PartEmu

• USENIX Security 2020
• Paper
• Source code unavailable

Traditional TrustZone OSes and Applications is not easy to fuzz because they cannot be instrumented or modified easily in the original hardware environment. So to emulate them for fuzzing purpose.

Targets

• Emulate TrustZone OSes(TZOS) and Trusted Applications (TAs)
• Abstract and reimplement a subset of hardware/software interfaces
• Fuzz these components
• TZOSes: QSEE, Huawei, OPTEE, Kinibi, TEEGRIS(Samsung) & TAs

Design & Method

• Re-host the TZOS frimware
• Choose the components to reuse/emulate carefully
  • Bootloader
  • Secure Monitor
  • TEE driver and TEE userspace
  • MMIO registers (easy to emulate)

Tools

• TriforceAFL + QEMU
• Manually written Interfaces

Results

Emulations works well. For upgraded TZOSes, only a few efforts are needed for compatibility.

TAs

Challenges

• Identifying the fuzzed target
• Result stability (migrate to hardware, reproducibility)
• Randomness

Class	Vulnerability Types	Crashes
Availability	Null-pointer dereferences	9
	Insufficient shared memory crashes	10
	Other	8
Confidentiality	Read from attacker-controlled pointer to shared memory	8
	Read from attacker-controlled	0
	OOB buffer length to shared memory
Integrity	Write to secure memory using attacker-controlled pointer	11
	Write to secure memory using	2
	attacker-controlled OOB buffer length

Just like the previous paper, the main causes of the crashes can be attributed to:

• Assumptions of Normal-World Call Sequence
• Unvalidated Pointers from Normal World
• Unvalidated Types

TZOSes

• Normal-World Checks
• Assumptions of Normal-World Call Sequence

Review

Strength

• Solid work
• Efforts taken to run TZOS and TA in emulation environment
• Acceptable performance

Weakness

• Low coverage
• Crashes -X-> vulnerabilities