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Subject[PATCH] (4/7) trust_pct
This allows adding a bit of entropy for a configurable percentage of
untrusted timer samples, controlled by a new sysctl. This defaults to
0 for safety, but can be used on headless machines without a hardware
RNG to continue to use /dev/random with some confidence.

RML's patch to turn on SA_SAMPLE_RANDOM for most NICs is safe after
this patch.

diff -ur a/drivers/char/random.c b/drivers/char/random.c
--- a/drivers/char/random.c 2002-08-23 12:43:24.000000000 -0500
+++ b/drivers/char/random.c 2002-08-23 12:43:27.000000000 -0500
@@ -694,6 +694,7 @@
static struct timer_rand_state extract_timer_state;
static struct timer_rand_state *irq_timer_state[NR_IRQS];
static struct timer_rand_state *blkdev_timer_state[MAX_BLKDEV];
+static int trust_break=50, trust_pct=0, trust_min=0, trust_max=100;

/*
* This function adds entropy to the entropy "pool" by using timing
@@ -761,6 +762,17 @@

entropy = int_log2_16bits(delta);
}
+ else if(trust_pct)
+ {
+ /* Count an untrusted bit as entropy trust_pct% of the time */
+ trust_break+=trust_pct;
+ if(trust_break >= 100)
+ {
+ entropy=1;
+ trust_break-=100;
+ }
+ }
+
batch_entropy_store(num^time, entropy);
}

@@ -1849,6 +1861,10 @@
{RANDOM_UUID, "uuid",
NULL, 16, 0444, NULL,
&proc_do_uuid, &uuid_strategy},
+ {RANDOM_TRUST_PCT, "trust_pct",
+ &trust_pct, sizeof(int), 0644, NULL,
+ &proc_dointvec_minmax, &sysctl_intvec, 0,
+ &trust_min, &trust_max},
{0}
};

diff -ur a/include/linux/sysctl.h b/include/linux/sysctl.h
--- a/include/linux/sysctl.h 2002-08-17 00:30:00.000000000 -0500
+++ b/include/linux/sysctl.h 2002-08-23 12:43:27.000000000 -0500
@@ -182,7 +182,8 @@
RANDOM_READ_THRESH=3,
RANDOM_WRITE_THRESH=4,
RANDOM_BOOT_ID=5,
- RANDOM_UUID=6
+ RANDOM_UUID=6,
+ RANDOM_TRUST_PCT=7
};

/* /proc/sys/bus/isa */

--
"Love the dolphins," she advised him. "Write by W.A.S.T.E.."
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