Commit 95b709b6 authored by Theodore Ts'o's avatar Theodore Ts'o
Browse files

random: drop trickle mode 

The add_timer_randomness() used to drop into trickle mode when entropy
pool was estimated to be 87.5% full.  This was important when
add_timer_randomness() was used to sample interrupts.  It's not used
for this any more --- add_interrupt_randomness() now uses fast_mix()
instead.  By elimitating trickle mode, it allows us to fully utilize
entropy provided by add_input_randomness() and add_disk_randomness()
even when the input pool is above the old trickle threshold of 87.5%.

This helps to answer the criticism in [1] in their hypothetical
scenario where our entropy estimator was inaccurate, even though the
measurements in [2] seem to indicate that our entropy estimator given
real-life entropy collection is actually pretty good, albeit on the
conservative side (which was as it was designed).

[1] http://eprint.iacr.org/2013/338.pdf
[2] http://eprint.iacr.org/2012/251.pdf



Signed-off-by: default avatar"Theodore Ts'o" <tytso@mit.edu>
parent 6e9fa2c8

drivers/char/random.c

+2 −17
Original line number Diff line number Diff line
@@ -282,10 +282,8 @@ 
#define LONGS(x) (((x) + sizeof(unsigned long) - 1)/sizeof(unsigned long))



/*

 * To allow fractional bits to be tracked, the following fields contain

 * this many fractional bits:

 *

 * entropy_count, trickle_thresh

 * To allow fractional bits to be tracked, the entropy_count field is

 * denominated in units of 1/8th bits.

 *

 * 2*(ENTROPY_SHIFT + log2(poolbits)) must <= 31, or the multiply in

 * credit_entropy_bits() needs to be 64 bits wide.
@@ -313,14 +311,6 @@ static int random_write_wakeup_thresh = 128; 
 */

static int random_min_urandom_seed = 60;



/*

 * When the input pool goes over trickle_thresh, start dropping most

 * samples to avoid wasting CPU time and reduce lock contention.

 */

static const int trickle_thresh = (INPUT_POOL_WORDS * 28) << ENTROPY_SHIFT;



static DEFINE_PER_CPU(int, trickle_count);



/*

 * Originally, we used a primitive polynomial of degree .poolwords

 * over GF(2).  The taps for various sizes are defined below.  They
@@ -755,10 +745,6 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num) 
	long delta, delta2, delta3;



	preempt_disable();

	/* if over the trickle threshold, use only 1 in 4096 samples */

	if (ENTROPY_BITS(&input_pool) > trickle_thresh &&

	    ((__this_cpu_inc_return(trickle_count) - 1) & 0xfff))

		goto out;



	sample.jiffies = jiffies;

	sample.cycles = random_get_entropy();
@@ -800,7 +786,6 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num) 
		credit_entropy_bits(&input_pool,

				    min_t(int, fls(delta>>1), 11));

	}

out:

	preempt_enable();

}

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