random32: add periodic reseeding

random32: add periodic reseeding

The current Tausworthe PRNG is never reseeded with truly random data after
the first attempt in late_initcall. As this PRNG is used for some critical
random data as e.g. UDP port randomization we should try better and reseed
the PRNG once in a while with truly random data from get_random_bytes().

When we reseed with prandom_seed we now make also sure to throw the first
output away. This suffices the reseeding procedure.

The delay calculation is based on a proposal from Eric Dumazet.

Joint work with Daniel Borkmann.

Cc: Eric Dumazet <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Signed-off-by: Hannes Frederic Sowa <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Signed-off-by: David S. Miller <[email protected]>