Messages in this thread | ![/](/images/icornerl.gif) | | Date | 22 Dec 2016 16:11:40 -0500 | From | "George Spelvin" <> | Subject | Re: George's crazy full state idea (Re: HalfSipHash Acceptable Usage) |
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> 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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