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>that ARM could do just about anything to the poor 486 without it ever being the wiser. Any device with DMA has that same issue, though. You could plug in a ha

>that ARM could do just about anything to the poor 486 without it ever being the wiser. Any device with DMA has that same issue, though. You could plug in a hard drive that takes control of the CPU by writing new instructions when certain conditions are met. Even if it doesn't have DMA, it could fulfill a request with crafted data. You can't defend against an adversary in your own machine.

astrange 1y ago
You can limit them with IOMMUs. It's reduced to the power of a hostile process. Well, that's still bad if you're booted off it.
- fluoridation 1y ago
  On i486?
  - toast0 1y ago
    An i486 certainly doesn't have an integrated iommu, and none of the chipsets for 486s had them either (afaik), but that doesn't mean you can't add one if you're building up your system from scratch.
    - jacquesm 1y ago
      Or you could forego DMA completely if you wanted. Almost all DMA capable devices have some kind of non-DMA route to access the data. It may be slower and it would of course still enable an adversary with access to your hardware to replace the device you boot from with one that has compromised data on it. But at that level it is usually game over anyway. I was thinking of just using an SD card, and if you're really paranoid about this (which I am!) you could glue it into the slot or make it physically impossible to replace it without damage to the case.
      - fluoridation 1y ago
        What's the goal here, out of curiosity?
        jacquesm 1y ago
        Good question :) Roughly what Bunny was after, not in laptop format, and running my own, recently revived OS. Effectively a machine of which I know each and every byte.
jacquesm 1y ago
> You can't defend against an adversary in your own machine. Not if you import large chunks of unknown hardware. But if you built the whole thing from scratch you could. And FPGA's with adversarial blocks in them (or a toolchain that would corrupt your own bitstream) are probably possible but I don't see these as realistic attacks against a one-off.

astrange 1y ago

You can limit them with IOMMUs. It's reduced to the power of a hostile process. Well, that's still bad if you're booted off it.

fluoridation 1y ago
On i486?
- toast0 1y ago
  An i486 certainly doesn't have an integrated iommu, and none of the chipsets for 486s had them either (afaik), but that doesn't mean you can't add one if you're building up your system from scratch.
  - jacquesm 1y ago
    Or you could forego DMA completely if you wanted. Almost all DMA capable devices have some kind of non-DMA route to access the data. It may be slower and it would of course still enable an adversary with access to your hardware to replace the device you boot from with one that has compromised data on it. But at that level it is usually game over anyway. I was thinking of just using an SD card, and if you're really paranoid about this (which I am!) you could glue it into the slot or make it physically impossible to replace it without damage to the case.
    - fluoridation 1y ago
      What's the goal here, out of curiosity?
      - jacquesm 1y ago
        Good question :) Roughly what Bunny was after, not in laptop format, and running my own, recently revived OS. Effectively a machine of which I know each and every byte.

jacquesm 1y ago

> You can't defend against an adversary in your own machine. Not if you import large chunks of unknown hardware. But if you built the whole thing from scratch you could. And FPGA's with adversarial blocks in them (or a toolchain that would corrupt your own bitstream) are probably possible but I don't see these as realistic attacks against a one-off.