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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 @danluu @taviso @chort0

      If by under load you mean not overclocked but under load. And as I'm sure you know, there was a fairly serious CPU correctness bug found by your employer during that timeframe, unsurprisingly against a whitelisted vendor.

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

      What percentage of chips couldn't handle lock cmpxchg8b eax? We're not talking about well formed code, we're talking about adversarial code from the web, designed to trigger edge cases or exercise obscure parts of the spec. Not sure spectre was relevant to NaCl, rowhammer was.

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

      what percentage of Intel chips or banned Centaur chips? I'm not saying that chips never fail, I'm just saying that your whitelisted vendors have the most complex and therefore the most likely to fail chips and this is what we've seen in practice.

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

      NaCl only depends on a small subset of the operation for security. Not sure why you keep talking about Spectre/Meltdown, it wasn't really relevant to NaCl (wasn't a free breakout, and you didn't need NaCl to exploit it, but rowhammer was).

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

      Then please enlighten me, what was the "fairly serious CPU correctness bug found by [my] employer"?

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

      As I'm sure you know, the details of these bugs are usually NDA'd when they're found outside of vendors. There's a public errata for it, but I'm not going to describe the finding of an NDA'd bug on public twitter.

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

      At my current (much smaller than Google) employer, we just found an issue that causes data corruption on a CPU from one of your whitelisted vendors. If you speak with people in the platforms group at any large company, you'll find people run into these with some regularity.

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

      Do you honestly think I don't read errata? Unless it affects the subset of functionality that NaCl relies on, please explain the relevance. CPUs have bugs, CPUs interpret the specs differently, behave differently on edge cases, etc. That's the whole point of the whitelist.

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

      No, and I never said that. The bug we ran into causes effectively arbitrary data corruption. I admit I haven't read all of the NaCl code, but I would be pretty surprised if it's robust against arbitrary data corruption.

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

      I'd be curious to know how reading CPU errata is helpful. A typical errata will say something like "under certain conditions, unexpected behavior may occur" and it will then describe corruption of {cache, registers, IP, flags, etc.} What can any software do to work around that?

      5:11 PM - 8 Dec 2019
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        2. Dan Luu‏ @danluu 8 Dec 2019
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          Replying to @danluu @taviso @chort0

          There are tens to hundreds of errata like this for a modern Intel CPU. When you say that NaCl depends on a limited subset of features, are you saying this subset of features does not include having correct values in registers, memory, flags, etc.?

          1 reply 0 retweets 2 likes
        3. Tavis Ormandy‏Verified account @taviso 8 Dec 2019
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          Replying to @danluu @chort0

          Because NaCl code isn't arbitrary, it has to follow certain rules so that it can be validated. If an errata requires a specific code sequence, and that code sequence is impossible to validate, then please explain how it's relevant.

          1 reply 0 retweets 1 like
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        2. Norman Yarvin‏ @NYarvin 8 Dec 2019
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          Replying to @danluu @taviso @chort0

          In the case of the bug you linked to earlier, the full phrase is "which may occur under complex microarchitectural conditions involving jump instructions that span 64-byte boundaries (cross cache lines)", which does tell you how to avoid it.

          2 replies 0 retweets 1 like
        3. Dan Luu‏ @danluu 8 Dec 2019
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          Replying to @NYarvin @taviso @chort0

          It was an example. If you search for "complex microarchitectural conditions" you can find plenty that don't have a reasonable software workaround (other than a ucode patch), the first document I found when searching for that has multiple examples.

          1 reply 0 retweets 0 likes
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