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Half-Double Rowhammer technique

A crew of safety researchers from Google has demonstrated yet one more variant of the Rowhammer assault that bypasses all present defenses to tamper with information saved in reminiscence.

Dubbed “Half-Double,” the brand new hammering approach hinges on the weak coupling between two reminiscence rows that aren’t instantly adjoining to one another however one row eliminated.

“In contrast to TRRespass, which exploits the blind spots of manufacturer-dependent defenses, Half-Double is an intrinsic property of the underlying silicon substrate,” the researchers noted.

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“That is doubtless a sign that {the electrical} coupling answerable for Rowhammer is a property of distance, successfully turning into stronger and longer-ranged as cell geometries shrink down. Distances higher than two are conceivable.”

Rowhammer assaults are much like speculative execution in that each break the basic safety ensures made by the underlying {hardware}. Found in 2014, Rowhammer refers to a category of DRAM vulnerabilities whereby repeated accesses to a reminiscence row (“aggressor”) can induce {an electrical} disturbance large enough to flip bits saved in an adjoining row (“sufferer”), thereby permitting untrusted code to flee its sandbox and take over management of the system.

Half-Double Rowhammer technique

Whereas DRAM producers deployed countermeasures like Goal Row Refresh (TRR) to thwart such assaults, the mitigations have been restricted to 2 speedy neighbors of an aggressor row, thus excluding reminiscence cells at a two-row distance. The imperfect protections meant TRR defenses in DDR4 playing cards could possibly be circumvented to stage new variants of Rowhammer assaults resembling TRRespass and SMASH.

The space-two assisted Rowhammer — aka Half-Double — now joins that record. “Given three consecutive rows A, B, and C, we have been capable of assault C by directing a really giant variety of accesses to A, together with only a handful (~dozens) to B,” the researchers defined. On this new setup, A is the “far aggressor,” B is the “close to aggressor,” and C is the “sufferer.”

Google stated it is at present working with the Joint Electron Machine Engineering Council (JEDEC), an impartial standardization physique and semiconductor engineering commerce group, together with different business companions, to establish doable options for Rowhammer exploits.

“To guage the effectiveness of a [SoC-level] mitigation, a DRAM vendor ought to check a mixture of hammering distances fairly than solely testing at particular person distances,” the researchers said. “In different phrases, hammering a single row or a pair of sandwiching rows on the uncooked medium is not going to present this impact. As a substitute, pairs of rows on one or each side of an supposed sufferer have to be hammered.”

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