Apple Private Cloud Compute Threat Modeling

Confidential computing in PCC involves implementing robust security measures to protect user data and maintain privacy. Threat modeling is an essential aspect of this, aiming to identify and mitigate potential attack surfaces. The following sections detail the key areas of concern and provide examples of specific attack techniques that could target these vulnerabilities.

Potential Attack Surfaces

1. Network Communications

• Load Balancer and Privacy Gateways: Load balancers and privacy gateways handle incoming requests and distribute them across the PCC infrastructure. Although they operate outside the PCC trust boundary, Apple did not specify how it is actually handled, especially when the load to the PCC server is high and the load balance is needed in many situations. If this happens, attackers could log or mishandle requests, leading to potential data leakage or unauthorized access

2. Hardware Compromise

• Physical Access to PCC Nodes: Physical security is critical as attackers with physical access to PCC nodes can compromise the hardware. Techniques such as cold boot attacks, where an attacker extracts data from RAM after rebooting the system, can be used to gain access to sensitive information. Apple’s key management and data encryption technique along with TEE may defend against this kind of attack. But some side channel attacks may be envisioned to bypass these defense measure if hacker are able to have direct access to the PCC nodes.
• Supply Chain Attacks: The hardware manufacturing and deployment process is another potential attack vector. Attackers could introduce vulnerabilities during the manufacturing process or compromise hardware during shipping and deployment. For example, malicious firmware could be installed, allowing the attacker to control or monitor the hardware remotely once deployed in data centers.

3. Software Vulnerabilities

• Operating System and Software Stack: Software vulnerabilities, including bugs and security flaws in the operating system or software stack, can be exploited by attackers to gain unauthorized access. Techniques such as ancient buffer overflow attacks, where the attacker exploits a buffer overflow vulnerability to execute arbitrary code, are still possible. Regular patching and code audits are essential to minimize these risks.
• Inference Control and Dispatch Layers: These layers handle user requests and manage the execution of operations. Attackers could target these components with code injection attacks, inserting malicious code into user requests to exploit vulnerabilities and gain control over the system. Apple stated that only attested code can run inside PCC, I am still not convinced that it is 100% bullet proofing. Some previous attacks on TEE exploited architectural weaknesses, implementation flaws, side-channels, and vulnerabilities in new CPU features. Some notable attacks include controlled-channel attacks targeting unprotected communication channels between the TEE and rich execution environment, side-channel attacks leveraging execution side-effects like timing and cache, fault attacks inducing faults during TEE operation, and memory safety violations exploiting bugs like buffer overflows in TEE firmware or trusted applications. For example, specifically targeting Intel’s SGX TEE, researchers have demonstrated attacks like the ROPFS attack using Return Oriented Programming to bypass SGX memory protections, the Foreshadow attack exploiting speculative execution to read SGX enclave memory from the L1 cache, and the Plundervolt attack using fault injection by undervolting the CPU to corrupt SGX enclave data. Additionally, implementation flaws have been discovered, such as the Qualcomm TEE vulnerability allowing any app to map host memory into the TEE. Ongoing research aims to identify and mitigate these attack vectors to enhance the security of TEEs.

4. Administrative Interfaces

• Privileged Access Interfaces: Administrative interfaces used for managing PCC infrastructure are high-value targets for attackers. Privilege escalation attacks, where an attacker exploits a vulnerability to gain higher-level access, can compromise the entire system. Apple has disabled remote SSH and console access to PCC environment for security reasons. Nevertheless, Apple also mentioned that there are some limited logging and admin capabilities. These limited logins and admin capability although do not involve user data, can be exploited if there is any bug or misconfigurations.

5. Request Routing and Target Diffusion

• Request Metadata Handling: While Apple’s documentation state whether routing app metadata de ont contain user information, it is a reasonable privacy concern that aggregated metadata and routing data could potentially reveal patterns of user behavior and movements over time, even if individual user identities are not directly associated with the data.

Frequent routing requests from the same area could indicate a user’s home location. Regular routes between two points could reveal a user’s commute. Routing data combined with metadata like time of day could expose a user’s daily routines and habits. Even aggregated metadata can enable re-identification of users when combined with other data sources.