CWE-1423: Exposure of Sensitive Information caused by Shared Microarchitectural Predictor State that Influences Transient Execution

MITRE CWE weakness

Description

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.

Common consequences

• Confidentiality: Read Memory

Mitigations

Architecture and Design — The hardware designer can attempt to prevent transient execution from causing observable discrepancies in specific covert channels.

Architecture and Design — 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 entries that influence transient execution in the kernel. Predictor entry tags can also be used to associate each predictor entry with the SMT thread that created it, and thus the processor can enforce that each predictor entry can only be used by the SMT thread that created it. This can prevent an SMT thread from using predictor entries crafted by a malicious sibling SMT thread.

Architecture and Design — 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 context switch from one process to another.

Implementation — 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.

Build and Compilation — 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]).

References

• CWE page: https://cwe.mitre.org/data/definitions/1423.html
• CWE list: https://cwe.mitre.org/data/index.html