In this paper, we analyze the hardware-based Meltdown mitigations in recent Intel microarchitectures, revealing that illegally accessed data is only zeroed out. Hence, while non-present loads stall the CPU, illegal loads are still executed. We present EchoLoad, a novel technique to distinguish load stalls from transiently executed loads. EchoLoad allows detecting physically-backed addresses from unprivileged applications, breaking KASLR in 40 µs on the newest Meltdown- and MDS-resistant Cascade Lake microarchitecture. As EchoLoad only relies on memory loads, it runs in highly-restricted environments, e.g., SGX or JavaScript, making it the first JavaScript based KASLR break. Based on EchoLoad, we demonstrate the first proof-of-concept Meltdown attack from JavaScript on systems that are still broadly not patched against Meltdown, i.e., 32-bit x86 OSs.

source: L