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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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    Jann Horn‏ @tehjh 10 May 2019
    • Report Tweet
    • Report NetzDG Violation

    this is amazing. https://lore.kernel.org/lkml/d204035e-6cf7-e7cb-85d2-cebf42d75852@infradead.org/T/ … apparently there is a perf counter for speculative execution of XABORT. and if you call XABORT through an indirect call whose target depends on a CMOV, XABORT execution depends on a value. so the kernel can peek inside speculation that way

    8:40 PM - 10 May 2019
    • 79 Retweets
    • 188 Likes
    • Ivan Kozik Mark Steward Mark Papadakis Eugene Pirogov Ben Cox Flameyyy Thierry Laurion Dmitry Kohmanyuk Kartik Null Cating-Subramanian
    8 replies 79 retweets 188 likes
      1. Jann Horn‏ @tehjh 10 May 2019
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        ... so this patch implements basically an assert() that triggers even if the condition is only violated speculatively

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      2. Brandon Falk‏ @gamozolabs 11 May 2019
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        Replying to @tehjh

        Here's what it looks like (given it's event number 0x5d umask value 0x01). I just use a faulting load and then speculation based on the result of the load. I added some loads to the abortfunc to show the speculative loads occurring as well. The TSX aborts line up close to clears.pic.twitter.com/yTvKEcMHJ8

        2 replies 1 retweet 7 likes
      3. Brandon Falk‏ @gamozolabs 11 May 2019
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        Replying to @gamozolabs @tehjh

        Let me know if you want any speculative behavior graphed. I love looking at what happens in the uarch during speculation!

        1 reply 1 retweet 4 likes
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      1. Dimitris Glynos‏ @dfunc 11 May 2019
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        Replying to @tehjh @jedisct1

        SPEC_WARN_ON() is amazing. Both in practical and educational value. Although I'm not completely sold on the latest efforts on proactive kernel protections, this is clearly on the useful side of things.

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      1. plutoo‏ @qlutoo 10 May 2019
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        Replying to @tehjh @moyix

        genius

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      1. Christian Brauner‏ @brau_ner 10 May 2019
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        Replying to @tehjh

        amaziny and crazy :)

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      1. Paul Vixie‏ @paulvixie 11 May 2019
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        Replying to @tehjh

        time travel!

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      1. Paul Muntean‏ @paulm1024 11 May 2019
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        Replying to @tehjh

        great, more stuff like this is needed, thanks for pointing out! :)

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      1. Amir‏ @disruptnhandlr 11 May 2019
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        Replying to @tehjh

        > by using CMOV which is not predicted

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