From: Matt Mackall 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 Signed-off-by: Theodore Ts'o Signed-off-by: Andrew Morton --- 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. _