  Existing attestation mechanisms lack scalability and support for
heterogeneous virtual execution environments (VEEs), such as virtual machines
and containers executed inside or outside hardware isolation on different
vendors' hardware in clouds managed by various organizations. To overcome these
limitations, hardware vendors and cloud providers implement proprietary
mechanisms (Intel DCAP, Amazon NitroTPM, Google Titan) to support their
offerings. However, due to their plurality, the attestation becomes cumbersome
because it increases maintenance and integration costs and reduces portability
required in hybrid- and multi-cloud deployments.
  We introduce WAWEL, a framework that enables scalable attestation of
heterogeneous VEEs. WAWEL can be plugged into existing hardware-specific
attestation mechanisms, offering a unified interface. WAWEL supports the widely
adopted trusted platform module (TPM) attestation standard. We implemented a
prototype and integrated it with three different VEEs. It supports runtime
integrity attestation with Linux integrity measurement architecture (IMA) and
legacy applications requiring zero-code changes. The evaluation demonstrated
that the WAWEL prototype achieves very good performance and scalability despite
the indirections between the VEE and hardware root of trust.
