On Sun, Apr 29, 2018 at 08:11:07PM -0400, Theodore Y. Ts'o wrote:
>
> What your patch does is assume that there is a full bit of uncertainty
> that can be obtained from the information gathered from each
> interrupt. I *might* be willing to assume that to be valid on x86
> systems that have a high resolution cycle counter. But on ARM
> platforms, especially during system bootup when the user isn't typing
> anything and SSD's and flash storage tend to have very predictable
> timing patterns? Not a bet I'd be willing to take. Even with a cycle
> counter, there's a reason why we assumed that we need to mix in timing
> results from 64 interrupts or one second's worth before we would give
> a single bit's worth of entropy credit.
>
> - Ted
What about abusing high-resolution timers to get entropy? Since hrtimers can't
make guarantees down to the nanosecond, there's always a skew between the
requested expiry time and the actual expiry time.
Please see the attached patch and let me know just how horrible it is.
Sultan
>From b0d21c38558c661531d4cb46816fbb36b874a169 Mon Sep 17 00:00:00 2001
From: Sultan Alsawaf <sultan...@gmail.com>
Date: Sun, 29 Apr 2018 21:28:08 -0700
Subject: [PATCH] random: use high-res timers to generate entropy until crng
init is done
---
drivers/char/random.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 47 insertions(+)
diff --git a/drivers/char/random.c b/drivers/char/random.c
index d9e38523b383..af2d60bbcec3 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -286,6 +286,7 @@
#define OUTPUT_POOL_WORDS (1 << (OUTPUT_POOL_SHIFT-5))
#define SEC_XFER_SIZE 512
#define EXTRACT_SIZE 10
+#define ENTROPY_GEN_INTVL_NS (1 * NSEC_PER_MSEC)
#define LONGS(x) (((x) + sizeof(unsigned long) - 1)/sizeof(unsigned long))
@@ -408,6 +409,8 @@ static struct fasync_struct *fasync;
static DEFINE_SPINLOCK(random_ready_list_lock);
static LIST_HEAD(random_ready_list);
+static struct hrtimer entropy_gen_hrtimer;
+
struct crng_state {
__u32 state[16];
unsigned long init_time;
@@ -2287,3 +2290,47 @@ void add_hwgenerator_randomness(const char *buffer,
size_t count,
credit_entropy_bits(poolp, entropy);
}
EXPORT_SYMBOL_GPL(add_hwgenerator_randomness);
+
+/*
+ * Generate entropy on init using high-res timers. Although high-res timers
+ * provide nanosecond precision, they don't actually honor requests to the
+ * nanosecond. The skew between the expected time difference in nanoseconds and
+ * the actual time difference can be used as a way to generate entropy on boot
+ * for machines that lack sufficient boot-time entropy.
+ */
+static enum hrtimer_restart entropy_timer_cb(struct hrtimer *timer)
+{
+ static u64 prev_ns;
+ u64 curr_ns, delta;
+
+ if (crng_ready())
+ return HRTIMER_NORESTART;
+
+ curr_ns = ktime_get_mono_fast_ns();
+ delta = curr_ns - prev_ns;
+
+ add_interrupt_randomness(delta);
+
+ /* Use the hrtimer skew to make the next interval more unpredictable */
+ if (likely(prev_ns))
+ hrtimer_add_expires_ns(timer, delta);
+ else
+ hrtimer_add_expires_ns(timer, ENTROPY_GEN_INTVL_NS);
+
+ prev_ns = curr_ns;
+ return HRTIMER_RESTART;
+}
+
+static int entropy_gen_hrtimer_init(void)
+{
+ if (!IS_ENABLED(CONFIG_HIGH_RES_TIMERS))
+ return 0;
+
+ hrtimer_init(&entropy_gen_hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+
+ entropy_gen_hrtimer.function = entropy_timer_cb;
+ hrtimer_start(&entropy_gen_hrtimer, ns_to_ktime(ENTROPY_GEN_INTVL_NS),
+ HRTIMER_MODE_REL);
+ return 0;
+}
+core_initcall(entropy_gen_hrtimer_init);
--
2.14.1
