> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
