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A bare-metal x86 instruction set fuzzer a la Sandsifter
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Baresifter is a 64-bit x86 instruction set fuzzer modeled after Sandsifter. In contrast to Sandsifter, Baresifter is intended to run bare-metal without any operating system.

When loaded, the main fuzzing logic runs in ring0 as a tiny kernel. To safely execute arbitrary instructions, baresifter creates a single executable page in ring3 user space. For every instruction candidate, baresifter writes the instruction bytes to this user space page and attempts to execute it by exiting to user space. It follows the same algorithm as outlined in the original Sandsifter paper to find interesting instructions and guess instruction length.

Building and running

The build is currently tested on Fedora 29. The build requirements are

  • clang++ 5.0 or later,
  • scons, and
  • qemu with KVM support (for easy testing).

To start the build execute scons.

Baresifter can be run in KVM with ./ and will output its results to the console.

To run baresifter bare-metal, use either grub or syslinux and boot baresifter.elf32 as multiboot kernel. It will dump instruction traces on the serial port. The serial port is hardcoded, so you might need to change that: git grep serial_output.

Interpreting results

Baresifter outputs data in a tabular format that looks like:

E <exc> O <capstone-instruction-id> <status> | <instruction hex bytes>

exc is the CPU exception that was triggered, when baresifter tried to execute the instruction. Exception 1 (#DB) indicates that an instruction was successfully executed. The capstone-instruction-id is an integer that represents the instruction that Capstone decoded. A zero in this field means that Capstone could not decode the instruction. status is currently one of BUG (indicating a capstone bug), UNKN (indicating an undocumented instruction), or OK (nothing interesting was found).

A concrete example looks like this:

E 0E O 0008 OK   | 00 14 6D 00 00 00 00
E 01 O 0000 UNKN | 0F 0D 3E
E 01 O 010A BUG  | 66 E9 00 00 00 00

The first line is an instruction that decoded successfully and generated a page fault when executing (exception 0xE). Capstone knows this instruction.

The second line is an undocumented instruction, i.e. the CPU executed it successfully (or at least didn't throw an undefined opcode exception), but Capstone has no idea what it is.

The second line is a Capstone bug. Here both the CPU and Capstone both decoded an instruction, the CPU was able to execute it, but Capstone and the CPU disagree on the length of that instruction.

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