  We introduce a new timing side-channel attack on Intel CPU processors. Our
Frontal attack exploits timing differences that arise from how the CPU frontend
fetches and processes instructions while being interrupted. In particular, we
observe that in modern Intel CPUs, some instructions' execution times will
depend on which operations precede and succeed them, and on their virtual
addresses. Unlike previous attacks that could only profile branches if they
contained different code or had known branch targets, the Frontal attack allows
the adversary to distinguish between instruction-wise identical branches. As
the attack requires OS capabilities to set the interrupts, we use it to exploit
SGX enclaves. Our attack further demonstrates that secret-dependent branches
should not be used even alongside defenses to current controlled-channel
attacks. We show that the adversary can use the Frontal attack to extract a
secret from an SGX enclave if that secret was used as a branching condition for
two instruction-wise identical branches. We successfully tested the attack on
all the available Intel CPUs with SGX (until 10th gen) and used it to leak
information from two commonly used cryptographic libraries.
