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.

kernel: Arm Processors: Privilege escalation or information disclosure via writes to higher exception level resources

Arm C1-Ultra, C1-Premium, Neoverse V3 & V3AE, Neoverse V2, Neoverse V1, Neoverse-N2, Neoverse-N1, Cortex-X925, Cortex-X4, Cortex-X3, Cortex-X2, Cortex-X1 & X1C, Cortex-A710, Cortex-A78, A78AE & A78C, Cortex-A77, Cortex-A76 & A76A may allow writes to resources owned by a higher exception level.

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.