|Q: Can you explain the problem in layman's terms?|
|Q: What is a cache side-channel attack?|
|Q: What does this mean for the average mobile user?|
|Q: Can you describe the variants of Spectre and Meltdown?|
|Q: How is TrustZone affected?|
Spectre Variant 1
|Q: Will the Cortex-A cores impacted by Spectre variant 1 be updated for resiliency against the attack?|
Spectre Variant 2
|Q: Is Google's Retpoline considered effective for mitigating variant 2 vulnerability on Arm based systems?|
|Q: Will compilers provide mitigations for variant 2?|
Meltdown Variant 3
|Q: Is Arm impacted by Meltdown 3a? (CVE-2018-3640)?|
Spectre Variant 4
Q: Can you explain the problem in layman's terms?
Both attacks exploit existing side-channel techniques and could result in small bits of data being accessed through use of malware.
Malware using this method and running locally could potentially allow malicious actors to improperly gather small bits of sensitive data from privileged memory (DRAM or CPU cache).
Arm believes these exploits do not have the potential to corrupt, modify or delete data.
All variants are based on speculative memory access causing cache allocation. Timing analysis of memory accesses can then be used to reveal data that would otherwise be kept secret. Each mechanism is slightly different, and some are microarchitecture dependent. The impact of each variant on Arm cores, plus the solutions and how they should be applied, are described in the Software Overview for Arm Cores Paper.
Q: What is a cache side-channel attack?
When using the time taken to execute instructions that access the cache (such as loads or stores) it's possible to infer what items are in the cache. By being able to infer such information, the values of addresses that have been used to allocate items into the cache can be deduced.
Q: What does this mean for the average mobile user?
Malicious actors would need to install malware on devices to execute any of these attacks. Users will greatly reduce their risk by following good security practices by avoiding suspicious links and downloads, and immediately installing any software updates when available from device makers.
Q: Can you describe the variants of Spectre and Meltdown?
Variant 1 (Spectre): Bounds Check Bypass - Use existing code with access to secrets by making it speculatively execute memory operations with out-of-range arguments.
Variant 2 (Spectre): Branch Target Injection - Malicious code usurps properties of CPU branch prediction features to speculatively run code.
Variant 3 (Meltdown): Rogue Data Load - Access memory controlled by the OS while running a malicious application.
Variant 3a (Meltdown) - A different form of Variant 3 identified by Arm. Variant 3a uses speculation to access a privileged system register.
Variant 4 (Spectre) - An attack using a CPU technology known as memory disambiguation.
Detailed CVE descriptions for all known variants can be found at www.arm.com/security-update.
Q: How is TrustZone affected?
Arm Trusted Firmware has been updated to mitigate the known Spectre variants (1, 2 and 4) which could potentially create different levels of privilege.
For Meltdown, this variant is only exploitable between Exception Levels within the same translation regime, for example between EL0 and EL1, therefore this variant cannot be used to access secure memory from the non-secure world and is not applicable for Arm Trusted Firmware.
The link to the updated Trusted Firmware can be found at www.arm.com/security-update.
Q: Is the Cortex-A65AE vulnerable to SMT-based attacks such as PortSmash or TLBleed?
As a multi-threaded processor, the Cortex-A65AE is vulnerable to attacks that are based on using one hardware thread to expose data in another. However, there are software mitigations available for those attacks. Arm will communicate that information to licensees of the Cortex-A65AE, and as details of that CPU become public, that information will be made available here.
Q: Which Arm processors are affected by Spectre and Meltdown variants?
A comprehensive and updated list of all Arm processors impacted by all known variants can be found at www.developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability.
Q: Will future Arm cores or architectures address this?
Future Arm cores and architectures will provide a range of features to address the issues that can be addressed within hardware and will provide more optimized mechanisms to allow software to address the issues that need to be addressed in software.
Q: My Arm processor operates with branch prediction and speculative execution, why is my processor not on the list of affected processors?
Power efficient processor implementations tend to have shorter pipelines with fewer backend stages, and may also lack more complex micro-architectural features such as out-of-order execution. While these processors do still use speculation, they tend to be less aggressively optimized which makes these attacks unfeasible.
Arm have looked at all processors in all families to assess their vulnerability and compiled the table of processors with their perceived vulnerability that can be seen on the Speculative Processor Vulnerability overview page. Any processor not on the table has been analyzed and deemed not susceptible.
Q: Can you comment on the susceptibility of Arm architecture partner implementations?
Arm cannot comment on architecture partner implementations. Please contact those partners directly with any questions.
Q: How have affected Arm CPUs been updated to address these threats?
For details on mitigations added to Armv8.5-A, view the Armv8.5-A CPU Updates document.
Q: Is there a fix for all the vulnerabilities?
Each variant will have a different set of mitigations. For all known variants impacting Arm cores, Arm has completed initial kernel patching, compiler work and firmware updates. We continue to monitor and work on mitigations. Detailed information on mitigations for all known variants can be found at www.arm.com/security-update.
Q: Are software mitigations available, and will I get them?
The major operating systems running on Arm are aware of these issues and are deploying software mitigations. Please contact the suppliers of that software for their plans in this regard.
Linux patches are now under community review and can be found here: https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=kpti.
Arm trusted firmware patches are being made available through GitHub:
Specifications of the changes required in Firmware are available here.
Q: How can mobile or PC users know whether the mitigation measure has been applied to them or not?
We encourage end-users to frequently check the security update sites for their respective operator or OEM or inquire directly as to when security updates will be available for Spectre and Meltdown.
Q: What is Arm doing to ensure existing Android devices are updated with mitigations?
Arm has pushed out the necessary mitigations for all known Meltdown and Spectre variants to our ecosystem. In some cases, updates have been pushed to existing telephones that address the original vulnerabilities published in January 2018.
Questions about specific devices or timing for deploying security updates should be directed to OEMs.
Q: Are all the fixes applicable to Arm architecture partner designs?
The vulnerabilities are dependent on processor micro-architecture and Arm cannot comment on how architecture partner designs are impacted. We advise you to contact Arm architecture partners directly with questions related to mitigations for their affected designs.
Spectre Variant 1
Q: Will the Cortex-A cores impacted by Spectre variant 1 be updated for resiliency against the attack?
Spectre variant 1 is an issue that a usually safe hardware mechanism becomes exploitable in a very small number of cases that need to be identified by the software. As such, it’s not possible for hardware to distinguish between apparently identical code sequences which are safe and those which are exploitable, and so it not practical to implement the generic fixes in the hardware without incurring significant performance degradation on safe code sequences. Therefore we regard it as being necessary for the software to be modified to avoid the issue.
Spectre Variant 2
Q: Is Google's Retpoline considered effective for mitigating variant 2 vulnerability on Arm based systems?
Arm has investigated the use of retpoline and has concluded that it doesn't provide effective mitigation on Arm-based systems. Retpoline relies on specific aspects of the design of the branch prediction logic in the CPU, which do not apply to Arm-based systems.
Variant 2 mitigations for Arm systems have been implemented in the Linux kernel and Arm Trusted Firmware and can be implemented in other operating systems. For details see www.arm.com/security-update.
See our FAQ "Are software mitigations available, and will I get them?" for more information.
Q: Will compilers provide mitigations for variant 2?
Arm is not currently aware of any effective compiler-based mitigation techniques (such as retpolines) for variant 2.
Variant 2 mitigations for Arm systems have been implemented in the Linux kernel and Arm Trusted Firmware, and can be implemented in other operating systems. For details see www.arm.com/security-update.
Meltdown Variant 3
Q: Is Arm impacted by Meltdown 3a? (CVE-2018-3640)?
In January 2018, Arm provided details on the Cortex-A cores (A15, A57, and A72) impacted. Arm believes that software mitigations for Meltdown 3a are not required for the impacted Arm cores. However, an update for the latest release of the Cortex-A72 does include a hardware mitigation for this issue. For more details, please refer to our whitepaper at www.arm.com/security-update.
Spectre Variant 4
Q: Please explain how Spectre Variant 4 differs from the other variants?
Variant 4 is a Spectre-type attack utilizing a CPU technology known as memory disambiguation, a technology used in high-end CPUs to enable greater out-of-order execution and higher performance.
Simply put, this is a race between a store and following load that target the same memory location whereby under specific conditions, a speculative load can overtake a store, resulting in the load returning stale data.
If a chain of suitable instructions can be constructed, this stale data can be used to construct an address that drives cache allocation. This can be used to leak data to an attacker across a privilege boundary in a similar way to existing variants.
Q: What Arm cores are impacted by Spectre Variant 4?
Four cores from the existing Cortex-A family are impacted: Cortex-A57, A72, A73 and A75. All cores impacted by all Spectre and Meltdown variants are listed at www.arm.com/security-update.
Q: Will the Cortex-A cores impacted by Spectre variant 4 be updated for resiliency against the attack?
The mitigation for existing cores impacted by Spectre variant 4 is a straightforward configuration setting at boot and, therefore, does not require hardware-level modifications. Arm has introduced new architectural features that make implementing software mitigations more straightforward and common across different CPUs.
Q: Can Spectre variant 4 attacks be executed through a web browser?
Q: Are any Arm cores affected by CVE-2018-3665?
No Arm Cortex cores are affected by the FPU state information attack described in CVE-2018-3665.
Q: Are any Arm cores affected by Foreshadow or Foreshadow-NG (L1 Terminal Fault)?
(CVE-2018-3615, CVE-2018-3620, CVE-2018-3646)
No Arm Cortex cores are affected by the speculative data leaks known as Foreshadow and Foreshadow-NG.
Q: Are Arm CPUs affected by the seven Spectre and Meltdown variants identified by academic researchers and announced in November 2018, which are described in this paper: https://arxiv.org/abs/1811.05441?
Arm CPUs affected by Spectre variant 1 and variant 2 are also affected by their derivatives described in the paper, however the existing mitigation techniques, including the use of DSB SY + ISB when deployed as directed by the Cache Speculation Side-channels Whitepaper are effective against these additional vulnerabilities.
We are thankful to the teams at Graz University of Technology, imec-DistriNet, KU Leuven, and the College of William and Mary for their ongoing research and interaction with Arm.
Q: Are Arm CPUs affected by the SplitSpectre variant, identified by academic researchers, and announced in December 2018?
Arm does not speculatively take exception returns, and so SplitSpectre cannot be exhibited across different exception levels on Arm. Therefore current Arm CPUs are not vulnerable to SplitSpectre attacks where the malicious software and victim software are running at separate levels of hardware enforced privilege.
However, where privilege is only enforced by software, SplitSpectre is effectively another presentation of Spectre variant 1 and attacks are possible on current Arm CPUs. In these cases Arm continues to recommend techniques described previously in the Cache Speculation Side-channels Whitepaper, such as site isolation, and/or use of DSB;ISB sequences when returning to a less privileged context.