On 22.12.2016 22:11, George Spelvin wrote: >> 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? We have spin_lock_irq* locks on the way. Of course they can hurt in when contended. The situation should be the same as with get_random_bytes, callable from every possible situation in the kernel without fear, so this should also work for get_random_int. A lockdep test should still be done. ;) > 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) Yes, that does look nice indeed. Accounting for bits instead of bytes shouldn't be a huge problem either. Maybe it gets a bit more verbose in case you can't satisfy a request with one batched entropy block and have to consume randomness from two. >> 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.) Can you quickly explain why we lose the backtracking capability? ChaCha as a block cipher gives a "perfect" permutation from the output of either the CRNG or the CPRNG, which actually itself has backtracking protection. Thanks for explaining, Hannes -- 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
