  The transient-execution attack Meltdown leaks sensitive information by
transiently accessing inaccessible data during out-of-order execution. Although
Meltdown is fixed in hardware for recent CPU generations, most
currently-deployed CPUs have to rely on software mitigations, such as KPTI.
Still, Meltdown is considered non-exploitable on current systems. In this
paper, we show that adding another layer of indirection to Meltdown transforms
a transient-execution attack into a side-channel attack, leaking metadata
instead of data. We show that despite software mitigations, attackers can still
leak metadata from other security domains by observing the success rate of
Meltdown on non-secret data. With LeakIDT, we present the first cache-line
granular monitoring of kernel addresses. LeakIDT allows an attacker to obtain
cycle-accurate timestamps for attacker-chosen interrupts. We use our attack to
get accurate inter-keystroke timings and fingerprint visited websites. While we
propose a low-overhead software mitigation to prevent the exploitation of
LeakIDT, we emphasize that the side-channel aspect of transient-execution
attacks should not be underestimated.
