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Date: 22 Dec 2016 16:11:40 -0500
From: "George Spelvin" <linux@...encehorizons.net>
To: linux@...encehorizons.net, luto@...nel.org
Cc: ak@...ux.intel.com, davem@...emloft.net, David.Laight@...lab.com,
djb@...yp.to, ebiggers3@...il.com, eric.dumazet@...il.com,
hannes@...essinduktion.org, Jason@...c4.com,
jeanphilippe.aumasson@...il.com, kernel-hardening@...ts.openwall.com,
linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
netdev@...r.kernel.org, tom@...bertland.com, torvalds@...ux-foundation.org,
tytso@....edu, vegard.nossum@...il.com
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),
get_random_bits(mmap_rnd_bits),
u32 imm_rnd = get_random_bits(32)
unsigned get_random_mod(unsigned modulus)
E.g. get_random_mod(hole) & ~(alignment - 1);
get_random_mod(port_scan_backoff)
(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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