  The security of computer systems fundamentally relies on memory isolation,
e.g., kernel address ranges are marked as non-accessible and are protected from
user access. In this paper, we present Meltdown. Meltdown exploits side effects
of out-of-order execution on modern processors to read arbitrary kernel-memory
locations including personal data and passwords. Out-of-order execution is an
indispensable performance feature and present in a wide range of modern
processors. The attack works on different Intel microarchitectures since at
least 2010 and potentially other processors are affected. The root cause of
Meltdown is the hardware. The attack is independent of the operating system,
and it does not rely on any software vulnerabilities. Meltdown breaks all
security assumptions given by address space isolation as well as
paravirtualized environments and, thus, every security mechanism building upon
this foundation. On affected systems, Meltdown enables an adversary to read
memory of other processes or virtual machines in the cloud without any
permissions or privileges, affecting millions of customers and virtually every
user of a personal computer. We show that the KAISER defense mechanism for
KASLR has the important (but inadvertent) side effect of impeding Meltdown. We
stress that KAISER must be deployed immediately to prevent large-scale
exploitation of this severe information leakage.
