hw: cpu: arm64: Spectre-BHB

Certain Arm Cortex and Neoverse processors through 2022-03-08 do not properly restrict cache speculation, aka Spectre-BHB. An attacker can leverage the shared branch history in the Branch History Buffer (BHB) to influence mispredicted branches. Then, cache allocation can allow the attacker to obtain sensitive information.

In certain Arm CPUs, a CPP RCTX instruction executed on one Processing Element (PE) may inhibit TLB invalidation when a TLBI is issued to the PE, either by the same PE or another PE in the shareability domain. In this case, the PE may retain stale TLB entries which should have been invalidated by the TLBI.

Use of Hardware Page Aggregation (HPA) and Stage-1 and/or Stage-2 translation on Cortex-A77, Cortex-A78, Cortex-A78C, Cortex-A78AE, Cortex-A710, Cortex-X1, Cortex-X1C, Cortex-X2, Cortex-X3, Cortex-X4, Cortex-X925, Neoverse V1, Neoverse V2, Neoverse V3, Neoverse V3AE, Neoverse N2 may permit bypass of Stage-2 translation and/or GPT protection.

Spectre BHB is a variant of Spectre-v2 in which malicious code uses the shared branch history (stored in the CPU BHB) to influence mispredicted branches in the victim's hardware context. Speculation caused by these mispredicted branches can then potentially be used to cause cache allocation, which can then be used to infer information that should be protected.