Patch "random: use chacha20 for get_random_int/long" has been added to the 4.10-stable tree

PATCH [email protected] 1 participant 1 message Apr 10, 2017

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Stable branch Random number driver

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2017-04-10 15:09 UTC #0 lore raw

This is a note to let you know that I've just added the patch titled

random: use chacha20 for get_random_int/long

to the 4.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
random-use-chacha20-for-get_random_int-long.patch
and it can be found in the queue-4.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

Quoted 1 line

From f5b98461cb8167ba362ad9f74c41d126b7becea7 Mon Sep 17 00:00:00 2001

From: "Jason A. Donenfeld" <[email protected]>
Date: Fri, 6 Jan 2017 19:32:01 +0100
Subject: random: use chacha20 for get_random_int/long

From: Jason A. Donenfeld <[email protected]>

commit f5b98461cb8167ba362ad9f74c41d126b7becea7 upstream.

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 <[email protected]>

Cc Theodore Ts'o <[email protected]>

Cc Hannes Frederic Sowa <[email protected]>

Cc Andy Lutomirski <[email protected]>

Signed-off-by Theodore Ts'o <[email protected]>

Signed-off-by Greg Kroah-Hartman <[email protected]>

---
 drivers/char/random.c  |   84 +++++++++++++++++++++++++------------------------
 include/linux/random.h |    1 
 init/main.c            |    1 
 3 files changed, 43 insertions(+), 43 deletions(-)

drivers/char/random.c +43 ~~-41~~

@@ -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

include/linux/random.h +0 -1

@@ -37,7 +37,6 @@ extern void get_random_bytes(void *buf,
 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;

init/main.c +0 -1

@@ -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)

Patches currently in stable-queue which might be from [email protected] are

queue-4.10/random-use-chacha20-for-get_random_int-long.patch