random: don't reset crng_init_cnt on urandom_read()

random: don't reset crng_init_cnt on urandom_read()

At the moment, urandom_read() (used for /dev/urandom) resets crng_init_cnt
to zero when it is called at crng_init<2. This is inconsistent: We do it
for /dev/urandom reads, but not for the equivalent
getrandom(GRND_INSECURE).

(And worse, as Jason pointed out, we're only doing this as long as
maxwarn>0.)

crng_init_cnt is only read in crng_fast_load(); it is relevant at
crng_init==0 for determining when to switch to crng_init==1 (and where in
the RNG state array to write).

As far as I understand:

 - crng_init==0 means "we have nothing, we might just be returning the same
   exact numbers on every boot on every machine, we don't even have
   non-cryptographic randomness; we should shove every bit of entropy we
   can get into the RNG immediately"
 - crng_init==1 means "well we have something, it might not be
   cryptographic, but at least we're not gonna return the same data every
   time or whatever, it's probably good enough for TCP and ASLR and stuff;
   we now have time to build up actual cryptographic entropy in the input
   pool"
 - crng_init==2 means "this is supposed to be cryptographically secure now,
   but we'll keep adding more entropy just to be sure".

The current code means that if someone is pulling data from /dev/urandom
fast enough at crng_init==0, we'll keep resetting crng_init_cnt, and we'll
never make forward progress to crng_init==1. It seems to be intended to
prevent an attacker from bruteforcing the contents of small individual RNG
inputs on the way from crng_init==0 to crng_init==1, but that's misguided;
crng_init==1 isn't supposed to provide proper cryptographic security
anyway, RNG users who care about getting secure RNG output have to wait
until crng_init==2.

This code was inconsistent, and it probably made things worse - just get
rid of it.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>