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Message-Id: <20170106183202.5035-1-Jason@zx2c4.com>
Date: Fri, 6 Jan 2017 19:32:01 +0100
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: tytso@....edu, linux-kernel@...r.kernel.org
Cc: "Jason A. Donenfeld" <Jason@...c4.com>,
Hannes Frederic Sowa <hannes@...essinduktion.org>,
Andy Lutomirski <luto@...capital.net>
Subject: [PATCH 1/2] 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@...c4.com>
Suggested-by: Theodore Ts'o <tytso@....edu>
Cc: Theodore Ts'o <tytso@....edu>
Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc: Andy Lutomirski <luto@...capital.net>
---
drivers/char/random.c | 84 ++++++++++++++++++++++++++------------------------
include/linux/random.h | 1 -
init/main.c | 1 -
3 files changed, 43 insertions(+), 43 deletions(-)
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 1ef26403bcc8..433facfd6cb8 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2042,63 +2042,65 @@ 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
diff --git a/include/linux/random.h b/include/linux/random.h
index 7bd2403e4fef..16ab429735a7 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -37,7 +37,6 @@ extern void get_random_bytes(void *buf, int nbytes);
extern int add_random_ready_callback(struct random_ready_callback *rdy);
extern void del_random_ready_callback(struct random_ready_callback *rdy);
extern void get_random_bytes_arch(void *buf, int nbytes);
-extern int random_int_secret_init(void);
#ifndef MODULE
extern const struct file_operations random_fops, urandom_fops;
diff --git a/init/main.c b/init/main.c
index b0c9d6facef9..09beb7fc6e8c 100644
--- a/init/main.c
+++ b/init/main.c
@@ -879,7 +879,6 @@ static void __init do_basic_setup(void)
do_ctors();
usermodehelper_enable();
do_initcalls();
- random_int_secret_init();
}
static void __init do_pre_smp_initcalls(void)
--
2.11.0
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