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danluu's profile
Dan Luu
Dan Luu
Dan Luu
@danluu

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Dan Luu

@danluu

https://patreon.com/danluu 

danluu.com
Joined December 2008

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    1. Dan Luu‏ @danluu 8 Dec 2019
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      Replying to @taviso @chort0

      Are you saying this couldn't happen on whitelisted CPUs? As a hardware engineer, I don't see how you could expect that. You couldn't possibly guarantee this without information the vendor won't give you (detailed timing models w/manufacturing variation).

      2 replies 0 retweets 6 likes
    2. Dan Luu‏ @danluu 8 Dec 2019
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      Replying to @danluu @taviso @chort0

      You can maybe get a vendor to tell you they tested for this, but most likely they'll just lie to you and roll their eyes internally. Even without overclocking, I've seen this kind of thing happen on server chips (which have more rigorous testing done on them than consumer chips)

      1 reply 0 retweets 5 likes
    3. Dan Luu‏ @danluu 8 Dec 2019
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      Replying to @danluu @taviso @chort0

      You can't reasonably test for this yourself in any way. Even if you sample 1M chips uniformly across wafers, fabs, etc., a new stepping, could be a simple 1-layer fix, could completely change time and invalidate all of your testing, you'd have to test another 1M sampled uniformly

      2 replies 0 retweets 2 likes
    4. Tavis Ormandy‏Verified account @taviso 8 Dec 2019
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      Replying to @danluu @chort0

      You misunderstand, we check the whitelisted subset of functionality we rely on, of course we can check it works as we expect?

      1 reply 0 retweets 0 likes
    5. Dan Luu‏ @danluu 8 Dec 2019
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      Replying to @taviso @chort0

      You specifically said you're concerned about things like overclocking causing incorrect behavior on branches How exactly do you check that doesn't happen and branches are executed properly on overlocked CPUs?

      1 reply 0 retweets 1 like
    6. Tavis Ormandy‏Verified account @taviso 8 Dec 2019
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      Replying to @danluu @chort0

      We test it. I think you're asking how can I be certain that it will work on every unit of that model ever produced, but obviously the answer is we can't, but we have higher confidence after checking?

      1 reply 0 retweets 0 likes
    7. Dan Luu‏ @danluu 8 Dec 2019
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      Replying to @taviso @chort0

      Since you're whitelisting based on vendor (?), no. Testing one uarch shouldn't give you any more that another uarch works than that a Transmeta chip works. And within one uarch, how exactly are you testing? I suspect the answer is technically yes, but only in a meaningless way.

      1 reply 0 retweets 1 like
    8. Tavis Ormandy‏Verified account @taviso 8 Dec 2019
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      Replying to @danluu @chort0

      It's a compromise, I would prefer a more specific check. Not sure what alternative you're proposing, just start depending on how obscure parts of the spec works under attack without even testing?

      1 reply 0 retweets 0 likes
    9. Dan Luu‏ @danluu 8 Dec 2019
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      Replying to @taviso @chort0

      IMO, this answer sounds like 1. Something must be done 2. This is something 3. We should do this I think almost any competent CPU engineer would tell you that this actually has no meaningful impact. You're pointing out a real problem, but that doesn't mean this helps at all.

      2 replies 1 retweet 6 likes
    10. Dan Luu‏ @danluu 8 Dec 2019
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      Replying to @danluu @taviso @chort0

      Well, that's half of it. The other half of your response is that if we don't do this test that does nothing, we also shouldn't test anything? But no one is proposing removing CPUID feature checks for features you're actually using.

      1 reply 0 retweets 0 likes
      Dan Luu‏ @danluu 8 Dec 2019
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      Replying to @danluu @taviso @chort0

      What's your mental model of the failure mode you're going to catch? The reason a competent CPU engineer is going to tell you this is useless is that their model of what might fail will expect that you get no new information from running the test even on thousands of chips.

      4:06 PM - 8 Dec 2019
      2 replies 0 retweets 0 likes
        1. New conversation
        2. Tavis Ormandy‏Verified account @taviso 8 Dec 2019
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          Replying to @danluu @chort0

          Interesting, so if I test 1000 chips, and they all fail, your saying that it's still safe to make this security sensitive? 🤔

          1 reply 0 retweets 0 likes
        3. Dan Luu‏ @danluu 8 Dec 2019
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          Replying to @taviso @chort0

          Come on, do you want to have a discussion or score points? Seems like you're more interesting in just scoring points?

          2 replies 0 retweets 1 like
        4. 18 more replies
        1. New conversation
        2. Dan Luu‏ @danluu 8 Dec 2019
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          Replying to @danluu @taviso @chort0

          What specifically are you expecting to find out about overclocked branch behavior when you test 1k overclocked chips from Intel?

          1 reply 0 retweets 0 likes
        3. Tavis Ormandy‏Verified account @taviso 8 Dec 2019
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          Replying to @danluu @chort0

          That if all 1000 fail, that we shouldn't make the security of millions users depend on it?

          0 replies 0 retweets 0 likes
        4. End of conversation

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