# A Survey of of Side-Channel Attacks and Mitigation for Processor Interconnects
## 4 Interconnect Contention Side Channel
### Mitigation  
**Ring Interconnect. ** Side-channel attacks based on ring bus contention share many characteristics with traditional microarchitectural side-channel attacks, the most fundamental of which is the exploitation of architecturally promised operations to create observable timing differences. Current mitigation strategies for such side channels are predominantly software-based, with the primary approach being adherence to constant-time programming principles. Although there is no precedent for attacking constant-time code in the existing Interconnect-based side channel literature, it is important to note that current constant-time programming techniques are mainly designed to counter side channels related to cache and instruction execution paths. Their effectiveness against interconnect-based microarchitectural side channels remains questionable. Furthermore, the implementation of truly constant-time code requires programmers to have a comprehensive and in-depth understanding of hardware optimizations, significantly increasing the complexity of constant-time code implementation. Reports indicate that Intel CPUs perform hardware memory  
Figure 4: Mesh interconnect contention. disambiguation between private caches and the ring interconnect [70]. This optimization can potentially disrupt constant-time programming by making ring contention a function of cache line content. In pure hardware mitigation, designs based on spatial partitioning and static scheduling arbitration policies can ensure that there is no ring contention between processes from different security domains [71]. However, this approach does not address slice contention issues and still requires the introduction of additional mechanisms for mitigation. Another approach is to limit the range of cores capable of executing certain specific code, such as deploying trusted and untrusted code on separate core clusters while isolating the LLC slices of the two clusters. This less intrusive hardware-software co-design requires careful restriction of cross-cluster traffic generation, such as LLC miss, for ring bus architectures. In terms of software mitigations, since proposed ring contention side channels heavily rely on the attacker (or receiver in a covert channel scenario) constantly missing in the private cache to load from the target LLC slice, purely software-based anomaly detection techniques can be developed. These techniques use hardware performance counters to monitor bursts of load requests to individual LLC slices. However, these techniques are only useful if they maintain a very low false-positive rate [20]. Some research has considered mitigations under multi-level security models, using principles similar to Quality of Service (QoS) optimization in the Internet by assigning higher priority to low-security domain traffic to ensure it is not affected by high-security domain traffic [72]. Nonetheless, this approach offers one-way protection against information leakage and does not meet the security requirements of today's commonly used cloud server trusted execution environments. **Mesh Interconnect. ** Mitigation measures against mesh interconnect contention can be also categorized into software and hardware approaches. Software mitigation strategies, based on reverse engineering of the mesh interconnect, propose placing victim programs on cores less susceptible to such side-channel attacks, such as centrally located cores [21], or isolating the LLC to prevent attackers from generating mesh traffic [22]. Additionally, restricting the location of attacker cores can also be effective. Hardware-level mitigation measures propose segregating traffic flows from different security domains. [PERSON] and [PERSON] studied a domain-aware priority arbitration strategy [72], which prioritizes low-security traffic over high-security traffic on routers. [PERSON] et al. proposed a time-multiplexed scheduling strategy [71], where network links can only carry traffic from predefined security domains at each moment.