Half-Double: Hammering From the Next Row Over

Rowhammer is a vulnerability in modern DRAM where repeated accesses to one row (the aggressor) give off electrical disturbance whose cumulative effect flips the bits in an adjacent row (the victim). Consequently, Rowhammer defenses presuppose the adjacency of aggressor-victim pairs, including those in LPDDR4 and DDR4, most notably TRR. In this paper, we present Half-Double(1), an escalation of Rowhammer to rows beyond immediate neighbors. Using Half-Double, we induce errors in a victim by combining many accesses to a distance-2 row with just a few to a distance-1 row. Our experiments show that the cumulative effect of these leads to a sufficient electrical disturbance in the victim row, inducing bit flips. We demonstrate the practical relevance of Half-Double in a proof-of-concept attack on a fully up-to-date system. We use side channels, a new technique called Blind-Hammering, a new spraying technique, and a Spectre attack in our end-to-end Half-Double Attack. On recent Chromebooks with ECC- and TRR-protected LPDDR4x memory, the attack takes less than 45 minutes on average.