Let /dev/urandom to initialize from the input pool

[nelhage]

I dunno if this is right but it makes my VM boot with your patch if I have a virtio rng

[alex]

Can you add a comment for this? I'm not quite sure I can follow the logic of this function.

[nelhage]

This function is called before extracting entropy from a pool, in order to move entropy from the "input" pool into the desired pool.

In order to prevent /dev/urandom from starving /dev/random, urandom will only reseed from the input pool once ever 60s (by default, it's tunable). This conditional block implements that ratelimit.

However, if it hasn't been initialized yet (doesn't have at least 128 bits of entropy), we should still greedily grab entropy until it has been, which is what my change does. My rationale is that we won't risk long-term starvation, since we'll only need to suck ~128 bits out, ever, and that ensuring the validity of the randomness in /dev/urandom ASAP is more important than keeping a few extra bits around to replenish /dev/urandom

Which part of that would help you as a comment? :) I don't think that entire graf merits inclusion…

[alex]

Just the highest of high levels. perhaps: "Only attempt to rate limit
transfers of entropy if we're initialized. Until we are initialized it
makes sense to transfer all the entropy we can."

On Tue, Jun 14, 2016 at 6:58 PM, Nelson Elhage [email protected]
wrote:

In drivers/char/random.c
#2 (comment):

@@ -981,7 +981,7 @@ static void xfer_secondary_pool(struct entropy_store *r, size_t nbytes)
r->entropy_count > r->poolinfo->poolfracbits)
return;

• if (r->limit == 0 && random_min_urandom_seed) {
• if (r->limit == 0 && r->initialized && random_min_urandom_seed) {

This function is called before extracting entropy from a pool, in order to
move entropy from the "input" pool into the desired pool.

In order to prevent /dev/urandom from starving /dev/random, urandom will
only reseed from the input pool once ever 60s (by default, it's tunable).
This conditional block implements that ratelimit.

However, if it hasn't been initialized yet (doesn't have at least 128 bits
of entropy), we should still greedily grab entropy until it has been, which
is what my change does. My rationale is that we won't risk long-term
starvation, since we'll only need to suck ~128 bits out, ever, and that
ensuring the validity of the randomness in /dev/urandom ASAP is more
important than keeping a few extra bits around to replenish /dev/urandom

Which part of that would help you as a comment? :) I don't think that
entire graf merits inclusion…

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