Messages in this thread Patch in this message | | | Date | Fri, 10 Dec 2004 16:28:15 -0500 | From | Theodore Ts'o <> | Subject | Re: Concurrent access to /dev/urandom |
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On Fri, Dec 10, 2004 at 10:28:04AM -0800, Matt Mackall wrote: > > Fair enough. s/__add/mix/, please. >
Why? Fundamentally, it's all about adding entropy to the pool. I don't have an strong objection to calling it __mix_entropy_words, but if we're going to change it, we should change the non-__ variant for consistency's sake, and I'd much rather do that in a separate patch if we're going to do it all. I don't see the point of the rename, though.
> It won't work as in we'll still get duplicates? I don't see how that > happens. The polynomial for the output pools is dense enough that even > on the very next one word mix, we're getting 96 bits changed in the > output and 32 new ones shifted in. And we're always doing at least > three adds for each pull.
You're right, it should be OK. I was concerned about the case where some percentage of the pool was still all zero's, and what might happen if two adjacent add_entropy_words() mixed in the same data (as could still happen). But one of the add_entropy_words() will be mixed in first, and even if the other add_entropy_words mixes in the exact same data, the data[] returned by first and second add_entropy_words() will be different, and we should be OK.
Still, I'd feel better if we did initialize more data via init_std_data(), and then cranked the LFSR some number of times so that we don't have to worry about analyzing the case where a good portion of the pool might contain consecutive zero values. But yeah, we can save that for another patch, as it's not absolutely essential.
Are we converging here?
- Ted
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>
--- 1.60/drivers/char/random.c 2004-11-18 17:23:14 -05:00 +++ edited/drivers/char/random.c 2004-12-10 16:26:51 -05:00 @@ -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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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