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RichFelker's profile
Rich Felker
Rich Felker
Rich Felker
@RichFelker

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Rich Felker

@RichFelker

Yeah, I do @musllibc, FOSS & infosec stuff. But now is not the time for a mostly-/only-tech Twitter feed.

musl-libc.org
Joined March 2014

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    Rich Felker‏ @RichFelker Aug 26

    Reminder: all Spectre, except v1 style, should be fully fixable, without trusting vendor ucode updates to do it right, by flushing whole cache hierarchy at every kernelspace entry and exit and turning off HT. Anyone tried this and measured cost?

    9:41 AM - 26 Aug 2018
    • 2 Likes
    • Kirti Rathore le tiañ
    3 replies 0 retweets 2 likes
      1. New conversation
      2. @landley‏ @landley Aug 26
        Replying to @RichFelker

        Presumably you could safely HT threads of the same process?

        1 reply 0 retweets 0 likes
      3. Koral‏ @Koral_001 Aug 26
        Replying to @landley @RichFelker

        I don't think so. Security boundary is not just at ht these days. If your application is sandboxing the execution of something else like browsers do with js this is a problem.

        1 reply 0 retweets 0 likes
      4. Rich Felker‏ @RichFelker Aug 26
        Replying to @Koral_001 @landley

        Right, this is why I said except v1 style.

        1 reply 0 retweets 0 likes
      5. @landley‏ @landley Aug 26
        Replying to @RichFelker @Koral_001

        Presumably process could provide a HT ok flag, or disable it when call to mprotect or namespace or something? Hmmm...

        1 reply 0 retweets 0 likes
      6. Rich Felker‏ @RichFelker Aug 27
        Replying to @landley @Koral_001

        Running tasks sharing same vm space on both HT threads is no loss in protection if you only flush cache at vm context switch, but doesn't allow you to protect kernel memory from the user task.

        1 reply 0 retweets 0 likes
      7. @landley‏ @landley Aug 27
        Replying to @RichFelker @Koral_001

        So you'd need to halt the other thread in the pair when either entered the kernel.

        1 reply 0 retweets 0 likes
      8. Rich Felker‏ @RichFelker Aug 27
        Replying to @landley @Koral_001

        Yeah. You could do it with IPI's but then you're just going to make syscall entry/exit even more expensive...

        1 reply 0 retweets 0 likes
      9. @landley‏ @landley Aug 27
        Replying to @RichFelker @Koral_001

        Except IPI between HT should be trivial?

        1 reply 0 retweets 0 likes
      10. 5 more replies
      1. New conversation
      2. Siddhesh Poyarekar‏ @siddhesh_p Aug 26
        Replying to @RichFelker

        Not sure about flushing at every context switch, but doing so whenever a different process is scheduled on a CPU (or core complex, when a complex shares L2) might make sense because there's probably some amount of eviction happening in such cases anyway.

        1 reply 0 retweets 1 like
      3. Rich Felker‏ @RichFelker Aug 26
        Replying to @siddhesh_p

        Every entry/exit is needed to avoid kernel leaks if you don't trust mitigations on kernel side. Otherwise just at vm context switch is ok.

        1 reply 0 retweets 0 likes
      4. Rich Felker‏ @RichFelker Aug 26
        Replying to @RichFelker @siddhesh_p

        It would be nice to see both tested and compared.

        0 replies 0 retweets 0 likes
      5. End of conversation
      1. New conversation
      2. Kirti Rathore‏ @kirtivr Aug 27
        Replying to @RichFelker

        I wonder if flushing the TLB would be enough since L1 is physically tagged on popular CPUs

        1 reply 0 retweets 0 likes
      3. Rich Felker‏ @RichFelker Aug 27
        Replying to @kirtivr

        No, because you can look for evictions of your own cached data to determine what speculative accesses happened in the victim.

        0 replies 0 retweets 1 like
      4. End of conversation

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