parallel-af DEFCON 2020 Finals

This challenge is an implementation of the Manchester Dataflow Machine.

There's an excellent overview of dataflow machine architecture in this survey from '86.

Machine Information

Opcodes

There are 34 opcodes in the machine, and they're defined in types.h.

I/O

Output is done to stdout through the two (essentially MMIO) destinations OUTPUTD_DESTINATION and OUTPUTS_DESTINATION (see types.h).

Input is more complicated.

There's four instructions that handle output:

1. Open, which will open a file: <fd> = OPN <filename> <flags>
2. Read, which will read one byte from an FD, and -1 on EOF: <byte> = RED <fd>
3. Write, which will write one byte to an FD: <success> = WRT <fd> <byte>
4. Close, which will close the file: <success> = CLS <fd>

An important note is that these functions can only be used by the OS. The code loader will change all of these instructions to a DUP

Trap mode

The processing_unit has a trap_flag that, when set to true, allows an x86-like trap mode single-step operation.

The trap-handeling code lives from instruction 0 to 100.

After another instruction executes (but before the execution_results are sent on the outgoing_token_packet), the following is sent to the following addresses (destination 0 for all):

• inst[0] = hash(src)
• inst[1] = hash(destination)
• inst[2] = return_location

If 1 is sent to the return_location, then the execution result will be sent, if anything else, then it will not.

Assembler

assembler.py has the assembler (modeled after the TASS language presented in the Manchester Dataflow Machine paper).

Compiler

compiler.py is the compiler for a higher level language that I called force (programming in TASS for things like functions and if statements is painful). One thing that's very annoying about the dataflow languages is guarding statements, so that's why there's a lot of foo = foo ^ foo and other things, to ensure that instructions are properly guarded (even if they don't have a direct dataflow dependency). Probably a better approach would have been to create syntax in the language to enforce that, but sometimes you just gotta yolo.

OS

The os.force is the OS of the system. It's the first file that boots up by the machine, so it's the only program that can use privileged instructions (defined in types.c). It also has library functions that help in actually using the machine (similar to libc).

userspace

There's a number of userspace applications written in the force language.

Bugs

Phase 1

In phase 1, there is no ability for the teams to write, so there's only one bug:

• Backdoor in OS

In phase 1, the backdoor was removed.

Phase 2

In phase 2, they can use the shell redirection and cat to create their own files that are loaded.

There's a few bugs:

• Another (different) backdoor in the OS. This can't be directly called, but the teams can directly call the OPN instruction.

• Bypass the check in open_file by directly calling OPN instruction.

• Write to shared memory queues.

• Loader overwrites opcode with external symbol with undocumented/unused flag.

The backdoor can be patched by removing that functionality.

The OPN instruction in open_file cannot be removed, so there's two options: (1) randomize the location of OPN (with cat able to read, this doesn't hide it from a good team) and (2) use Trap Mode (see below) to perform data flow protect to prevent the direct calls.

An attacker can then mess with Trap Mode in two ways:

• Use hash collisions to trick the trap mode code into thinking that the call to OPN is valid.

• The check for trap-mode code is any code in the first 100 instructions of the OS. The check for this incorrectly casts the instruction number to a 16-bit integer, which means that a clever attacker can overflow it.

To trigger the third bug (writing to the shared memory queues) the teams could use lseek and write on the FDs of the shared memory queues that act as the buses between the hardware components, thus executing their own instructions (bypassing trap mode and all). This can be patched by checking in the WRT function the FDs.

To trigger the fourth bug (loader overwrites opcode with symbol), if the highest bit is set in the external references flags, the loader will overwrite the opcode with the current destination of the symbol. In this way, an attacker can create privileged opcodes in userspace.