On November 13, 2018, researchers from Graz University of Technology*, imec*-DistriNet, KU Leuven*, and College of William and Mary* released a paper entitled A Systematic Evaluation of Transient Execution Attacks and Defenses. In this paper they document the following seven exploits related to Spectre and Meltdown:
Meltdown-PK – exploitation of Memory Protection Keys
Meltdown-BR (MPX/BND) – exploitation of Memory Protection Extensions
Spectre-PHT-CA-OP—exploitation of the pattern history table across address spaces
Spectre-PHT-SA-IP—exploitation of the pattern history table in the same address space
Spectre-PHT-SA-OP—exploitation of the pattern history table in a different branch address as the branch target
Spectre-BTB-SA-IP—exploitation of the branch target buffer in the same branch address as the branch target
Spectre-BTP-SA-OP—exploitation of the branch target buffer in a different branch address as the branch target
After careful assessment, Intel determined that existing software guidance for mitigating previously disclosed Spectre and Meltdown vulnerabilities provides protection against the exploits documented in this paper.
Meltdown-PK and Meltdown-BR Mitigations
Meltdown-PK is mitigated using address space isolation, as with speculative store bypass.
Future Intel processors will be able to mitigate Meltdown-PK using a combination of hardware support that also covers rogue data cache load (IA32_ARCH_CAPABILITIES[RDCL_NO]) and protection keys support (CPUID.7.0.ECX[3]), which can limit the memory addresses that could be revealed by these vulnerabilities.
Meltdown-BR is addressed by mitigations for bounds check bypass and bounds check bypass store, which is accomplished by modifying software to insert LFENCE or other serializing instructions to constrain speculation in confused deputy code. These instructions suffice regardless of whether the bounds checking is implemented using conditional branches or through the use of bound-checking instructions (BNDCL and BNDCU) that are part of the Intel® Memory Protection Extensions (Intel® MPX).
Spectre-related Mitigations
Previous disclosures of Spectre vulnerabilities name the exploits by the mechanism by which data can be accessed by a malicious actor. In the A Systematic Evaluation of Transient Execution Attacks and Defenses paper, the researchers instead have created an alternative classification system to identify the theoretically possible Spectre variants.
These variants do not identify any new microarchitectural structures that can be exploited, just new ways to train the microarchitectural structures. Existing mitigation strategies will continue to protect code as follows:
Spectre-PHT and Spectre-BTB can be mitigated through software-based approaches, including LFENCE, Indirect Branch Restricted Speculation (IBRS) and retpoline. Developers should review their code, identify secrets, and insert LFENCE appropriately, as advised for Bounds Check Bypass. Details on implementing IBRS and retpoline can be found under Branch Target Injection.
Stay Updated and Informed
Intel and the ecosystem have released microcode updates and software mitigations for Spectre and Meltdown, and these mitigations are effective for the exploits described in A Systematic Evaluation of Transient Execution Attacks and Defenses.
We will continue to provide software developer guidance when appropriate. Please check back for the latest information.
Spectre-BTB-SA-IP—exploitation of the branch target buffer in the same branch address as the branch target
Spectre-BTP-SA-OP—exploitation of the branch target buffer in a different branch address as the branch target
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