  Smart devices produce security-sensitive data and keep them in on-device
storage for persistence. The current storage stack on smart devices, however,
offers weak security guarantees: not only because the stack depends on a
vulnerable commodity OS, but also because smart device deployment is known weak
on security measures.
  To safeguard such data on smart devices, we present a novel storage stack
architecture that i) protects file data in a trusted execution environment
(TEE); ii) outsources file system logic and metadata out of TEE; iii) running a
metadata-only file system replica in the cloud for continuously verifying the
on-device file system behaviors. To realize the architecture, we build
Overwatch, aTrustZone-based storage stack. Overwatch addresses unique
challenges including discerning metadata at fine grains, hiding network delays,
and coping with cloud disconnection. On a suite of three real-world
applications, Overwatch shows moderate security overheads.
