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authorJason A. Donenfeld <Jason@zx2c4.com>2017-01-06 19:32:01 +0100
committerTheodore Ts'o <tytso@mit.edu>2017-01-27 14:25:06 -0500
commitf5b98461cb8167ba362ad9f74c41d126b7becea7 (patch)
treea03acbf2b6426dc27d642bacedd567611896d709 /drivers
parent5d0e5ea343a0f70351428476bcf8715e0731f26a (diff)
downloadleg-kernel-f5b98461cb8167ba362ad9f74c41d126b7becea7.tar.gz
random: use chacha20 for get_random_int/long
Now that our crng uses chacha20, we can rely on its speedy characteristics for replacing MD5, while simultaneously achieving a higher security guarantee. Before the idea was to use these functions if you wanted random integers that aren't stupidly insecure but aren't necessarily secure either, a vague gray zone, that hopefully was "good enough" for its users. With chacha20, we can strengthen this claim, since either we're using an rdrand-like instruction, or we're using the same crng as /dev/urandom. And it's faster than what was before. We could have chosen to replace this with a SipHash-derived function, which might be slightly faster, but at the cost of having yet another RNG construction in the kernel. By moving to chacha20, we have a single RNG to analyze and verify, and we also already get good performance improvements on all platforms. Implementation-wise, rather than use a generic buffer for both get_random_int/long and memcpy based on the size needs, we use a specific buffer for 32-bit reads and for 64-bit reads. This way, we're guaranteed to always have aligned accesses on all platforms. While slightly more verbose in C, the assembly this generates is a lot simpler than otherwise. Finally, on 32-bit platforms where longs and ints are the same size, we simply alias get_random_int to get_random_long. Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> Suggested-by: Theodore Ts'o <tytso@mit.edu> Cc: Theodore Ts'o <tytso@mit.edu> Cc: Hannes Frederic Sowa <hannes@stressinduktion.org> Cc: Andy Lutomirski <luto@amacapital.net> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Diffstat (limited to 'drivers')
-rw-r--r--drivers/char/random.c84
1 files changed, 43 insertions, 41 deletions
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 9d147d456598..b800e5479b7d 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2016,63 +2016,65 @@ struct ctl_table random_table[] = {
};
#endif /* CONFIG_SYSCTL */
-static u32 random_int_secret[MD5_MESSAGE_BYTES / 4] ____cacheline_aligned;
-
-int random_int_secret_init(void)
-{
- get_random_bytes(random_int_secret, sizeof(random_int_secret));
- return 0;
-}
-
-static DEFINE_PER_CPU(__u32 [MD5_DIGEST_WORDS], get_random_int_hash)
- __aligned(sizeof(unsigned long));
+struct batched_entropy {
+ union {
+ unsigned long entropy_long[CHACHA20_BLOCK_SIZE / sizeof(unsigned long)];
+ unsigned int entropy_int[CHACHA20_BLOCK_SIZE / sizeof(unsigned int)];
+ };
+ unsigned int position;
+};
/*
- * Get a random word for internal kernel use only. Similar to urandom but
- * with the goal of minimal entropy pool depletion. As a result, the random
- * value is not cryptographically secure but for several uses the cost of
- * depleting entropy is too high
+ * Get a random word for internal kernel use only. The quality of the random
+ * number is either as good as RDRAND or as good as /dev/urandom, with the
+ * goal of being quite fast and not depleting entropy.
*/
-unsigned int get_random_int(void)
+static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_long);
+unsigned long get_random_long(void)
{
- __u32 *hash;
- unsigned int ret;
+ unsigned long ret;
+ struct batched_entropy *batch;
- if (arch_get_random_int(&ret))
+ if (arch_get_random_long(&ret))
return ret;
- hash = get_cpu_var(get_random_int_hash);
-
- hash[0] += current->pid + jiffies + random_get_entropy();
- md5_transform(hash, random_int_secret);
- ret = hash[0];
- put_cpu_var(get_random_int_hash);
-
+ batch = &get_cpu_var(batched_entropy_long);
+ if (batch->position % ARRAY_SIZE(batch->entropy_long) == 0) {
+ extract_crng((u8 *)batch->entropy_long);
+ batch->position = 0;
+ }
+ ret = batch->entropy_long[batch->position++];
+ put_cpu_var(batched_entropy_long);
return ret;
}
-EXPORT_SYMBOL(get_random_int);
+EXPORT_SYMBOL(get_random_long);
-/*
- * Same as get_random_int(), but returns unsigned long.
- */
-unsigned long get_random_long(void)
+#if BITS_PER_LONG == 32
+unsigned int get_random_int(void)
{
- __u32 *hash;
- unsigned long ret;
+ return get_random_long();
+}
+#else
+static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_int);
+unsigned int get_random_int(void)
+{
+ unsigned int ret;
+ struct batched_entropy *batch;
- if (arch_get_random_long(&ret))
+ if (arch_get_random_int(&ret))
return ret;
- hash = get_cpu_var(get_random_int_hash);
-
- hash[0] += current->pid + jiffies + random_get_entropy();
- md5_transform(hash, random_int_secret);
- ret = *(unsigned long *)hash;
- put_cpu_var(get_random_int_hash);
-
+ batch = &get_cpu_var(batched_entropy_int);
+ if (batch->position % ARRAY_SIZE(batch->entropy_int) == 0) {
+ extract_crng((u8 *)batch->entropy_int);
+ batch->position = 0;
+ }
+ ret = batch->entropy_int[batch->position++];
+ put_cpu_var(batched_entropy_int);
return ret;
}
-EXPORT_SYMBOL(get_random_long);
+#endif
+EXPORT_SYMBOL(get_random_int);
/**
* randomize_page - Generate a random, page aligned address