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  • Hacker News
  • I'm getting 16-bit zeros on my Zen 3 chip (+1:3821, 0:3893, -1:3895), I will wait to get some statistically significant samples for the 32-bit values and update the forum thread. Maybe it was fixed after Zen 2?
    by matja
  • I always wonder how hardware bugs like this happen with the sheer amount of hardware validation that's done. It'd be fascinating to know how it slipped through the cracks, though I know almost nothing about this side of the industry sadly
    by 20k
  • I remember setting up a new git CI build server many years ago, which at the time rather quickly started failing build pipelines in a nodejs css frontend build script, turns out there was something funky with the AMD processor's RDRAND. A motherboard BIOS flash update fixed it.

    https://github.com/sass/libsass/issues/3151

    by 0x0
  • Not sure if this finding is new, but the author from FASM Thread apparently has a Zen 2 (He doesn't directly mention anything besides "Ryzen 7", some other poster mentions it is a 4800HS). There were a number of articles about RDRAND being broken on Zen 2 and earlier generations but fixed via Microcode from about 6 years ago:

    https://www.phoronix.com/news/AMD-Releases-Linux-Zen2-Fix

    https://arstechnica.com/gadgets/2019/10/how-a-months-old-amd...

    No idea what happened after. And that also means that you suddently need information about user systems BIOS/Microcode.

    by zir_blazer
  • https://www.amd.com/en/resources/product-security/bulletin/a...
    by 349ru3h4f03
  • Embarrassing, but probably little practical impact, since these hardware random numbers are typically not used directly and instead seed a CSPRNG.
    by CodesInChaos
  • This is why I use, in security critical contents of my software (where the numbers have to be computationally infeasible to produce), a type of random number generator called an XOF (extendable-output function).

    It takes entropy from multiple different sources, makes it all input to the XOF, then the XOF uses cryptography to output a stream that has as much entropy as the combined entropy of all of its sources of randomness. So if an XOF, for example, takes 100 runs of rdrand16, along with the system time in microseconds and the number of milliseconds between receiving 100 packets over the network, the XOF will output a completely random stream without artifacts like never returning 0x0000, even if rdrand16 never outputs 0x0000.

    by strenholme
  • This is not the first RNG bug on Zen 2, I recall after I first got mine that some application or other would quit immediately at startup because rdrand always returned -1, i.e. all 1s. It was fixed with a microcode update.

    Do we now learn that they fixed "always generate all 1s" with "never generate all 0s"??

    EDIT: I've been unable to reproduce the problem on my CPU, FWIW. It's a Ryzen 5 3600.

    EDIT2: OK, update, I can reproduce it with rdrand16, rdrand32 is fine but rdrand16 can never generate all 0s. So my CPU does have this problem!

    by jstanley

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