TOCTOU, Traps, and Trusted Computing
The security of the standard TCG architecture depends on whether the values in the PCRs match the actual platform configuration. However, this design admits potential for time-of-check time-of-use vulnerabilities: a PCR reflects the state of code and data when it was measured, not when the TPM uses a credential or signs an attestation based on that measurement. We demonstrate how an attacker with sufficient privileges can compromise the integrity of a TPM-protected system by modifying critical loaded code and static data after measurement has taken place. To solve this problem, we explore using the MMU and the TPM in concert to provide a memory event trapping framework, in which trap handlers perform TPM operations to enforce a security policy. Our framework proposal includes modifying the MMU to support selective memory immutability and generate higher granularity memory access traps. To substantiate our ideas, we designed and implemented a software prototype system employing the monitoring capabilities of the Xen virtual machine monitor.
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