Shared microarchitectural predictor state may allow code to influence transient execution across a hardware boundary, potentially exposing data that is accessible beyond the boundary over a covert channel.
Many commodity processors have Instruction Set Architecture (ISA) features that protect software components from one another. These features can include memory segmentation, virtual memory, privilege rings, trusted execution environments, and virtual machines, among others. For example, virtual memory provides each process with its own address space, which prevents processes from accessing each other's private data. Many of these features can be used to form hardware-enforced security boundaries between software components.
When separate software components (for example, two processes) share microarchitectural predictor state across a hardware boundary, code in one component may be able to influence microarchitectural predictor behavior in another component. If the predictor can cause transient execution, the shared predictor state may allow an attacker to influence transient execution in a victim, and in a manner that could allow the attacker to infer private data from the victim by monitoring observable discrepancies (CWE-203) in a covert channel [REF-1400].
Predictor state may be shared when the processor transitions from one component to another (for example, when a process makes a system call to enter the kernel). Many commodity processors have features which prevent microarchitectural predictions that occur before a boundary from influencing predictions that occur after the boundary.
Predictor state may also be shared between hardware threads, for example, sibling hardware threads on a processor that supports simultaneous multithreading (SMT). This sharing may be benign if the hardware threads are simultaneously executing in the same software component, or it could expose a weakness if one sibling is a malicious software component, and the other sibling is a victim software component. Processors that share microarchitectural predictors between hardware threads may have features which prevent microarchitectural predictions that occur on one hardware thread from influencing predictions that occur on another hardware thread.
Features that restrict predictor state sharing across transitions or between hardware threads may be always-on, on by default, or may require opt-in from software.
The hardware designer can attempt to prevent transient execution from causing observable discrepancies in specific covert channels.
Hardware designers may choose to use microarchitectural bits to tag predictor entries. For example, each predictor entry may be tagged with a kernel-mode bit which, when set, indicates that the predictor entry was created in kernel mode. The processor can use this bit to enforce that predictions in the current mode must have been trained in the current mode. This can prevent malicious cross-mode training, such as when user-mode software attempts to create predictor ...
Hardware designers may choose to sanitize microarchitectural predictor state (for example, branch prediction history) when the processor transitions to a different context, for example, whenever a system call is invoked. Alternatively, the hardware may expose instruction(s) that allow software to sanitize predictor state according to the user's threat model. For example, this can allow operating system software to sanitize predictor state when performing a cont...
System software can mitigate this weakness by invoking predictor-state-sanitizing operations (for example, the indirect branch prediction barrier on Intel x86) when switching from one context to another, according to the hardware vendor's recommendations.
If the weakness is exposed by a single instruction (or a small set of instructions), then the compiler (or JIT, etc.) can be configured to prevent the affected instruction(s) from being generated. One prominent example of this mitigation is retpoline ([REF-1414]).
Use control-flow integrity (CFI) techniques to constrain the behavior of instructions that redirect the instruction pointer, such as indirect branch instructions.
Use software techniques (including the use of serialization instructions) that are intended to reduce the number of instructions that can be executed transiently after a processor event or misprediction.
Some systems may allow the user to disable predictor sharing. For example, this could be a BIOS configuration, or a model-specific register (MSR) that can be configured by the operating system or virtual machine monitor.
The hardware vendor may provide a patch to, for example, sanitize predictor state when the processor transitions to a different context, or to prevent predictor entries from being shared across SMT threads. A patch may also introduce new ISA that allows software to toggle a mitigation.
If a hardware feature can allow microarchitectural predictor state to be shared between contexts, SMT threads, or other architecturally defined boundaries, the hardware designer may opt to disclose this behavior in architecture documentation. This documentation can inform users about potential consequences and effective mitigations.
Processor designers, system software vendors, or other agents may choose to restrict the ability of unprivileged software to access to high-resolution timers that are commonly used to monitor covert channels.
This weakness can be detected in hardware by manually inspecting processor specifications. Features that exhibit this weakness may have microarchitectural predictor state that is shared between hardware threads, execution contexts (for example, user and kernel), or other components that may host mutually distrusting software (or firmware, etc.).
Software vendors can release tools that detect presence of known weaknesses on a processor. For example, some of these tools can attempt to transiently execute a vulnerable code sequence and detect whether code successfully leaks data in a manner consistent with the weakness under test. Alternatively, some hardware vendors provide enumeration for the presence of a weakness (or lack of a weakness). These enumeration bits can be checked and reported by system sof...
This weakness can be detected in hardware by employing static or dynamic taint analysis methods [REF-1401]. These methods can label each predictor entry (or prediction history, etc.) according to the processor context that created it. Taint analysis or information flow analysis can then be applied to detect when predictor state created in one context can influence predictions made in another context.
CVE-2017-5754(Branch Target Injection, BTI, Spectre v2). Shared microarchitectural indirect branch predictor state may allow code to influence transient execution across a process, VM, or privilege boundary, potentially exposing data that is accessible beyond the boundary.
CVE-2022-0001(Branch History Injection, BHI, Spectre-BHB). Shared branch history state may allow user-mode code to influence transient execution in the kernel, potentially exposing kernel data over a covert channel.
CVE-2021-33149(RSB underflow, Retbleed). Shared return stack buffer state may allow code that executes before a prediction barrier to influence transient execution after the prediction barrier, potentially exposing data that is accessible beyond the barrier over a covert channel.