This paper introduces novel attack primitives that enable adversaries to leak (read) and manipulate (write) the path history register (PHR) and the prediction history tables (PHTs) of the conditional branch predictor in high-performance CPUs. These primitives enable two new classes of attacks: first, it can recover the entire control flow history of a victim program by exploiting read primitives, as demonstrated by a practical secret-image recovery based on capturing the entire control flow of libjpeg routines. Second, it can launch extremely high-resolution transient attacks by exploiting write primitives. We demonstrate this with a key recovery attack against AES based on extracting intermediate values.

Spectre-type attacks have exposed a major class of vulnerabilities arising from speculative execution of instructions, the main performance enabler of modern CPUs. These attacks speculatively leak secrets that have been either speculatively loaded (seen in sand-boxed programs) or non-speculatively loaded (seen in constant-time programs). Various hardware-only defenses have been proposed to mitigate both speculative and non-speculative secrets via all potential transmission channels. However, limited program knowledge is exposed to the hardware and these solutions conservatively restrict the execution of all instructions that can potentially leak.