Sequence of Processor Instructions Leads to Unexpected Behavior

Incomplete Base

Structure: Simple

Description

Certain sequences of valid and invalid processor instructions can cause the CPU to lock up or behave unpredictably, often requiring a hard reset to recover.

Extended Description

This issue arises when a processor's instruction set and internal logic aren't rigorously designed and tested. When the CPU encounters specific, problematic combinations of instructions—even if individual instructions are legal—it can enter a locked state or exhibit other erratic behavior instead of safely throwing an exception. This flaw sits at the intersection of hardware design and software execution, where the processor fails to handle edge-case instruction sequences gracefully. From a security perspective, this creates a critical vulnerability. An unprivileged user or program could deliberately craft these harmful instruction sequences to trigger a denial-of-service condition by freezing the CPU. Effective mitigation relies on hardware vendors identifying and correcting these logic flaws through microcode updates or processor revisions, as software workarounds are often limited.

Common Consequences 1

Scope: IntegrityAvailability

Impact: Varies by Context

Potential Mitigations 2

Phase: Testing

Implement a rigorous testing strategy that incorporates randomization to explore instruction sequences that are unlikely to appear in normal workloads in order to identify halt and catch fire instruction sequences.

Phase: Patching and Maintenance

Patch operating system to avoid running Halt and Catch Fire type sequences or to mitigate the damage caused by unexpected behavior. See [REF-1108].

Demonstrative Examples 4

The Pentium F00F bug is a real-world example of how a sequence of instructions can lock a processor. The "cmpxchg8b" instruction compares contents of registers with a memory location. The operand is expected to be a memory location, but in the bad code snippet it is the eax register. Because the specified operand is illegal, an exception is generated, which is the correct behavior and not a security issue in itself. However, when prefixed with the "lock" instruction, the processor deadlocks because locked memory transactions require a read and write pair of transactions to occur before the lock on the memory bus is released. The exception causes a read to occur but there is no corresponding write, as there would have been if a legal operand had been supplied to the cmpxchg8b instruction. [REF-1331]

Code Example:

Bad

x86 Assembly

lock cmpxchg8b eax

The Cyrix Coma bug was capable of trapping a Cyrix 6x86, 6x86L, or 6x86MX processor in an infinite loop. An infinite loop on a processor is not necessarily an issue on its own, as interrupts could stop the loop. However, on select Cyrix processors, the x86 Assembly 'xchg' instruction was designed to prevent interrupts. On these processors, if the loop was such that a new 'xchg' instruction entered the instruction pipeline before the previous one exited, the processor would become deadlocked. [REF-1323]

The Motorola MC6800 microprocessor contained the first documented instance of a Halt and Catch Fire instruction - an instruction that causes the normal function of a processor to stop. If the MC6800 was given the opcode 0x9D or 0xDD, the processor would begin to read all memory very quickly, in sequence, and without executing any other instructions. This will cause the processor to become unresponsive to anything but a hard reset. [REF-1324]

The example code is taken from the commit stage inside the processor core of the HACK@DAC'19 buggy CVA6 SoC [REF-1342]. To ensure the correct execution of atomic instructions, the CPU must guarantee atomicity: no other device overwrites the memory location between the atomic read starts and the atomic write finishes. Another device may overwrite the memory location only before the read operation or after the write operation, but never between them, and finally, the content will still be consistent.

Atomicity is especially critical when the variable to be modified is a mutex, counting semaphore, or similar piece of data that controls access to shared resources. Failure to ensure atomicity may result in two processors accessing a shared resource simultaneously, permanent lock-up, or similar disastrous behavior.

Code Example:

Bad

Verilog

if (csr_exception_i.valid && csr_exception_i.cause[63] && commit_instr_i[0].fu != CSR) begin** ``` exception_o = csr_exception_i; exception_o.tval = commit_instr_i[0].ex.tval; end

The above vulnerable code checks for CSR interrupts and gives them precedence over any other exception. However, the interrupts should not occur when the processor runs a series of atomic instructions. In the above vulnerable code, the required check must be included to ensure the processor is not in the middle of a series of atomic instructions.

Refrain from interrupting if the intention is to commit an atomic instruction that should not be interrupted. This can be done by adding a condition to check whether the current committing instruction is atomic. [REF-1343]

Code Example:

Good

Verilog

if (csr_exception_i.valid && csr_exception_i.cause[63] && !amo_valid_commit_o && commit_instr_i[0].fu != CSR) begin** ``` exception_o = csr_exception_i; exception_o.tval = commit_instr_i[0].ex.tval; end

Observed Examples 2

CVE-2021-26339A bug in AMD CPU's core logic allows a potential DoS by using a specific x86 instruction sequence to hang the processor

CVE-1999-1476A bug in some Intel Pentium processors allow DoS (hang) via an invalid "CMPXCHG8B" instruction, causing a deadlock

References 7

Breaking the x86 ISA

Christopher Domas

https://github.com/xoreaxeaxeax/sandsifter/blob/master/references/domas_breaking_the_x86_isa_wp.pdf

ID: REF-1094

Deep Dive: Retpoline: A Branch Target Injection Mitigation

Intel Corporation

https://www.intel.com/content/www/us/en/developer/topic-technology/software-security-guidance/overview.html(2023-04-07)

ID: REF-1108