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Rowhammer is a hardware vulnerability in DRAM memory, where repeated accessto memory can induce bit flips in neighboring memory locations. Being ahardware vulnerability, rowhammer bypasses all of the system memory protection,allowing adversaries to compromise the integrity and confidentiality of data.Rowhammer attacks have shown to enable privilege escalation, sandbox escape,and cryptographic key disclosures. Recently, several proposals suggestexploiting the spatial proximity between the accessed memory location and thelocation of the bit flip for a defense against rowhammer. These all aim to denythe attacker's permission to access memory locations near sensitive data. Inthis paper, we question the core assumption underlying these defenses. Wepresent PThammer, a confused-deputy attack that causes accesses to memorylocations that the attacker is not allowed to access. Specifically, PThammerexploits the address translation process of modern processors, inducing theprocessor to generate frequent accesses to protected memory locations. Weimplement PThammer, demonstrating that it is a viable attack, resulting in asystem compromise (e.g., kernel privilege escalation). We further evaluate theeffectiveness of proposed software-only defenses showing that PThammer canovercome those.

PThammer: Cross-User-Kernel-Boundary Rowhammer through Implicit Accesses