Modern CPUs employ Speculative Store Bypass (SSB) to reduce load latency and improve performance. In response to transient attacks such as Spectre, CPU vendors have also introduced mitigations to prevent incorrect speculation from leaking data. In this work, we show that the SSB on Armv9 CPUs introduces a previously unexplored form of non-speculative data leakage. Specifically, we find that the SSB on Armv9 performance cores is governed by an undocumented predictor. Through reverse engineering, we uncover the design of this predictor and show that it lacks isolation across security domains. Furthermore, existing mitigations such as SSBS are insufficient to prevent leaks. Based on this, we present SSBleed, the first non-speculative side-channel attack via SSB on Armv9 CPUs. We validate the practicality of SSBleed through five case studies, including crossprocess RSA signature and key generation attacks on the latest version of MbedTLS and WolfSSL, interrupt detection, and improved data transmission in two transient attacks. Finally, we propose a flush-based mitigation through a kernel patch, which incurs an average performance overhead of 0.46 %.

The vulnerabilities of transient execution have been exploited in many side-channel attacks (SCA). We report Whisper, a novel transient execution timing (TET) side channel, which is based on the execution time difference of transient execution under different conditions. We develop TET version of SCAs including Meltdown, Zombieload, and Spectre-RSB that use Whisper as covert channel to leak information. We further propose TET-KASLR to break the kernel address space layout randomization (KASLR) mechanism under the protection of KPTI and FLARE. These attacks are simple to implement and can bypass the existing mitigation methods because the TET side channel relies on execution time that can be conveniently obtained by architectural level timing analysis. We demonstrate the correctness and effectiveness of these attacks on various x86-64 CPUs. The root cause of Whisper is analyzed with our toolset built on performance monitor unit (PMU) and potential defense against Whisper is also discussed.