As ARM is becoming more popular in today’s processor market, the OS kernel on ARM is gradually bloated to meet the market demand for more sophisticated services by absorbing diverse kernel extensions. Since this kernel bloating inevitably increases the attack surface, there has been a continuous effort to decrease the surface by dissociating or isolating untrusted extensions from the kernel. One approach in this effort is using software fault isolation (SFI) that instruments memory and control-transfer instructions to prevent isolated extensions from having unauthorized accesses to memory regions of the core kernel. Being implementable in pure software has been considered the greatest strength of SFI and thus popularly adopted by engineers to isolate kernel extensions, but software versions of SFI mostly suffer from high performance overhead, which can be a critical drawback for performance-sensitive mobile devices that overwhelmingly use ARM CPUs. The purpose of our work, named as Sfitag, is to make SFI for ARM kernel extensions more efficient by leveraging the hardware support from the latest ARM AArch64 architecture, called the ARM8.5-A memory tagging extension (MTE). For efficiency, Sfitag relies on MTE support when it allocates a tag value different from the core kernel for untrusted extensions and enforces extensions to use that value as a tag for pointers and memory objects. Consequently, in Sfitag, accessing the core kernel memory is legitimate only when the tag of a pointer matches the value of the kernel tag, which by means of MTE in effect enables us to safely confine unexpected and buggy behaviors of extensions within the space isolated from the kernel. Through our evaluation, we prove the effectiveness of Sfitag by showing that our MTE-supported SFI efficiently enforces isolation for extensions just with 1% slowdown on the throughput of a network driver and 5.7% on a block device driver.

Clustered federated learning (CFL) is an advanced technique in the field of federated learning (FL) that addresses the issue of catastrophic forgetting caused by non-independent and identically distributed (non-IID) datasets. CFL achieves this by clustering clients based on the similarity of their datasets and training a global model for each cluster. Despite the effectiveness of CFL in mitigating performance degradation resulting from non-IID datasets, the potential risk of privacy leakages in CFL has not been thoroughly studied. Previous work evaluated the risk of privacy leakages in FL using the property inference attack (PIA), which extracts information about unintended properties (i.e., attributes that differ from the target attribute of the global model’s main task). In this paper, we explore the potential risk of unintended property leakage in CFL by subjecting it to both passive and active PIAs. Our empirical analysis shows that the passive PIA performance on CFL is substantially better than that on FL in terms of the attack AUC score. Moreover, we propose an enhanced active PIA method tailored for CFL to improve the attack performance. Our method introduces a scale-up parameter that amplifies the impact of malicious local updates, resulting in better performance than the previous technique. Furthermore, we demonstrate that the vulnerability of CFL can be alleviated by applying differential privacy (DP) mechanisms at the client-level. Unlike previous works, which have shown that applying DP to FL can induce a high utility loss, our empirical results indicate that DP can be used as a defense mechanism in CFL, leading to a better trade-off between privacy and utility.