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SubjectRe: /dev/random entropy calculations broken?
On Oct 02, 2001  10:37 -0500, Oliver Xymoron wrote:
> On Tue, 2 Oct 2001, Andreas Dilger wrote:
> > Fifth fix: in extract_entropy() the "redundant but just in case" check was
> > wrong, comparing entropy bit count and pool words. This had the effect
> > of losing 31/32 of the random pool on each access. BAD, BAD program!
>
> > + if (r->entropy_count > r->poolinfo.poolwords * 32)
> > + r->entropy_count = r->poolinfo.poolwords * 32;
>
> Damnit, I read that line 30 times yesterday!

So did I. It wasn't till my system was spewing debugging output that I
saw where it was going.

> While we're on words/bytes/bits confusion, add_entropy_words() seems to
> get called with number of bytes rather than words.

Makes it that much more random, doesn't it ;-). OK, here is a new version
of the patch. It clears up the parameters to the functions, and makes sure
that we pass the right values to each.

Cheers, Andreas
===========================================================================
--- linux.orig/drivers/char/random.c Tue Jul 10 17:05:24 2001
+++ linux/drivers/char/random.c Tue Oct 2 14:50:19 2001
@@ -272,8 +272,8 @@
static int random_write_wakeup_thresh = 128;

/*
- * A pool of size POOLWORDS is stirred with a primitive polynomial
- * of degree POOLWORDS over GF(2). The taps for various sizes are
+ * A pool of size .poolwords is stirred with a primitive polynomial
+ * of degree .poolwords over GF(2). The taps for various sizes are
* defined below. They are chosen to be evenly spaced (minimum RMS
* distance from evenly spaced; the numbers in the comments are a
* scaled squared error sum) except for the last tap, which is 1 to
@@ -281,49 +281,47 @@
*/
static struct poolinfo {
int poolwords;
+ int poolbits; /* poolwords * 32 */
int tap1, tap2, tap3, tap4, tap5;
} poolinfo_table[] = {
/* x^2048 + x^1638 + x^1231 + x^819 + x^411 + x + 1 -- 115 */
- { 2048, 1638, 1231, 819, 411, 1 },
+ { 2048, 65536, 1638, 1231, 819, 411, 1 },

/* x^1024 + x^817 + x^615 + x^412 + x^204 + x + 1 -- 290 */
- { 1024, 817, 615, 412, 204, 1 },
-
+ { 1024, 32768, 817, 615, 412, 204, 1 },
#if 0 /* Alternate polynomial */
/* x^1024 + x^819 + x^616 + x^410 + x^207 + x^2 + 1 -- 115 */
- { 1024, 819, 616, 410, 207, 2 },
+ { 1024, 32768, 819, 616, 410, 207, 2 },
#endif
-
- /* x^512 + x^411 + x^308 + x^208 + x^104 + x + 1 -- 225 */
- { 512, 411, 308, 208, 104, 1 },

+ /* x^512 + x^411 + x^308 + x^208 + x^104 + x + 1 -- 225 */
+ { 512, 16384, 411, 308, 208, 104, 1 },
#if 0 /* Alternates */
/* x^512 + x^409 + x^307 + x^206 + x^102 + x^2 + 1 -- 95 */
- { 512, 409, 307, 206, 102, 2 },
+ { 512, 16384, 409, 307, 206, 102, 2 },
/* x^512 + x^409 + x^309 + x^205 + x^103 + x^2 + 1 -- 95 */
- { 512, 409, 309, 205, 103, 2 },
+ { 512, 16384, 409, 309, 205, 103, 2 },
#endif

/* x^256 + x^205 + x^155 + x^101 + x^52 + x + 1 -- 125 */
- { 256, 205, 155, 101, 52, 1 },
-
- /* x^128 + x^103 + x^76 + x^51 +x^25 + x + 1 -- 105 */
- { 128, 103, 76, 51, 25, 1 },
+ { 256, 8192, 205, 155, 101, 52, 1 },

+ /* x^128 + x^103 + x^76 + x^51 +x^25 + x + 1 -- 105 */
+ { 128, 4096, 103, 76, 51, 25, 1 },
#if 0 /* Alternate polynomial */
/* x^128 + x^103 + x^78 + x^51 + x^27 + x^2 + 1 -- 70 */
- { 128, 103, 78, 51, 27, 2 },
+ { 128, 4096, 103, 78, 51, 27, 2 },
#endif

/* x^64 + x^52 + x^39 + x^26 + x^14 + x + 1 -- 15 */
- { 64, 52, 39, 26, 14, 1 },
+ { 64, 2048, 52, 39, 26, 14, 1 },

/* x^32 + x^26 + x^20 + x^14 + x^7 + x + 1 -- 15 */
- { 32, 26, 20, 14, 7, 1 },
+ { 32, 1024, 26, 20, 14, 7, 1 },
+
+ { 0, 0, 0, 0, 0, 0, 0 },
+};

- { 0, 0, 0, 0, 0, 0 },
-};
-
/*
* For the purposes of better mixing, we use the CRC-32 polynomial as
* well to make a twisted Generalized Feedback Shift Reigster
@@ -461,6 +459,12 @@
}
#endif

+#if 0
+#define DEBUG_ENT(fmt, arg...) printk(KERN_DEBUG "random: " fmt, ## arg)
+#else
+#define DEBUG_ENT(fmt, arg...) do {} while (0)
+#endif
+
/**********************************************************************
*
* OS independent entropy store. Here are the functions which handle
@@ -545,18 +549,19 @@
* the entropy is concentrated in the low-order bits.
*/
static void add_entropy_words(struct entropy_store *r, const __u32 *in,
- int num)
+ int nwords)
{
static __u32 const twist_table[8] = {
0, 0x3b6e20c8, 0x76dc4190, 0x4db26158,
0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278 };
unsigned i;
int new_rotate;
+ int wordmask = r->poolinfo.poolwords - 1;
__u32 w;

- while (num--) {
+ while (nwords--) {
w = rotate_left(r->input_rotate, *in);
- i = r->add_ptr = (r->add_ptr - 1) & (r->poolinfo.poolwords-1);
+ i = r->add_ptr = (r->add_ptr - 1) & wordmask;
/*
* Normally, we add 7 bits of rotation to the pool.
* At the beginning of the pool, add an extra 7 bits
@@ -569,11 +574,11 @@
r->input_rotate = new_rotate & 31;

/* XOR in the various taps */
- w ^= r->pool[(i+r->poolinfo.tap1)&(r->poolinfo.poolwords-1)];
- w ^= r->pool[(i+r->poolinfo.tap2)&(r->poolinfo.poolwords-1)];
- w ^= r->pool[(i+r->poolinfo.tap3)&(r->poolinfo.poolwords-1)];
- w ^= r->pool[(i+r->poolinfo.tap4)&(r->poolinfo.poolwords-1)];
- w ^= r->pool[(i+r->poolinfo.tap5)&(r->poolinfo.poolwords-1)];
+ w ^= r->pool[(i + r->poolinfo.tap1) & wordmask];
+ w ^= r->pool[(i + r->poolinfo.tap2) & wordmask];
+ w ^= r->pool[(i + r->poolinfo.tap3) & wordmask];
+ w ^= r->pool[(i + r->poolinfo.tap4) & wordmask];
+ w ^= r->pool[(i + r->poolinfo.tap5) & wordmask];
w ^= r->pool[i];
r->pool[i] = (w >> 3) ^ twist_table[w & 7];
}
@@ -582,16 +587,22 @@
/*
* Credit (or debit) the entropy store with n bits of entropy
*/
-static void credit_entropy_store(struct entropy_store *r, int num)
+static void credit_entropy_store(struct entropy_store *r, int nbits)
{
- int max_entropy = r->poolinfo.poolwords*32;
-
- if (r->entropy_count + num < 0)
+ if (r->entropy_count + nbits < 0) {
+ DEBUG_ENT("negative entropy/overflow (%d+%d)\n",
+ r->entropy_count, nbits);
r->entropy_count = 0;
- else if (r->entropy_count + num > max_entropy)
- r->entropy_count = max_entropy;
- else
- r->entropy_count = r->entropy_count + num;
+ } else if (r->entropy_count + nbits > r->poolinfo.poolbits) {
+ r->entropy_count = r->poolinfo.poolbits;
+ } else {
+ r->entropy_count += nbits;
+ if (nbits)
+ DEBUG_ENT("%s added %d bits, now %d\n",
+ r == sec_random_state ? "secondary" :
+ r == random_state ? "primary" : "unknown",
+ nbits, r->entropy_count);
+ }
}

/**********************************************************************
@@ -627,6 +638,12 @@
return 0;
}

+/*
+ * Changes to the entropy data is put into a queue rather than being added to
+ * the entropy counts directly. This is presumably to avoid doing heavy
+ * hashing calculations during an interrupt in add_timer_randomness().
+ * Instead, the entropy is only added to the pool once per timer tick.
+ */
void batch_entropy_store(u32 a, u32 b, int num)
{
int new;
@@ -643,32 +660,33 @@
queue_task(&batch_tqueue, &tq_timer);
batch_head = new;
} else {
-#if 0
- printk(KERN_NOTICE "random: batch entropy buffer full\n");
-#endif
+ DEBUG_ENT("batch entropy buffer full\n");
}
}

+/*
+ * Flush out the accumulated entropy operations, adding entropy to the passed
+ * store (normally random_state). If that store has enough entropy, alternate
+ * between randomizing the data of the primary and secondary stores.
+ */
static void batch_entropy_process(void *private_)
{
- int num = 0;
- int max_entropy;
struct entropy_store *r = (struct entropy_store *) private_, *p;
-
+
if (!batch_max)
return;

- max_entropy = r->poolinfo.poolwords*32;
+ p = r;
while (batch_head != batch_tail) {
+ if (r->entropy_count >= r->poolinfo.poolbits) {
+ r = (r == sec_random_state) ? random_state :
+ sec_random_state;
+ }
add_entropy_words(r, batch_entropy_pool + 2*batch_tail, 2);
- p = r;
- if (r->entropy_count > max_entropy && (num & 1))
- r = sec_random_state;
credit_entropy_store(r, batch_entropy_credit[batch_tail]);
batch_tail = (batch_tail+1) & (batch_max-1);
- num++;
}
- if (r->entropy_count >= random_read_wakeup_thresh)
+ if (p->entropy_count >= random_read_wakeup_thresh)
wake_up_interruptible(&random_read_wait);
}

@@ -1210,14 +1228,26 @@
{
__u32 tmp[TMP_BUF_SIZE];

- if (r->entropy_count < nbytes*8) {
- extract_entropy(random_state, tmp, sizeof(tmp), 0);
- add_entropy_words(r, tmp, TMP_BUF_SIZE);
- credit_entropy_store(r, TMP_BUF_SIZE*8);
+ if (r->entropy_count < nbytes * 8 &&
+ r->entropy_count < r->poolinfo.poolbits) {
+ int nwords = MIN(r->poolinfo.poolwords - r->entropy_count/32,
+ sizeof(tmp) / 4);
+
+ DEBUG_ENT("xfer %d from primary to %s (have %d need %d)\n",
+ nwords * 32,
+ r == sec_random_state ? "secondary" : "unknown",
+ r->entropy_count, nbytes * 8);
+
+ extract_entropy(random_state, tmp, nwords, 0);
+ add_entropy_words(r, tmp, nwords);
+ credit_entropy_store(r, nwords * 32);
}
if (r->extract_count > 1024) {
+ DEBUG_ENT("reseeding %s with %d from primary\n",
+ r == sec_random_state ? "secondary" : "unknown",
+ sizeof(tmp) * 8);
extract_entropy(random_state, tmp, sizeof(tmp), 0);
- add_entropy_words(r, tmp, TMP_BUF_SIZE);
+ add_entropy_words(r, tmp, sizeof(tmp) / 4);
r->extract_count = 0;
}
}
@@ -1228,9 +1258,12 @@
* bits of entropy are left in the pool, but it does not restrict the
* number of bytes that are actually obtained. If the EXTRACT_ENTROPY_USER
* flag is given, then the buf pointer is assumed to be in user space.
- * If the EXTRACT_ENTROPY_SECONDARY flag is given, then this function will
*
- * Note: extract_entropy() assumes that POOLWORDS is a multiple of 16 words.
+ * If the EXTRACT_ENTROPY_SECONDARY flag is given, then we are actually
+ * extracting entropy from the secondary pool, and can refill from the
+ * primary pool if needed.
+ *
+ * Note: extract_entropy() assumes that .poolwords is a multiple of 16 words.
*/
static ssize_t extract_entropy(struct entropy_store *r, void * buf,
size_t nbytes, int flags)
@@ -1240,14 +1273,19 @@
__u32 x;

add_timer_randomness(&extract_timer_state, nbytes);
-
+
/* Redundant, but just in case... */
- if (r->entropy_count > r->poolinfo.poolwords)
- r->entropy_count = r->poolinfo.poolwords;
+ if (r->entropy_count > r->poolinfo.poolbits)
+ r->entropy_count = r->poolinfo.poolbits;

if (flags & EXTRACT_ENTROPY_SECONDARY)
xfer_secondary_pool(r, nbytes);

+ DEBUG_ENT("%s has %d bits, want %d bits\n",
+ r == sec_random_state ? "secondary" :
+ r == random_state ? "primary" : "unknown",
+ r->entropy_count, nbytes * 8);
+
if (r->entropy_count / 8 >= nbytes)
r->entropy_count -= nbytes*8;
else
@@ -1543,9 +1581,7 @@
c -= bytes;
p += bytes;

- /* Convert bytes to words */
- bytes = (bytes + 3) / sizeof(__u32);
- add_entropy_words(random_state, buf, bytes);
+ add_entropy_words(random_state, buf, (bytes + 3) / 4);
}
if (p == buffer) {
return (ssize_t)ret;
@@ -1589,7 +1625,7 @@
ent_count = random_state->entropy_count;
if (put_user(ent_count, p++))
return -EFAULT;
-
+
if (get_user(size, p))
return -EFAULT;
if (put_user(random_state->poolinfo.poolwords, p++))
@@ -1598,7 +1634,7 @@
return -EINVAL;
if (size > random_state->poolinfo.poolwords)
size = random_state->poolinfo.poolwords;
- if (copy_to_user(p, random_state->pool, size*sizeof(__u32)))
+ if (copy_to_user(p, random_state->pool, size * 4))
return -EFAULT;
return 0;
case RNDADDENTROPY:
@@ -1845,8 +1881,7 @@
{
min_read_thresh = 8;
min_write_thresh = 0;
- max_read_thresh = max_write_thresh =
- random_state->poolinfo.poolwords * 32;
+ max_read_thresh = max_write_thresh = random_state->poolinfo.poolbits;
random_table[1].data = &random_state->entropy_count;
}
#endif /* CONFIG_SYSCTL */
@@ -2238,4 +2273,5 @@
EXPORT_SYMBOL(add_interrupt_randomness);
EXPORT_SYMBOL(add_blkdev_randomness);
EXPORT_SYMBOL(batch_entropy_store);
+EXPORT_SYMBOL(generate_random_uuid);

--
Andreas Dilger \ "If a man ate a pound of pasta and a pound of antipasto,
\ would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/ -- Dogbert
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