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random: allow architectures to optionally define random_get_entropy()

Allow architectures which have a disabled get_cycles() function to
provide a random_get_entropy() function which provides a fine-grained,
rapidly changing counter that can be used by the /dev/random driver.

For example, an architecture might have a rapidly changing register
used to control random TLB cache eviction, or DRAM refresh that
doesn't meet the requirements of get_cycles(), but which is good
enough for the needs of the random driver.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org
master
Theodore Ts'o 8 years ago
parent
commit
61875f30da
  1. 8
      drivers/char/random.c
  2. 14
      include/linux/timex.h

8
drivers/char/random.c

@ -643,7 +643,7 @@ struct timer_rand_state {
*/
void add_device_randomness(const void *buf, unsigned int size)
{
unsigned long time = get_cycles() ^ jiffies;
unsigned long time = random_get_entropy() ^ jiffies;
mix_pool_bytes(&input_pool, buf, size, NULL);
mix_pool_bytes(&input_pool, &time, sizeof(time), NULL);
@ -680,7 +680,7 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
goto out;
sample.jiffies = jiffies;
sample.cycles = get_cycles();
sample.cycles = random_get_entropy();
sample.num = num;
mix_pool_bytes(&input_pool, &sample, sizeof(sample), NULL);
@ -747,7 +747,7 @@ void add_interrupt_randomness(int irq, int irq_flags)
struct fast_pool *fast_pool = &__get_cpu_var(irq_randomness);
struct pt_regs *regs = get_irq_regs();
unsigned long now = jiffies;
__u32 input[4], cycles = get_cycles();
__u32 input[4], cycles = random_get_entropy();
input[0] = cycles ^ jiffies;
input[1] = irq;
@ -1485,7 +1485,7 @@ unsigned int get_random_int(void)
hash = get_cpu_var(get_random_int_hash);
hash[0] += current->pid + jiffies + get_cycles();
hash[0] += current->pid + jiffies + random_get_entropy();
md5_transform(hash, random_int_secret);
ret = hash[0];
put_cpu_var(get_random_int_hash);

14
include/linux/timex.h

@ -64,6 +64,20 @@
#include <asm/timex.h>
#ifndef random_get_entropy
/*
* The random_get_entropy() function is used by the /dev/random driver
* in order to extract entropy via the relative unpredictability of
* when an interrupt takes places versus a high speed, fine-grained
* timing source or cycle counter. Since it will be occurred on every
* single interrupt, it must have a very low cost/overhead.
*
* By default we use get_cycles() for this purpose, but individual
* architectures may override this in their asm/timex.h header file.
*/
#define random_get_entropy() get_cycles()
#endif
/*
* SHIFT_PLL is used as a dampening factor to define how much we
* adjust the frequency correction for a given offset in PLL mode.

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