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Subject[PATCH] [3/4] /dev/random: Use separate entropy store for /dev/urandom
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This patch adds a separate pool for use with /dev/urandom. This
prevents a /dev/urandom read from being able to completely drain the
entropy in the /dev/random pool, and also makes it much more difficult
for an attacker to carry out a state extension attack.

patch-random-3-urandom-pool

--- random.c 2004/08/19 22:49:48 1.3
+++ random.c 2004/08/19 22:50:19 1.4
@@ -401,6 +401,7 @@
*/
static struct entropy_store *random_state; /* The default global store */
static struct entropy_store *sec_random_state; /* secondary store */
+static struct entropy_store *urandom_state; /* For urandom */
static DECLARE_WAIT_QUEUE_HEAD(random_read_wait);
static DECLARE_WAIT_QUEUE_HEAD(random_write_wait);

@@ -1474,14 +1475,21 @@
*/
void get_random_bytes(void *buf, int nbytes)
{
- if (sec_random_state)
- extract_entropy(sec_random_state, (char *) buf, nbytes,
- EXTRACT_ENTROPY_SECONDARY);
- else if (random_state)
- extract_entropy(random_state, (char *) buf, nbytes, 0);
- else
+ struct entropy_store *r = urandom_state;
+ int flags = EXTRACT_ENTROPY_SECONDARY;
+
+ if (!r)
+ r = sec_random_state;
+ if (!r) {
+ r = random_state;
+ flags = 0;
+ }
+ if (!r) {
printk(KERN_NOTICE "get_random_bytes called before "
"random driver initialization\n");
+ return;
+ }
+ extract_entropy(r, (char *) buf, nbytes, flags);
}

EXPORT_SYMBOL(get_random_bytes);
@@ -1532,8 +1540,12 @@
if (create_entropy_store(SECONDARY_POOL_SIZE, "secondary",
&sec_random_state))
goto err;
+ if (create_entropy_store(SECONDARY_POOL_SIZE, "urandom",
+ &urandom_state))
+ goto err;
clear_entropy_store(random_state);
clear_entropy_store(sec_random_state);
+ clear_entropy_store(urandom_state);
init_std_data(random_state);
#ifdef CONFIG_SYSCTL
sysctl_init_random(random_state);
@@ -1667,9 +1679,15 @@
urandom_read(struct file * file, char __user * buf,
size_t nbytes, loff_t *ppos)
{
- return extract_entropy(sec_random_state, buf, nbytes,
- EXTRACT_ENTROPY_USER |
- EXTRACT_ENTROPY_SECONDARY);
+ int flags = EXTRACT_ENTROPY_USER;
+ unsigned long cpuflags;
+
+ spin_lock_irqsave(&random_state->lock, cpuflags);
+ if (random_state->entropy_count > random_state->poolinfo.POOLBITS)
+ flags |= EXTRACT_ENTROPY_SECONDARY;
+ spin_unlock_irqrestore(&random_state->lock, cpuflags);
+
+ return extract_entropy(urandom_state, buf, nbytes, flags);
}

static unsigned int
-
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