Efficient Code Certification for Open Firmware

1. Efficient Code Certification for Open Firmware OASIS PI Meeting, Santa Fe NM July 25, 2001 Matt Stillerman, PhD Odyssey Research Associates 33 Thornwood Drive, Suite 500 Ithaca, NY 14850 matt@oracorp.com

2. Collaborators Dexter Kozen, Cornell University Thomas Merritt, CodeGen, Inc.

3. Problem: Malicious Firmware Information systems are vulnerable when booting. Security measures are not started yet. Boot software runs in a very privileged mode – it can “do anything.” This vulnerability would be exploited by inserting malicious code into the boot program (firmware). Odyssey’s Solution: BootSafe will detect all non-TCB firmware programs with the potential to violate the security policy, at boot-time, before they run.

4. Why worry about boot firmware? The boot program: tells the operating system about the hardware configuration. It could lie. (e.g. “Here is an approved cryptographic device.”) initializes all hardware devices. It could operate those devices maliciously, fail to initialize them, or damage them. (e.g. “It’s time to erase all keys.”) loads the operating system. It could hack the OS, and thus is capable of disabling, circumventing, or subverting all trusted host software. (e.g. Substitute a spoof version of the login module for the real one.)

5. Practical, Exploitable Weakness • Within the means of a nation-state or well-funded organization. • Several routes for insertion of malicious code: • Firmware patches and upgrades • Device drivers for peripheral devices • Console boot prompt gives full access to an interpreter. • User/Administrator community generally not aware of this danger – systems are wide-open to arbitrary harm.

6. Expected Benefits of BootSafe • Directly detects what users care about: potential violations of the security policy. • Malicious code is identified before it runs. • Code is rechecked before each boot cycle. • All trust resides in end-user systems. Can accept code updates from untrusted suppliers. • End-user gains well-founded trust without source code. • Complements code-signing integrity approaches. • Based on a rigorous formal analysis, thus can achieve high assurance.

7. Scope: Open Firmware • BootSafe will detect malicious code in Open Firmware-based systems. • Open Firmware is a widely used standard “platform” for boot firmware (IEEE-1275). • Standardizes the execution environment, the device API, the operating system API, and the user interface. • Popular because it enables reusability and portability of boot code. • Used by Sun Microsystems, Apple, and many embedded system vendors. • Used in DoD and US Government information systems.

8. Forth-based Solution Open Firmware Boot Host Software Developer Verifier Other Software Fcode Interpreter Forth Source program certificate Fcode programs Certifying tokenizer BootSafe

9. Java-based Solution Java Program javac ROM Storage certificate J2F certifying compiler JVM Bytecode Fcode Firmware Developer

10. Fcode Loading and Verification Verifier Other Software Fcode Interpreter certificate “Loading” Fcode programs Fcode Boot Program ROM Storage

11. Advantages of Java • Java is strongly typed. • JAVA bytecode is strongly typed -- can carry typing down to Fcode type annotations that reflect JAVA typing • Fcode verification mimics JAVA bytecode verification • Open Firmware is naturally object-oriented • The device tree has a natural object-oriented inheritance structure -- can provide templates with general functionality for each device type that can be subclassed • Static/instance structure already present in Open Firmware

12. Advantages of Java • Thus our security policy will be very naturally expressed in terms of Java, as: • Type checking. • Safety of Java namespace. Non-malleable class definitions. • Requirement to implement specific standard interfaces. • Liberal use of final and private attributes. • Restrictions on lexical references

13. First Level Policy: Type Safety • Memory safety • Control flow • Stack safety • Compiling down from a type safe language ensures this. • Enforced in Fcode by the analog of Java bytecode verification.

14. Second Level Policy: Device Encapsulation • Each physical device may only be accessed through its device driver using the published driver interface. • Each device driver interface conforms to the standard. • No additional public interface is defined. • No external access to internal methods or data structures of the device driver. • All calls to driver interface methods are well-formed.

15. Third Level Policies Threat categories they address: • Malicious inter-device access • Resource exhaustion • Incorrect device alias or name • Malformed device tree • Corruption of the operating system • As it is loaded • As it runs • Wrong OS boot device.

16. Preventing Malicious Inter-device Access • Allow only plausible forms of access, by device type. • Enforce additional site-specific restrictions on inter-device access. • Enforcement: • Check explicit calls in device driver code against policy. • Restrict “dynamic” inter-device calls where the target or method is computed in a non-obvious way. • Mediate calls by a run-time check. Verify that the correct enforcement code (boiler-plate) is in place.

17. Status • Phase I SBIR, nearly completed • Accomplishments • Forth-based and Java-based approaches. • Feasibility • Architecture • Draft security policy • J2F program • Potential • Addresses a real vulnerability • Commercially attractive

18. Eager Java Class Initialization Eager class loading and initialization would be preferable to Java’s lazy class loading policy. Problem: What order to initialize classes with cyclic class initializer dependencies. class A { static int a = B.b + 1; ... 2 } class B { static int b = A.a + 1; ... 1 }