called the sufferer row.

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