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You need a compiler to compile anything, including a compiler, so if your compiler was tampered with then anything you compile can be tampered with as well. Th

You need a compiler to compile anything, including a compiler, so if your compiler was tampered with then anything you compile can be tampered with as well. The only solution is be to download a binary compiler from someone you trust.

stefs 8y ago
yes, but ken thompsons evil compiler theory states, that a trusted compiler is technically impossible (in the paranoid world of theoretical security where all compiler binaries are possibly tainted already) except if you write your own bootstrap compiler chain in machine code. still, all other running software (OS/drivers/the intel management engine) might be able to modify the binary in memory on the fly. the rabbit hole goes down to the hardware level.
- emilfihlman 8y ago
  It doesn't even stop at hardware exactly (mcu/cpu), it only stops when you assemble the mcu/cpu yourself.
  - antsar 8y ago
    What if one of your transistors has a tiny logic board inside?
    - TheArcane 8y ago
      Logic boards can't be smaller than a modern transistor. Logic boards contain transistors.
      - antsar 8y ago
        "modern transistors" are available in differently-sized packages. I assumed someone building their own CPU would be using larger transistors than those normally used inside a production CPU. You sure one of these[0] couldn't contain a tiny processor inside? [0] https://images-na.ssl-images-amazon.com/images/I/41bBdtuYF0L._SX342_.jpg https://images-na.ssl-images-amazon.com/images/I/41bBdtuYF0L...
    - emilfihlman 8y ago
      In theory yeah that's possible. But given that you'd have to cram quite a lot of logic inside to analyse and influence all custom systems when all you are given is a single line that's just a single line in a huge system.
waterhouse 8y ago
I think another solution would be to insert and compile the source of a different compiler, hopefully a rare one like TCC, or even a C interpreter of some sort. The evil compiler can't be programmed to recognize all conceivable C compilers; there must be a small finite list it understands. (This was probably a difficult option back then.) Depending on how advanced the evil compiler is... Plan A would be to use it to compile TCC, then TCC to compile the original compiler. If the evil compiler inserts into every binary the generic command sequence "search for files that look like the original compiler and evil-ify them", then Plan B is to use evil to compile TCC, then use the resulting slightly-evil-TCC to compile TCC, then use the resulting clean TCC to compile the original. If someone has also modified the kernel such that anytime someone uses the syscall "chmod" to mark a file as executable, it'll add evil instructions to that file... Well, perhaps you could recompile the kernel, but it'd be easy for evil instructions to cover that too... finding a new independent kernel source that runs on your machine is probably pretty hard... it'd also kind of interfere with downloading good binaries onto the machine, if the process was "download to a file, then chmod", though if it was "chmod, then write to the file", the evil kernel would have to be clever enough to make it instead append to after the evil instructions; you also could attach a disk and mount it and run executables already on it, and that would be pretty complicated for the evil code to transparently handle (of course, it could just refuse to run such things, but there are lots of things it could refuse to do).

stefs 8y ago

yes, but ken thompsons evil compiler theory states, that a trusted compiler is technically impossible (in the paranoid world of theoretical security where all compiler binaries are possibly tainted already) except if you write your own bootstrap compiler chain in machine code. still, all other running software (OS/drivers/the intel management engine) might be able to modify the binary in memory on the fly. the rabbit hole goes down to the hardware level.

emilfihlman 8y ago
It doesn't even stop at hardware exactly (mcu/cpu), it only stops when you assemble the mcu/cpu yourself.
- antsar 8y ago
  What if one of your transistors has a tiny logic board inside?
  - TheArcane 8y ago
    Logic boards can't be smaller than a modern transistor. Logic boards contain transistors.
    - antsar 8y ago
      "modern transistors" are available in differently-sized packages. I assumed someone building their own CPU would be using larger transistors than those normally used inside a production CPU. You sure one of these[0] couldn't contain a tiny processor inside? [0] https://images-na.ssl-images-amazon.com/images/I/41bBdtuYF0L._SX342_.jpg https://images-na.ssl-images-amazon.com/images/I/41bBdtuYF0L...
  - emilfihlman 8y ago
    In theory yeah that's possible. But given that you'd have to cram quite a lot of logic inside to analyse and influence all custom systems when all you are given is a single line that's just a single line in a huge system.

waterhouse 8y ago

I think another solution would be to insert and compile the source of a different compiler, hopefully a rare one like TCC, or even a C interpreter of some sort. The evil compiler can't be programmed to recognize all conceivable C compilers; there must be a small finite list it understands. (This was probably a difficult option back then.) Depending on how advanced the evil compiler is... Plan A would be to use it to compile TCC, then TCC to compile the original compiler. If the evil compiler inserts into every binary the generic command sequence "search for files that look like the original compiler and evil-ify them", then Plan B is to use evil to compile TCC, then use the resulting slightly-evil-TCC to compile TCC, then use the resulting clean TCC to compile the original. If someone has also modified the kernel such that anytime someone uses the syscall "chmod" to mark a file as executable, it'll add evil instructions to that file... Well, perhaps you could recompile the kernel, but it'd be easy for evil instructions to cover that too... finding a new independent kernel source that runs on your machine is probably pretty hard... it'd also kind of interfere with downloading good binaries onto the machine, if the process was "download to a file, then chmod", though if it was "chmod, then write to the file", the evil kernel would have to be clever enough to make it instead append to after the evil instructions; you also could attach a disk and mount it and run executables already on it, and that would be pretty complicated for the evil code to transparently handle (of course, it could just refuse to run such things, but there are lots of things it could refuse to do).