Agner`s CPU blog

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Proposal for instruction set - now on Github
Author: Agner Date: 2017-09-23 00:43
Enforcing instruction alignment at cache boundaries doesn't help much, because malicious code can jump over the boundaries. You would have to enforce alignment of all jump targets as well, which means a lot of wasteful NOPs in the code.

Another possible solution is to use the same principle as in UTF-8 code: single-byte UTF-8 codes and continuation bytes can be distinguished by the first bit. A similar principle could be implemented in ForwardCom. The first word of a multi-word instruction has the most significant bit = 1. The last word of a multi-word instruction has the most significant bit = 0. A single word instruction has the most significant bit = 0. This will make it impossible to execute stealth code by misaligning an entry. To implement this feature we need an extra bit in the first word of each multi-word instruction to replace the bit we have used in the last word. If the last word contains a 32-bit payload such as a 32-bit address or a floating point number, then all binary tools will need to have the extra complication of inserting the missing bit. This is a clumsy solution, but possible. The first word of multi-word instructions has no unused bits so we will have to sacrifice some other feature if we want to make space for this extra bit.

We have to make a cost benefit analysis to see if this feature is worth the costs. The costs of making a bit to distinguish start words from end words are: fewer bits for instruction features, clumsy and complicated binary tools, and slower emulation. The advantage is easier analysis of potentially malicious code.

I wonder it the kind of analysis you are talking about actually is an efficient weapon against malicious code. It is not easy to jump to an unauthorized address on ForwardCom without detection. All jump tables and function pointers are in read-only memory, and the call stack is separate from the data stack and isolated from everything else. The only way a malicious code could jump into an unrecognized address is by jumping to a calculated address and make this calculation difficult to trace. A scanner for malicious code should look for any jump to a calculated address and do a more thorough analysis only if such a jump occurs. This extra analysis wouldn't be so time consuming after all because disassembling is fast.

I would be more concerned about malicious code calling a system function with a calculated ID. Running in a sandbox, the code would call a benign system function, while under certain conditions or on a certain date, the malicious code would call a different system function with different parameters. An intelligent analysis tool could detect that the code uses a calculated system function ID and calculated parameters, but it may be impossible for even the best analysis tool to detect what the code can do if the calculation of system function ID uses cryptographic techniques. We cannot prevent this kind of attack by code alignment techniques and analysis of binary code. I think the best prevention against threats like these is to make restrictions on the access rights of each executable file. I have proposed the following security principle for ForwardCom: An executable file is not allowed to run unless it has been installed by a standardized installation procedure. This procedure includes the assignment of access right to the executable file, such as whether the file is allowed to access various resources.

 
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