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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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    1. Rich Felker‏ @RichFelker Feb 21

      OK, finally an actually-working Spectre v1 mitigation: mov %cr0,%eax or $0x40000000,%eax mov %eax,%cr0 wbinvd

      1 reply 0 retweets 5 likes
    2. friend void‏ @volatile_void Feb 22
      Replying to @RichFelker

      is that the “disable branch prediction altogether” flag that you said existed and I claimed might revert the processor to static prediction instead?

      1 reply 0 retweets 1 like
    3. Rich Felker‏ @RichFelker Feb 22
      Replying to @volatile_void

      No, it's "disable cache". ;-)

      1 reply 0 retweets 4 likes
    4. Rich Felker‏ @RichFelker Feb 22
      Replying to @RichFelker @volatile_void

      This probably kills 5 9s of performance whereas timer interrupt every 100ns to clobber BTB would only kill 4...

      1 reply 0 retweets 1 like
    5. Rich Felker‏ @RichFelker Feb 22
      Replying to @RichFelker @volatile_void

      It's really bad when your unit of performance loss is "nines"...

      1 reply 0 retweets 2 likes
    6. Jonas Termansen‏ @sortiecat Feb 22
      Replying to @RichFelker @volatile_void

      lol. "This spectre fix only has five nines of high unavailability."

      1 reply 0 retweets 3 likes
    7. Rich Felker‏ @RichFelker Feb 22
      Replying to @sortiecat @volatile_void

      With µc that adds the "flush btb" insn, a timer interrupt that's just "ibpb;iret" might not be in the "nines" level of horribleness, but I doubt ibpb is sufficient to clobber all branch prediction, just indirect.

      1 reply 0 retweets 1 like
    8. Rich Felker‏ @RichFelker Feb 22
      Replying to @RichFelker @sortiecat @volatile_void

      Oh wait. If the cpu has HT, can you just steal a whole HT thread to constantly clobber BTBs?

      2 replies 0 retweets 0 likes
    9. Alexander Tarasikov‏ @astarasikov Feb 22
      Replying to @RichFelker @sortiecat @volatile_void

      ugh. How about just putting *secret* data into the uncached/device memory? And not care about the non-secret?

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker Feb 22
      Replying to @astarasikov @sortiecat @volatile_void

      It doesn't scale. Requires hacks in every application that might have secrets. It's like foregoing MMU: instead of a general-purpose safety mechanism you have to manually ensure every check is right.

      1:28 PM - 22 Feb 2018
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Alexander Tarasikov‏ @astarasikov Feb 22
          Replying to @RichFelker @sortiecat @volatile_void

          If you unmap userspace memory upon kernel entry (aka Meltdown) fix and remove physmap from kernel, you shall be able to mitigate cross-app leakage. Am I right?

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker Feb 22
          Replying to @astarasikov @sortiecat @volatile_void

          No. The idea of Spectre v1 is that you trick a process to leak parts of its own memory via cache side channels. Malicious code in JIT is the obvious/easy way but far from the only one.

          2 replies 0 retweets 0 likes
        4. Rich Felker‏ @RichFelker Feb 22
          Replying to @RichFelker @astarasikov and

          I suspect it's even possible to make a jpeg or png whose decoding time (thus time between network requests) leaks auth tokens out of the browser.

          1 reply 0 retweets 0 likes
        5. Alexander Tarasikov‏ @astarasikov Feb 22
          Replying to @RichFelker @sortiecat @volatile_void

          Ah I see, you are talking about v1 and I was thinking mostly about v2. I'll take a pause to think more about it

          1 reply 0 retweets 0 likes
        6. Rich Felker‏ @RichFelker Feb 22
          Replying to @astarasikov @sortiecat @volatile_void

          Yes. Spectre v2 mitigation is a solved problem except for making it less costly. Spectre v1 has no solution so far except using unaffected hardware or the above "4-5 nines" jokes.

          0 replies 0 retweets 0 likes
        7. End of conversation

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