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tehjh's profile
Jann Horn
Jann Horn
Jann Horn
@tehjh

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Jann Horn

@tehjh

works at Google Project Zero. personal account.

Joined August 2011

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    1. Fiora @ mandatory homestuck!‏ @FioraAeterna 3 Jan 2018
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      if i understand right: the simplest variant works like this. basically all modern CPUs speculate loads far beyond the point where they know it's safe. this is necessary for even half-decent performance in a big pipeline.

      5 replies 44 retweets 190 likes
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    2. Fiora @ mandatory homestuck!‏ @FioraAeterna 3 Jan 2018
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      so if you do this: a = x[4]; b = y[a]; c = z[b]; it may load "c" long before it even knows the load of "a" was safe! this is fine as long as it can roll things back in the case the first load failed. completely normal

      3 replies 47 retweets 208 likes
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    3. Fiora @ mandatory homestuck!‏ @FioraAeterna 3 Jan 2018
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      the catch here is: doing the load puts that data in the cache. so... imagine you're not supposed to be able to know "b" because it's in kernel memory. it'll load b, and then load z[b] to get c. then at some point it will fail and roll back.

      2 replies 17 retweets 147 likes
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    4. Fiora @ mandatory homestuck!‏ @FioraAeterna 3 Jan 2018
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      but this will have a side effect: the chunk of memory containing "c" will end up getting loaded into cache. the rollback *isn't total*. it's like the CPU went ahead to the next page of an assignment before it was supposed to, but didn't completely hide that fact.

      8 replies 24 retweets 197 likes
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    5. Fiora @ mandatory homestuck!‏ @FioraAeterna 3 Jan 2018
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      and given the right circumstances, you can use this to recover "b", one bit at a time.

      4 replies 15 retweets 134 likes
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    6. Fiora @ mandatory homestuck!‏ @FioraAeterna 3 Jan 2018
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      you can now recover arbitrary data from any memory in the system. you win.

      2 replies 22 retweets 187 likes
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    7. Fiora @ mandatory homestuck!‏ @FioraAeterna 3 Jan 2018
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      (there's other variants that use branch prediction, etc to apply the approximate same attack. but the core idea is the same: construct a case where the CPU leaves a visible trail of its speculative execution)

      1 reply 9 retweets 129 likes
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    8. Fiora @ mandatory homestuck!‏ @FioraAeterna 3 Jan 2018
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      someone who read more of the article than me tell me if i'm wrong tho

      10 replies 2 retweets 84 likes
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    9. Jann Horn‏ @tehjh 3 Jan 2018
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      Replying to @FioraAeterna

      yup, that's the basic idea behind all three variants

      1 reply 4 retweets 26 likes
    10. Fiora @ mandatory homestuck!‏ @FioraAeterna 3 Jan 2018
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      Replying to @tehjh

      (fangirl noises) approval by the actual author!!! i absolutely *love* this bug though. out of curiosity, how confident are you in AMD's claimed safety (due to refusal to speculate past privilege boundaries)?

      2 replies 0 retweets 7 likes
      Jann Horn‏ @tehjh 3 Jan 2018
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      Replying to @FioraAeterna

      I don't know enough about how CPUs actually work to be able to say anything useful about that

      4:42 PM - 3 Jan 2018
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      • @mikko Fiora @ mandatory homestuck!
      0 replies 0 retweets 2 likes

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