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Date: 22 Dec 2016 16:11:40 -0500
From: "George Spelvin" <>
Subject: Re: George's crazy full state idea (Re: HalfSipHash Acceptable Usage)

> I do tend to like Ted's version in which we use batched
> get_random_bytes() output.  If it's fast enough, it's simpler and lets
> us get the full strength of a CSPRNG.

With the ChaCha20 generator, that's fine, although note that this abandons
anti-backtracking entirely.

It also takes locks, something the previous get_random_int code
path avoided.  Do we need to audit the call sites to ensure that's safe?

And there is the issue that the existing callers assume that there's a
fixed cost per word.  A good half of get_random_long calls are followed by
"& ~PAGE_MASK" to extract the low 12 bits.  Or "& ((1ul << mmap_rnd_bits)
- 1)" to extract the low 28.  If we have a buffer we're going to have to
pay to refill, it would be nice to use less than 8 bytes to satisfy those.

But that can be a followup patch.  I'm thinking

unsigned long get_random_bits(unsigned bits)
	E.g. get_random_bits(PAGE_SHIFT),
	u32 imm_rnd = get_random_bits(32)

unsigned get_random_mod(unsigned modulus)
	E.g. get_random_mod(hole) & ~(alignment - 1);
	(Althogh probably drivers/s390/scsi/zfcp_fc.c should be changed
	to prandom.)

with, until the audit is completed:
#define get_random_int() get_random_bits(32)
#define get_random_long() get_random_bits(BITS_PER_LONG)

> It could only mix the output back in every two calls, in which case
> you can backtrack up to one call but you need to do 2^128 work to
> backtrack farther.  But yes, this is getting excessively complicated.

No, if you're willing to accept limited backtrack, this is a perfectly
acceptable solution, and not too complicated.  You could do it phase-less
if you like; store the previous output, then after generating the new
one, mix in both.  Then overwrite the previous output.  (But doing two
rounds of a crypto primtive to avoid one conditional jump is stupid,
so forget that.)

>> Hmm, interesting.  Although, for ASLR, we could use get_random_bytes()
>> directly and be done with it.  It won't be a bottleneck.

Isn't that what you already suggested?

I don't mind fewer primtives; I got a bit fixated on "Replace MD5 with
SipHash".  It's just the locking that I want to check isn't a problem.

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