Deep Dive into FreeBSD's Kernel RNG, John-Mark Gurney and Dean Freeman

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Uploaded on 03/12/2018

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By loeg    2018-05-04

It's worth noting that FreeBSD also had some RNG troubles quite recently (2017)[0] (talk: [1]). Personally, I don't think they were as severe as this. (But I am biased, I do FreeBSD development.) What I'm trying to say is, getting random right is hard. It's easy to produce something random-looking that isn't good enough.

JMG's FreeBSD 2017 RNG talk Tl;dr:

1. Boot-time entropy might leak on either UFS or ZFS; might be re-used in a byzantine ZFS environment. (Not addressed, as far as I know.)

2. Input entropy data (pre-whitening) had only about 0.18 bits of entropy per byte of input. NIST likes to see 4-6 bits per byte. (Not really an issue due to sufficient input volume.) Partially due to:

3. All the fast, high-quality sources of random entropy were accidentally disabled(!). (All the PURE_* ones, including x86 RDRAND.) Fixed in r324394.

4. Other low entropy structures were getting mixed in. Probably harmless, but reduces entropy-per-byte measure that NIST cares about. Fixed in r324372.

The earlier 2015 FreeBSD RNG issues were probably as severe as this Linux issue[2]:

> URGENT: RNG broken for last 4 months

> If you are running a current kernel r273872 or later, please upgrade your kernel to r278907 or later immediately and regenerate keys.

> I discovered an issue where the new framework code was not calling randomdev_init_reader, which means that read_random(9) was not returning good random data. read_random(9) is used by arc4random(9) which is the primary method that arc4random(3) is seeded from.

> This means most/all keys generated may be predictable and must be regenerated. This includes, but not limited to, ssh keys and keys generated by openssl. This is purely a kernel issue, and a simple kernel upgrade w/ the patch is sufficient to fix the issue.

[0]: https://www.funkthat.com/~jmg/vbsdcon_2017_ddfreebsdrng_slid...

[1]: https://www.youtube.com/watch?v=A41cDCE6pTc

[2]: https://lists.freebsd.org/pipermail/freebsd-current/2015-Feb...

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