By Adelin Travers

Earlier this week, Apple announced Private Cloud Compute (or PCC for short). Without deep context on the state of the art of Artificial Intelligence (AI) and Machine Learning (ML) security, some sensible design choices may seem surprising. Conversely, some of the risks linked to this design are hidden in the fine print. In this blog post, we’ll review Apple’s announcement, both good and bad, focusing on the context of AI/ML security. We recommend Matthew Green’s excellent thread on X for a more general security context on this announcement:

https://x.com/matthew_d_green/status/1800291897245835616

Disclaimer: This breakdown is based solely on Apple’s blog post and thus subject to potential misinterpretations of wording. We do not have access to the code yet, but we look forward to Apple’s public PCC Virtual Environment release to examine this further!

Review summary

This design is excellent on the conventional non-ML security side. Apple seems to be doing everything possible to make PCC a secure, privacy-oriented solution. However, the amount of review that security researchers can do will depend on what code is released, and Apple is notoriously secretive.

On the AI/ML side, the key challenges identified are on point. These challenges result from Apple’s desire to provide additional processing power for compute-heavy ML workloads today, which incidentally requires moving away from on-device data processing to the cloud. Homomorphic Encryption (HE) is a big hope in the confidential ML field but doesn’t currently scale. Thus, Apple’s choice to process data in its cloud at scale requires decryption. Moreover, the PCC guarantees vary depending on whether Apple will use a PCC environment for model training or inference. Lastly, because Apple is introducing its own custom AI/ML hardware, implementation flaws that lead to information leakage will likely occur in PCC when these flaws have already been patched in leading AI/ML vendor devices.

Running commentary

We’ll follow the release post’s text in order, section-by-section, as if we were reading and commenting, halting on specific passages.

Introduction

When I first read this post, I’ll admit that I misunderstood this passage as Apple starting an announcement that they had achieved end-to-end encryption in Machine Learning. This would have been even bigger news than the actual announcement.

That’s because Apple would need to use Homomorphic Encryption to achieve full end-to-end encryption in an ML context. HE allows computation of a function, typically an ML model, without decrypting the underlying data. HE has been making steady progress and is a future candidate for confidential ML (see for instance this 2018 paper). However, this would have been a major announcement and shift in the ML security landscape because HE is still considered too slow to be deployed at the cloud scale and in complex functions like ML. More on this later on.

Note that Multi-Party Computation (MPC)—which allows multiple agents, for instance the server and the edge device, to compute different parts of a function like an ML model and aggregate the result privately—would be a distributed scheme on both the server and edge device which is different from what is presented here.

The term “requires unencrypted access” is the key to the PCC design challenges. Apple could continue processing data on-device, but this means abiding by mobile hardware limitations. The complex ML workloads Apple wants to offload, like using Large Language Models (LLM), exceed what is practical for battery-powered mobile devices. Apple wants to move the compute to the cloud to provide these extended capabilities, but HE doesn’t currently scale to that level. Thus to provide these new capabilities of service presently, Apple requires access to unencrypted data.

This being said, Apple’s design for PCC is exceptional, and the effort required to develop this solution was extremely high, going beyond most other cloud AI applications to date.

Thus, the security and privacy of ML models in the cloud is an unsolved and active research domain when an auditor only has access to the model.

A good example of these difficulties can be found in Machine Unlearning—a privacy scheme that allows removing data from a model—that was shown to be impossible to formally prove by just querying a model. Unlearning must thus be proven at the algorithm implementation level.

When the underlying entirely custom and proprietary technical stack of Apple’s PCC is factored in, external audits become significantly more complex. Matthew Green notes that it’s unclear what part of the stack and ML code and binaries Apple will release to audit ML algorithm implementations.

This is also definitely true. Members of the ML Assurance team at Trail of Bits have been releasing attacks that modify the ML software stack at runtime since 2021. Our attacks have exploited the widely used pickle VM for traditional RCE backdoors and malicious custom ML graph operators on Microsoft’s ONNXRuntime. Sleepy Pickles, our most recent attack, uses a runtime attack to dynamically swap an ML model’s weights when the model is loaded.

This is also true; the design later introduced by Apple is far better than many other existing designs.

Designing Private Cloud compute

From an ML perspective, this claim depends on the intended use case for PCC, as it cannot hold true in general. This claim may be true if PCC is only used for model inference. The rest of the PCC post only mentions inference which suggests that PCC is not currently used for training.

However, if PCC is used for training, then data will be retained, and stateless computation that leaves no trace is likely impossible. This is because ML models retain data encoded in their weights as part of their training. This is why the research field of Machine Unlearning introduced above exists.

The big question that Apple needs to answer is thus whether it will use PCC for training models in the future. As others have noted, this is an easy slope to slip into.

Non-targetability is a really interesting design idea that hasn’t been applied to ML before. It also mitigates hardware leakage vulnerabilities, which we will see next.

Introducing Private Cloud Compute nodes

As others have noted, using Secure Enclaves and Secure Boot is excellent since it ensures only legitimate code is run. GPUs will likely continue to play a large role in AI acceleration. Apple has been building its own GPUs for some time, with its M series now in the third generation rather than using Nvidia’s, which are more pervasive in ML.

However, enclaves and attestation will provide only limited guarantees to end-users, as Apple effectively owns the attestation keys. Moreover, enclaves and GPUs have had vulnerabilities and side channels that resulted in exploitable leakage in ML. Apple GPUs have not yet been battle-tested in the AI domain as much as Nvidia’s; thus, these accelerators may have security issues that their Nvidia counterparts do not have. For instance, Apple’s custom hardware was and remains affected by the LeftoverLocals vulnerability when Nvidia’s hardware was not. LeftoverLocals is a GPU hardware vulnerability released by Trail of Bits earlier this year. It allows an attacker collocated with a victim on a vulnerable device to listen to the victim’s LLM output. Apple’s M2 processors are still currently impacted at the time of writing.

This being said, the PCC design’s non-targetability property may help mitigate LeftoverLocals for PCC since it prevents an attacker from identifying and achieving collocation to the victim’s device.

This is important as Swift is a compiled language. Swift is thus not prone to the dynamic runtime attacks that affect languages like Python which are more pervasive in ML. Note that Swift would likely only be used for CPU code. The GPU code would likely be written in Apple’s Metal GPU programming framework. More on dynamic runtime attacks and Metal in the next section.

Stateless computation and enforceable guarantees

Apple’s solution is not end-to-end encrypted but rather an enclave-based solution. Thus, it does not represent an advancement in HE for ML but rather a well-thought-out combination of established technologies. This is, again, impressive, but the data is decrypted on Apple’s server.

As presented in the introduction, using compiled Swift and signed code throughout the stack should prevent attacks on ML software stacks at runtime. Indeed, the ONNXRuntime attack defines a backdoored custom ML primitive operator by loading an adversary-built shared library object, while the Sleepy Pickle attack relies on dynamic features of Python.

Just-in-Time (JIT) compiled code has historically been a steady source of remote code execution vulnerabilities. JIT compilers are notoriously difficult to implement and create new executable code by design, making them a highly desirable attack vector. It may surprise most readers, but JIT is widely used in ML stacks to speed up otherwise slow Python code. JAX, an ML framework that is the basis for Apple’s own AXLearn ML framework, is a particularly prolific user of JIT. Apple avoids the security issues of JIT by not using it. Apple’s ML stack is instead built in Swift, a memory safe ahead-of-time compiled language that does not need JIT for runtime performance.

As we’ve said, the GPU code would likely be written in Metal. Metal does not enforce memory safety. Without memory safety, attacks like LeftoverLocals are possible (with limitations on the attacker, like machine collocation).

No privileged runtime access

This is an interesting approach because it shows Apple is willing to trade off infrastructure monitoring capabilities (and thus potentially reduce PCC’s reliability) for additional security and privacy guarantees. To fully understand the benefits and limits of this solution, ML security researchers would need to know what exact information is captured in the structured logs. A complete analysis thus depends on Apple’s willingness or unwillingness to release the schema and pre-determined fields for these logs.

Interestingly, limiting the type of logs could increase ML model risks by preventing ML teams from collecting adequate information to manage these risks. For instance, the choice of collected logs and metrics may be insufficient for the ML teams to detect distribution drift—when input data no longer matches training data and the model performance decreases. If our understanding is correct, most of the collected metrics will be metrics for SRE purposes, meaning that data drift detection would not be possible. If the collected logs include ML information, accidental data leakage is possible but unlikely.

Non-targetability

This is excellent as lower levels of the ML stack, including the physical layer, are sometimes overlooked in ML threat models.

The term “metadata” is important here. Only the metadata can be filtered away in the manner Apple describes. However, there are virtually no ways of filtering out all PII in the body content sent to the LLM. Any PII in the body content will be processed unencrypted by the LLM. If PCC is used for inference only, this risk is mitigated by structured logging. If PCC is also used for training, which Apple has yet to clarify, we recommend not sharing PII with systems like these when it can be avoided.

It might be possible for an attacker to obtain identifying information in the presence of side channel vulnerabilities, for instance, linked to implementation flaws, that leak some information. However, this is unlikely to happen in practice: the cost placed on the adversary to simultaneously exploit both the load balancer and side channels will be prohibitive for non-nation state threat actors.

An adversary with this level of control should be able to spoof the statistical distribution of nodes unless the auditing and statistical analysis are done at the network level.

Verifiable transparency

This is nice to see! Of course, we do not know if these will need to be analyzed through extensive reverse engineering, which will be difficult, if not impossible, for Apple’s custom ML hardware. It is still a commendable rare occurrence for projects of this scale.

PCC: Security wins, ML questions

Apple’s design is excellent from a security standpoint. Improvements on the ML side are always possible. However, it is important to remember that those improvements are tied to some open research questions, like the scalability of homomorphic encryption. Only future vulnerability research will shed light on whether implementation flaws in hardware and software will impact Apple. Lastly, only time will tell if Apple continuously commits to security and privacy by only using PCC for inference rather than training and implementing homomorphic encryption as soon as it is sufficiently scalable.

*** This is a Security Bloggers Network syndicated blog from Trail of Bits Blog authored by Trail of Bits. Read the original post at: https://blog.trailofbits.com/2024/06/14/pcc-bold-step-forward-not-without-flaws/