Shedding Light on the x86 Black Box: Uncovering Secrets and Bugs with Sandsifter
- Stephanie Domas
- Jul 11
- 4 min read
Updated: Jul 16
For too long, we've treated our computer processors as infallible "black boxes," blindly trusting them to execute our code without question. However, the reality is far more complex: modern x86 chips, the very heart of our computing devices, are packed with secret instructions, undocumented features, and hardware bugs—much like the software we rigorously audit. This inherent lack of transparency in hardware has historically been a significant blind spot in cybersecurity.
The x86 architecture, despite its widespread use, has a lesser-known history rife with hidden functionalities and flaws. From well-known hardware bugs like the Pentium f00f and Cyrix comma bugs, to corporate secrets such as Intel's mysterious "Appendix H" and restricted backdoors found in AMD and VIA processors, the instruction set has never been fully transparent. This history fueled my research: an effort to systematically uncover these hidden aspects and validate the processors we implicitly rely upon.
The Challenge of Cracking the x86 Instruction Set
My goal was ambitious: to programmatically and exhaustively search the vast x86 instruction set to find hidden or undocumented instructions and instruction-level flaws, such as the infamous Pentium f00f bug. This task is immensely challenging due to the sheer complexity of x86 instructions. They can range from a single byte to a whopping 15 bytes in length, making simple iterative searches infeasible. Randomly generating instructions offers exceptionally poor coverage of the enormous search space (approximately 1.3 x 10^36 possible instructions). Furthermore, relying solely on official documentation is unreliable, as it often omits undocumented instructions and hardware errors resulting from invalid instruction sequences.
Introducing Sandsifter: Novel Approach to Processor Auditing
To overcome these hurdles, I developed a novel approach centered around two key techniques: Tunneling and Page Fault Analysis.
Tunneling: Navigating the Instruction Space
The "tunneling" algorithm intelligently reduces the search space by observing changes in instruction lengths. Instead of brute-forcing every possible byte combination, we generate candidate instructions, execute them, and observe their lengths. When an instruction's length changes as we increment specific bytes, we know those bytes are "meaningful." This allows us to quickly skip over bytes that don't influence instruction length or behavior, drastically reducing the search space to a manageable ~100 million instructions—something achievable within a day of scanning.
Page Fault Analysis: Decoding Instruction Lengths
A critical component of tunneling is accurately determining an instruction's true length, especially for instructions that might cause faults or operate at privileged levels (like Ring 0 or Ring -2 SMM instructions). We achieve this through "page fault analysis". By strategically placing an instruction's bytes across two consecutive memory pages—one executable, the other non-executable—we can observe the processor's page fault behavior. When the processor attempts to fetch a byte from the non-executable page, it generates a page fault, with the faulting address recorded in the CR2 register. By iteratively moving the instruction back one byte and observing repeated page faults until the instruction no longer causes one, we can precisely determine how many bytes the instruction decoder consumed, thus revealing its length.
Sandsifter: An Open-Source Auditing Tool
This research culminated in sandsifter, an open-source tool designed to systematically scan x86 processors for secrets and bugs.
Surviving the Hunt: How Sandsifter Stays Stable
One major challenge of fuzzing the same device you're running on is preventing crashes. Sandsifter employs several robust survival mechanisms:
Ring 3 Limitation: The fuzzer operates primarily within Ring 3 (user mode), preventing accidental total system failures, even though it can still resolve instructions in deeper rings.
Comprehensive Exception Handling: Sandsifter hooks all possible exceptions an instruction might generate (e.g., SIGSEGV, SIGILL) to ensure the process can clean up after itself.
Register Initialization and Maintenance: General-purpose registers are initialized to zero, and the tunneling approach constrains memory offsets. This prevents arbitrary memory writes from corrupting the injecting process's address space, ensuring that even faulting instructions are safely contained.
What Sandsifter Uncovered: Bugs and Hidden Gems
The "sifter" component of the tool analyzes the execution results from the "injector" (fuzzer), comparing them against a disassembler (like Capstone) as a "ground truth". Anomalies indicate different types of discoveries:
Undocumented Instructions: Sequences that the disassembler doesn't recognize, yet the processor executes them without generating an "invalid opcode" (#UD) exception.
Software Bugs: Cases where the disassembler recognizes an instruction, but the processor reports a different length.
Hardware Bugs: Investigated when unexpected failures occur without consistent heuristics.
Through sandsifter, we've successfully identified numerous hidden instructions across various processor architectures, including:
Intel Core i7-4650U CPU: Undocumented 0f0dxx, 0f18xx, 0f{1a-1f}xx, 0fae{e9-ef, f1-f7, f9-ff} fields, dbe0, dbe1, df{c0-c7}, f1, and specific c0-c1, d0-d1, d2-d3, f6 /1, f7 /1 patterns.
AMD Athlon (Geode NX1500): Undocumented 0f0f{40-7f}{80-ff}{xx} ranges, dbe0, dbe1, df{c0-c7}.
VIA Nano U3500, VIA C7-M: Undocumented 0f0dxx, 0f18xx, 0f{1a-1f}xx fields.
The Importance of Hardware Auditing
My work with sandsifter represents a significant step towards introspecting the "black box" that is the x86 processor. It empowers users to audit their own processors for bugs, backdoors, and hidden functionality, encouraging a shift away from blindly trusting hardware specifications. The discoveries highlight the critical need for continuous, in-depth security research into the foundational components of our computing infrastructure.
Christopher Domas (@xoreaxeaxeax)
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