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Subject[PATCH 4/11] Entropy fixes - trust_pct for headless
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.

diff -ur a/drivers/char/random.c b/drivers/char/random.c
--- a/drivers/char/random.c 2002-09-07 19:20:27.000000000 -0500
+++ b/drivers/char/random.c 2002-09-07 19:20:30.000000000 -0500
@@ -679,6 +679,7 @@
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
@@ -745,6 +746,17 @@

entropy = fls((delta>>3) & 0xfff);
}
+ 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);
}

@@ -1833,6 +1845,10 @@
{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 2002-09-04 11:28:54.000000000 -0500
+++ b/include/linux/sysctl.h 2002-09-07 19:20:30.000000000 -0500
@@ -183,7 +183,8 @@
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 */

--
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