From: Matt Mackall <mpm@selenic.com>
This patch fixes a problem where /dev/urandom can return duplicate values
when two processors read from it at the same time. It relies on the fact
that we already are taking a lock in add_entropy_words(), and atomically
hashes in some freshly mixed in data into the returned randomness.
Signed-off-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
25-akpm/drivers/char/random.c | 30 ++++++++++++++++++++++++++----
1 files changed, 26 insertions(+), 4 deletions(-)
diff -puN drivers/char/random.c~fix-concurrent-access-to-dev-urandom drivers/char/random.c
--- 25/drivers/char/random.c~fix-concurrent-access-to-dev-urandom Fri Dec 10 15:20:07 2004
+++ 25-akpm/drivers/char/random.c Fri Dec 10 15:20:07 2004
@@ -572,8 +572,8 @@ static void free_entropy_store(struct en
* it's cheap to do so and helps slightly in the expected case where
* the entropy is concentrated in the low-order bits.
*/
-static void add_entropy_words(struct entropy_store *r, const __u32 *in,
- int nwords)
+static void __add_entropy_words(struct entropy_store *r, const __u32 *in,
+ int nwords, __u32 out[16])
{
static __u32 const twist_table[8] = {
0, 0x3b6e20c8, 0x76dc4190, 0x4db26158,
@@ -626,9 +626,23 @@ static void add_entropy_words(struct ent
r->input_rotate = input_rotate;
r->add_ptr = add_ptr;
+ if (out) {
+ for (i = 0; i < 16; i++) {
+ out[i] = r->pool[add_ptr];
+ add_ptr = (add_ptr - 1) & wordmask;
+ }
+ }
+
spin_unlock_irqrestore(&r->lock, flags);
}
+static inline void add_entropy_words(struct entropy_store *r, const __u32 *in,
+ int nwords)
+{
+ __add_entropy_words(r, in, nwords, NULL);
+}
+
+
/*
* Credit (or debit) the entropy store with n bits of entropy
*/
@@ -1342,7 +1356,7 @@ static ssize_t extract_entropy(struct en
size_t nbytes, int flags)
{
ssize_t ret, i;
- __u32 tmp[TMP_BUF_SIZE];
+ __u32 tmp[TMP_BUF_SIZE], data[16];
__u32 x;
unsigned long cpuflags;
@@ -1422,7 +1436,15 @@ static ssize_t extract_entropy(struct en
HASH_TRANSFORM(tmp, r->pool+i);
add_entropy_words(r, &tmp[x%HASH_BUFFER_SIZE], 1);
}
-
+
+ /*
+ * To avoid duplicates, we atomically extract a
+ * portion of the pool while mixing, and hash one
+ * final time.
+ */
+ __add_entropy_words(r, &tmp[x%HASH_BUFFER_SIZE], 1, data);
+ HASH_TRANSFORM(tmp, data);
+
/*
* In case the hash function has some recognizable
* output pattern, we fold it in half.
_