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[PATCH 1/3] siphash: add cryptographically secure hashtable function

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  [PATCH 1/3] siphash: add cryptographically secure hashtable function "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-14 01:30 +0100
    [PATCH 4/3] random: use siphash24 instead of md5 for get_random_int/long "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-14 04:20 +0100
      Re: [PATCH 4/3] random: use siphash24 instead of md5 for  get_random_int/long Theodore Ts'o <tytso@mit.edu> - 2016-12-14 17:40 +0100
        Re: [kernel-hardening] Re: [PATCH 4/3] random: use siphash24 instead  of md5 for get_random_int/long "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-14 19:00 +0100
        Re: [kernel-hardening] Re: [PATCH 4/3] random: use siphash24 instead  of md5 for get_random_int/long "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-14 20:20 +0100
          Re: [kernel-hardening] Re: [PATCH 4/3] random: use siphash24 instead  of md5 for get_random_int/long "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-15 02:30 +0100
    RE: [PATCH 1/3] siphash: add cryptographically secure hashtable  function David Laight <David.Laight@ACULAB.COM> - 2016-12-14 11:00 +0100

#1541610 — [PATCH 1/3] siphash: add cryptographically secure hashtable function

SipHash is a 64-bit keyed hash function that is actually a
cryptographically secure PRF, like HMAC. Except SipHash is super fast,
and is meant to be used as a hashtable keyed lookup function.

SipHash isn't just some new trendy hash function. It's been around for a
while, and there really isn't anything that comes remotely close to
being useful in the way SipHash is. With that said, why do we need this?

There are a variety of attacks known as "hashtable poisoning" in which an
attacker forms some data such that the hash of that data will be the
same, and then preceeds to fill up all entries of a hashbucket. This is
a realistic and well-known denial-of-service vector.

Linux developers already seem to be aware that this is an issue, and
various places that use hash tables in, say, a network context, use a
non-cryptographically secure function (usually jhash) and then try to
twiddle with the key on a time basis (or in many cases just do nothing
and hope that nobody notices). While this is an admirable attempt at
solving the problem, it doesn't actually fix it. SipHash fixes it.

(It fixes it in such a sound way that you could even build a stream
cipher out of SipHash that would resist the modern cryptanalysis.)

There are a modicum of places in the kernel that are vulnerable to
hashtable poisoning attacks, either via userspace vectors or network
vectors, and there's not a reliable mechanism inside the kernel at the
moment to fix it. The first step toward fixing these issues is actually
getting a secure primitive into the kernel for developers to use. Then
we can, bit by bit, port things over to it as deemed appropriate.

Dozens of languages are already using this internally for their hash
tables. Some of the BSDs already use this in their kernels. SipHash is
a widely known high-speed solution to a widely known problem, and it's
time we catch-up.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>
Cc: Daniel J. Bernstein <djb@cr.yp.to>
---
 include/linux/siphash.h | 20 +++++++++++++
 lib/Kconfig.debug       |  6 ++--
 lib/Makefile            |  5 ++--
 lib/siphash.c           | 76 +++++++++++++++++++++++++++++++++++++++++++++++++
 lib/test_siphash.c      | 74 +++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 176 insertions(+), 5 deletions(-)
 create mode 100644 include/linux/siphash.h
 create mode 100644 lib/siphash.c
 create mode 100644 lib/test_siphash.c

diff --git a/include/linux/siphash.h b/include/linux/siphash.h
new file mode 100644
index 000000000000..6623b3090645
--- /dev/null
+++ b/include/linux/siphash.h
@@ -0,0 +1,20 @@
+/* Copyright (C) 2016 Jason A. Donenfeld <Jason@zx2c4.com>
+ *
+ * This file is provided under a dual BSD/GPLv2 license.
+ *
+ * SipHash: a fast short-input PRF
+ * https://131002.net/siphash/
+ */
+
+#ifndef _LINUX_SIPHASH_H
+#define _LINUX_SIPHASH_H
+
+#include <linux/types.h>
+
+enum siphash_lengths {
+	SIPHASH24_KEY_LEN = 16
+};
+
+u64 siphash24(const u8 *data, size_t len, const u8 key[SIPHASH24_KEY_LEN]);
+
+#endif /* _LINUX_SIPHASH_H */
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index a6c8db1d62f6..2a1797704b41 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1823,9 +1823,9 @@ config TEST_HASH
 	tristate "Perform selftest on hash functions"
 	default n
 	help
-	  Enable this option to test the kernel's integer (<linux/hash,h>)
-	  and string (<linux/stringhash.h>) hash functions on boot
-	  (or module load).
+	  Enable this option to test the kernel's integer (<linux/hash.h>),
+	  string (<linux/stringhash.h>), and siphash (<linux/siphash.h>)
+	  hash functions on boot (or module load).
 
 	  This is intended to help people writing architecture-specific
 	  optimized versions.  If unsure, say N.
diff --git a/lib/Makefile b/lib/Makefile
index 50144a3aeebd..71d398b04a74 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -22,7 +22,8 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
 	 sha1.o chacha20.o md5.o irq_regs.o argv_split.o \
 	 flex_proportions.o ratelimit.o show_mem.o \
 	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
-	 earlycpio.o seq_buf.o nmi_backtrace.o nodemask.o win_minmax.o
+	 earlycpio.o seq_buf.o siphash.o \
+	 nmi_backtrace.o nodemask.o win_minmax.o
 
 lib-$(CONFIG_MMU) += ioremap.o
 lib-$(CONFIG_SMP) += cpumask.o
@@ -44,7 +45,7 @@ obj-$(CONFIG_TEST_HEXDUMP) += test_hexdump.o
 obj-y += kstrtox.o
 obj-$(CONFIG_TEST_BPF) += test_bpf.o
 obj-$(CONFIG_TEST_FIRMWARE) += test_firmware.o
-obj-$(CONFIG_TEST_HASH) += test_hash.o
+obj-$(CONFIG_TEST_HASH) += test_hash.o test_siphash.o
 obj-$(CONFIG_TEST_KASAN) += test_kasan.o
 obj-$(CONFIG_TEST_KSTRTOX) += test-kstrtox.o
 obj-$(CONFIG_TEST_LKM) += test_module.o
diff --git a/lib/siphash.c b/lib/siphash.c
new file mode 100644
index 000000000000..7b55ad3a7fe9
--- /dev/null
+++ b/lib/siphash.c
@@ -0,0 +1,76 @@
+/* Copyright (C) 2015-2016 Jason A. Donenfeld <Jason@zx2c4.com>
+ * Copyright (C) 2012-2014 Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>
+ * Copyright (C) 2012-2014 Daniel J. Bernstein <djb@cr.yp.to>
+ *
+ * This file is provided under a dual BSD/GPLv2 license.
+ *
+ * SipHash: a fast short-input PRF
+ * https://131002.net/siphash/
+ */
+
+#include <linux/siphash.h>
+#include <linux/kernel.h>
+#include <asm/unaligned.h>
+
+#if defined(CONFIG_DCACHE_WORD_ACCESS) && BITS_PER_LONG == 64
+#include <linux/dcache.h>
+#include <asm/word-at-a-time.h>
+#endif
+
+#define SIPROUND \
+	do { \
+	v0 += v1; v1 = rol64(v1, 13); v1 ^= v0; v0 = rol64(v0, 32); \
+	v2 += v3; v3 = rol64(v3, 16); v3 ^= v2; \
+	v0 += v3; v3 = rol64(v3, 21); v3 ^= v0; \
+	v2 += v1; v1 = rol64(v1, 17); v1 ^= v2; v2 = rol64(v2, 32); \
+	} while(0)
+
+u64 siphash24(const u8 *data, size_t len, const u8 key[SIPHASH24_KEY_LEN])
+{
+	u64 v0 = 0x736f6d6570736575ULL;
+	u64 v1 = 0x646f72616e646f6dULL;
+	u64 v2 = 0x6c7967656e657261ULL;
+	u64 v3 = 0x7465646279746573ULL;
+	u64 b = ((u64)len) << 56;
+	u64 k0 = get_unaligned_le64(key);
+	u64 k1 = get_unaligned_le64(key + sizeof(u64));
+	u64 m;
+	const u8 *end = data + len - (len % sizeof(u64));
+	const u8 left = len & (sizeof(u64) - 1);
+	v3 ^= k1;
+	v2 ^= k0;
+	v1 ^= k1;
+	v0 ^= k0;
+	for (; data != end; data += sizeof(u64)) {
+		m = get_unaligned_le64(data);
+		v3 ^= m;
+		SIPROUND;
+		SIPROUND;
+		v0 ^= m;
+	}
+#if defined(CONFIG_DCACHE_WORD_ACCESS) && BITS_PER_LONG == 64
+	if (left)
+		b |= le64_to_cpu(load_unaligned_zeropad(data) & bytemask_from_count(left));
+#else
+	switch (left) {
+	case 7: b |= ((u64)data[6]) << 48;
+	case 6: b |= ((u64)data[5]) << 40;
+	case 5: b |= ((u64)data[4]) << 32;
+	case 4: b |= get_unaligned_le32(data); break;
+	case 3: b |= ((u64)data[2]) << 16;
+	case 2: b |= get_unaligned_le16(data); break;
+	case 1: b |= data[0];
+	}
+#endif
+	v3 ^= b;
+	SIPROUND;
+	SIPROUND;
+	v0 ^= b;
+	v2 ^= 0xff;
+	SIPROUND;
+	SIPROUND;
+	SIPROUND;
+	SIPROUND;
+	return (v0 ^ v1) ^ (v2 ^ v3);
+}
+EXPORT_SYMBOL(siphash24);
diff --git a/lib/test_siphash.c b/lib/test_siphash.c
new file mode 100644
index 000000000000..336298aaa33b
--- /dev/null
+++ b/lib/test_siphash.c
@@ -0,0 +1,74 @@
+/* Test cases for siphash.c
+ *
+ * Copyright (C) 2015-2016 Jason A. Donenfeld <Jason@zx2c4.com>
+ *
+ * This file is provided under a dual BSD/GPLv2 license.
+ *
+ * SipHash: a fast short-input PRF
+ * https://131002.net/siphash/
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/siphash.h>
+#include <linux/kernel.h>
+#include <linux/string.h>
+#include <linux/errno.h>
+#include <linux/module.h>
+
+/* Test vectors taken from official reference source available at:
+ *     https://131002.net/siphash/siphash24.c
+ */
+static const u64 test_vectors[64] = {
+	0x726fdb47dd0e0e31ULL, 0x74f839c593dc67fdULL, 0x0d6c8009d9a94f5aULL,
+	0x85676696d7fb7e2dULL, 0xcf2794e0277187b7ULL, 0x18765564cd99a68dULL,
+	0xcbc9466e58fee3ceULL, 0xab0200f58b01d137ULL, 0x93f5f5799a932462ULL,
+	0x9e0082df0ba9e4b0ULL, 0x7a5dbbc594ddb9f3ULL, 0xf4b32f46226bada7ULL,
+	0x751e8fbc860ee5fbULL, 0x14ea5627c0843d90ULL, 0xf723ca908e7af2eeULL,
+	0xa129ca6149be45e5ULL, 0x3f2acc7f57c29bdbULL, 0x699ae9f52cbe4794ULL,
+	0x4bc1b3f0968dd39cULL, 0xbb6dc91da77961bdULL, 0xbed65cf21aa2ee98ULL,
+	0xd0f2cbb02e3b67c7ULL, 0x93536795e3a33e88ULL, 0xa80c038ccd5ccec8ULL,
+	0xb8ad50c6f649af94ULL, 0xbce192de8a85b8eaULL, 0x17d835b85bbb15f3ULL,
+	0x2f2e6163076bcfadULL, 0xde4daaaca71dc9a5ULL, 0xa6a2506687956571ULL,
+	0xad87a3535c49ef28ULL, 0x32d892fad841c342ULL, 0x7127512f72f27cceULL,
+	0xa7f32346f95978e3ULL, 0x12e0b01abb051238ULL, 0x15e034d40fa197aeULL,
+	0x314dffbe0815a3b4ULL, 0x027990f029623981ULL, 0xcadcd4e59ef40c4dULL,
+	0x9abfd8766a33735cULL, 0x0e3ea96b5304a7d0ULL, 0xad0c42d6fc585992ULL,
+	0x187306c89bc215a9ULL, 0xd4a60abcf3792b95ULL, 0xf935451de4f21df2ULL,
+	0xa9538f0419755787ULL, 0xdb9acddff56ca510ULL, 0xd06c98cd5c0975ebULL,
+	0xe612a3cb9ecba951ULL, 0xc766e62cfcadaf96ULL, 0xee64435a9752fe72ULL,
+	0xa192d576b245165aULL, 0x0a8787bf8ecb74b2ULL, 0x81b3e73d20b49b6fULL,
+	0x7fa8220ba3b2eceaULL, 0x245731c13ca42499ULL, 0xb78dbfaf3a8d83bdULL,
+	0xea1ad565322a1a0bULL, 0x60e61c23a3795013ULL, 0x6606d7e446282b93ULL,
+	0x6ca4ecb15c5f91e1ULL, 0x9f626da15c9625f3ULL, 0xe51b38608ef25f57ULL,
+	0x958a324ceb064572ULL
+};
+
+static int __init siphash_test_init(void)
+{
+	u8 in[64], k[16], i;
+	int ret = 0;
+
+	for (i = 0; i < 16; ++i)
+		k[i] = i;
+	for (i = 0; i < 64; ++i) {
+		in[i] = i;
+		if (siphash24(in, i, k) != test_vectors[i]) {
+			pr_info("self-test %u: FAIL\n", i + 1);
+			ret = -EINVAL;
+		}
+	}
+	if (!ret)
+		pr_info("self-tests: pass\n");
+	return ret;
+}
+
+static void __exit siphash_test_exit(void)
+{
+}
+
+module_init(siphash_test_init);
+module_exit(siphash_test_exit);
+
+MODULE_AUTHOR("Jason A. Donenfeld <Jason@zx2c4.com>");
+MODULE_LICENSE("Dual BSD/GPL");
-- 
2.11.0

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#1541673 — [PATCH 4/3] random: use siphash24 instead of md5 for get_random_int/long

This duplicates the current algorithm for get_random_int/long, but uses
siphash24 instead. This comes with several benefits. It's certainly
faster and more cryptographically secure than MD5. This patch also
hashes the pid, entropy, and timestamp as fixed width fields, in order
to increase diffusion.

The previous md5 algorithm used a per-cpu md5 state, which caused
successive calls to the function to chain upon each other. While it's
not entirely clear that this kind of chaining is absolutely necessary
when using a secure PRF like siphash24, it can't hurt, and the timing of
the call chain does add a degree of natural entropy. So, in keeping with
this design, instead of the massive per-cpu 64-byte md5 state, there is
instead a per-cpu previously returned value for chaining.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>
---
 drivers/char/random.c | 50 +++++++++++++++++++++++++++++++-------------------
 1 file changed, 31 insertions(+), 19 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index d6876d506220..25f96f074da5 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -262,6 +262,7 @@
 #include <linux/syscalls.h>
 #include <linux/completion.h>
 #include <linux/uuid.h>
+#include <linux/siphash.h>
 #include <crypto/chacha20.h>
 
 #include <asm/processor.h>
@@ -2042,7 +2043,7 @@ struct ctl_table random_table[] = {
 };
 #endif 	/* CONFIG_SYSCTL */
 
-static u32 random_int_secret[MD5_MESSAGE_BYTES / 4] ____cacheline_aligned;
+static u8 random_int_secret[SIPHASH24_KEY_LEN];
 
 int random_int_secret_init(void)
 {
@@ -2050,8 +2051,7 @@ int random_int_secret_init(void)
 	return 0;
 }
 
-static DEFINE_PER_CPU(__u32 [MD5_DIGEST_WORDS], get_random_int_hash)
-		__aligned(sizeof(unsigned long));
+static DEFINE_PER_CPU(u64, get_random_int_chaining);
 
 /*
  * Get a random word for internal kernel use only. Similar to urandom but
@@ -2061,19 +2061,25 @@ static DEFINE_PER_CPU(__u32 [MD5_DIGEST_WORDS], get_random_int_hash)
  */
 unsigned int get_random_int(void)
 {
-	__u32 *hash;
+	uint64_t *chaining;
 	unsigned int ret;
+	struct {
+		uint64_t chaining;
+		unsigned long ts;
+		unsigned long entropy;
+		pid_t pid;
+	} __packed combined;
 
 	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 = hash[0];
-	put_cpu_var(get_random_int_hash);
-
+	chaining = &get_cpu_var(get_random_int_chaining);
+	combined.chaining = *chaining;
+	combined.ts = jiffies;
+	combined.entropy = random_get_entropy();
+	combined.pid = current->pid;
+	ret = *chaining = siphash24((u8 *)&combined, sizeof(combined), random_int_secret);
+	put_cpu_var(chaining);
 	return ret;
 }
 EXPORT_SYMBOL(get_random_int);
@@ -2083,19 +2089,25 @@ EXPORT_SYMBOL(get_random_int);
  */
 unsigned long get_random_long(void)
 {
-	__u32 *hash;
+	uint64_t *chaining;
 	unsigned long ret;
+	struct {
+		uint64_t chaining;
+		unsigned long ts;
+		unsigned long entropy;
+		pid_t pid;
+	} __packed combined;
 
 	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 = *(unsigned long *)hash;
-	put_cpu_var(get_random_int_hash);
-
+	chaining = &get_cpu_var(get_random_int_chaining);
+	combined.chaining = *chaining;
+	combined.ts = jiffies;
+	combined.entropy = random_get_entropy();
+	combined.pid = current->pid;
+	ret = *chaining = siphash24((u8 *)&combined, sizeof(combined), random_int_secret);
+	put_cpu_var(chaining);
 	return ret;
 }
 EXPORT_SYMBOL(get_random_long);
-- 
2.11.0

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#1542032 — Re: [PATCH 4/3] random: use siphash24 instead of md5 for get_random_int/long

On Wed, Dec 14, 2016 at 04:10:37AM +0100, Jason A. Donenfeld wrote:
> This duplicates the current algorithm for get_random_int/long, but uses
> siphash24 instead. This comes with several benefits. It's certainly
> faster and more cryptographically secure than MD5. This patch also
> hashes the pid, entropy, and timestamp as fixed width fields, in order
> to increase diffusion.
> 
> The previous md5 algorithm used a per-cpu md5 state, which caused
> successive calls to the function to chain upon each other. While it's
> not entirely clear that this kind of chaining is absolutely necessary
> when using a secure PRF like siphash24, it can't hurt, and the timing of
> the call chain does add a degree of natural entropy. So, in keeping with
> this design, instead of the massive per-cpu 64-byte md5 state, there is
> instead a per-cpu previously returned value for chaining.
> 
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> Cc: Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>

The original reason for get_random_int was because the original
urandom algorithms were too slow.  When we moved to chacha20, which is
must faster, I didn't think to revisit get_random_int() and
get_random_long().

One somewhat undesirable aspect of the current algorithm is that we
never change random_int_secret.  So I've been toying with the
following, which is 4 times faster than md5.  (I haven't tried
benchmarking against siphash yet.)

[    3.606139] random benchmark!!
[    3.606276] get_random_int # cycles: 326578
[    3.606317] get_random_int_new # cycles: 95438
[    3.607423] get_random_bytes # cycles: 2653388

     	       			  	  - Ted

P.S.  It's interesting to note that siphash24 and chacha20 are both
add-rotate-xor based algorithms.

diff --git a/drivers/char/random.c b/drivers/char/random.c
index d6876d506220..be172ea75799 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1681,6 +1681,38 @@ static int rand_initialize(void)
 }
 early_initcall(rand_initialize);
 
+static unsigned int get_random_int_new(void);
+
+static int rand_benchmark(void)
+{
+	cycles_t start,finish;
+	int i, out;
+
+	pr_crit("random benchmark!!\n");
+	start = get_cycles();
+	for (i = 0; i < 1000; i++) {
+		get_random_int();
+	}
+	finish = get_cycles();
+	pr_err("get_random_int # cycles: %llu\n", finish - start);
+
+	start = get_cycles();
+	for (i = 0; i < 1000; i++) {
+		get_random_int_new();
+	}
+	finish = get_cycles();
+	pr_err("get_random_int_new # cycles: %llu\n", finish - start);
+
+	start = get_cycles();
+	for (i = 0; i < 1000; i++) {
+		get_random_bytes(&out, sizeof(out));
+	}
+	finish = get_cycles();
+	pr_err("get_random_bytes # cycles: %llu\n", finish - start);
+	return 0;
+}
+device_initcall(rand_benchmark);
+
 #ifdef CONFIG_BLOCK
 void rand_initialize_disk(struct gendisk *disk)
 {
@@ -2064,8 +2096,10 @@ unsigned int get_random_int(void)
 	__u32 *hash;
 	unsigned int ret;
 
+#if 0	// force slow path
 	if (arch_get_random_int(&ret))
 		return ret;
+#endif
 
 	hash = get_cpu_var(get_random_int_hash);
 
@@ -2100,6 +2134,38 @@ unsigned long get_random_long(void)
 }
 EXPORT_SYMBOL(get_random_long);
 
+struct random_buf {
+	__u8 buf[CHACHA20_BLOCK_SIZE];
+	int ptr;
+};
+
+static DEFINE_PER_CPU(struct random_buf, batched_entropy);
+
+static void get_batched_entropy(void *buf, int n)
+{
+	struct random_buf *p;
+
+	p = &get_cpu_var(batched_entropy);
+
+	if ((p->ptr == 0) ||
+	    (p->ptr + n >= CHACHA20_BLOCK_SIZE)) {
+		extract_crng(p->buf);
+		p->ptr = 0;
+	}
+	BUG_ON(n > CHACHA20_BLOCK_SIZE);
+	memcpy(buf, p->buf, n);
+	p->ptr += n;
+	put_cpu_var(batched_entropy);
+}
+
+static unsigned int get_random_int_new(void)
+{
+	int	ret;
+
+	get_batched_entropy(&ret, sizeof(ret));
+	return ret;
+}
+
 /**
  * randomize_page - Generate a random, page aligned address
  * @start:	The smallest acceptable address the caller will take.

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#1542088 — Re: [kernel-hardening] Re: [PATCH 4/3] random: use siphash24 instead of md5 for get_random_int/long

Hey Ted,

On Wed, Dec 14, 2016 at 5:37 PM, Theodore Ts'o <tytso@mit.edu> wrote:
> One somewhat undesirable aspect of the current algorithm is that we
> never change random_int_secret.

Why exactly would this be a problem? So long as the secret is kept
secret, the PRF is secure. If an attacker can read arbitrary kernel
memory, there are much much bigger issues to be concerned about. As
well, the "chaining" variable I introduce ensures that the random
numbers are, per-cpu, related to the uniqueness of timing of
subsequent calls.

> So I've been toying with the
> following, which is 4 times faster than md5.  (I haven't tried
> benchmarking against siphash yet.)
>
> [    3.606139] random benchmark!!
> [    3.606276] get_random_int # cycles: 326578
> [    3.606317] get_random_int_new # cycles: 95438
> [    3.607423] get_random_bytes # cycles: 2653388

Cool, I'll benchmark it against the siphash implementation. I like
what you did with batching up lots of chacha output, and doling it out
bit by bit. I suspect this will be quite fast, because with chacha20
you get an entire block.

> P.S.  It's interesting to note that siphash24 and chacha20 are both
> add-rotate-xor based algorithms.

Quite! Lots of nice shiny things are turning out be be ARX -- ChaCha,
BLAKE2, Siphash, NORX. The simplicity is really appealing.

Jason

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#1542133 — Re: [kernel-hardening] Re: [PATCH 4/3] random: use siphash24 instead of md5 for get_random_int/long

Hi again,

On Wed, Dec 14, 2016 at 5:37 PM, Theodore Ts'o <tytso@mit.edu> wrote:
> [    3.606139] random benchmark!!
> [    3.606276] get_random_int # cycles: 326578
> [    3.606317] get_random_int_new # cycles: 95438
> [    3.607423] get_random_bytes # cycles: 2653388

Looks to me like my siphash implementation is much faster for
get_random_long, and more or less tied for get_random_int:

[    1.729370] random benchmark!!
[    1.729710] get_random_long # cycles: 349771
[    1.730128] get_random_long_chacha # cycles: 359660
[    1.730457] get_random_long_siphash # cycles: 94255
[    1.731307] get_random_bytes # cycles: 1354894
[    1.731707] get_random_int # cycles: 305640
[    1.732095] get_random_int_chacha # cycles: 80726
[    1.732425] get_random_int_siphash # cycles: 94265
[    1.733278] get_random_bytes # cycles: 1315873

Given the increasing usage of get_random_long for ASLR and related, I
think this makes the siphash approach worth pursuing. The chacha
approach is also not significantly different from the md5 approach in
terms of speed for get_rand_long. Additionally, since siphash is a
PRF, I think this opens up a big window for optimizing it even
further.

Benchmark here:
    https://git.zx2c4.com/linux-dev/commit/?h=rng-bench

Jason

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#1542425 — Re: [kernel-hardening] Re: [PATCH 4/3] random: use siphash24 instead of md5 for get_random_int/long

Hey Ted,

On Wed, Dec 14, 2016 at 8:12 PM, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> I think this opens up a big window for optimizing it even
> further.

I optimized it a bit further and siphash is now the clear winner over chacha:

[    1.784801] random benchmark!!
[    1.785161] get_random_long # cycles: 415983
[    1.785595] get_random_long_chacha # cycles: 242047
[    1.785997] get_random_long_siphash # cycles: 137130
[    1.787450] get_random_bytes # cycles: 1452985
[    1.787947] get_random_int # cycles: 343323
[    1.788282] get_random_int_chacha # cycles: 170767
[    1.788656] get_random_int_siphash # cycles: 86384
[    1.789764] get_random_bytes # cycles: 2279519

And even still, there is more that could be optimized. Therefore, I'll
continue to keep this patch in the series and will CC you on the next
patch set that goes out.

Jason

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#1541797 — RE: [PATCH 1/3] siphash: add cryptographically secure hashtable function

From: Jason A. Donenfeld
> Sent: 14 December 2016 00:17
> SipHash is a 64-bit keyed hash function that is actually a
> cryptographically secure PRF, like HMAC. Except SipHash is super fast,
> and is meant to be used as a hashtable keyed lookup function.
> 
> SipHash isn't just some new trendy hash function. It's been around for a
> while, and there really isn't anything that comes remotely close to
> being useful in the way SipHash is. With that said, why do we need this?
> 
> There are a variety of attacks known as "hashtable poisoning" in which an
> attacker forms some data such that the hash of that data will be the
> same, and then preceeds to fill up all entries of a hashbucket. This is
> a realistic and well-known denial-of-service vector.
> 
> Linux developers already seem to be aware that this is an issue, and
> various places that use hash tables in, say, a network context, use a
> non-cryptographically secure function (usually jhash) and then try to
> twiddle with the key on a time basis (or in many cases just do nothing
> and hope that nobody notices). While this is an admirable attempt at
> solving the problem, it doesn't actually fix it. SipHash fixes it.
> 
> (It fixes it in such a sound way that you could even build a stream
> cipher out of SipHash that would resist the modern cryptanalysis.)
> 
> There are a modicum of places in the kernel that are vulnerable to
> hashtable poisoning attacks, either via userspace vectors or network
> vectors, and there's not a reliable mechanism inside the kernel at the
> moment to fix it. The first step toward fixing these issues is actually
> getting a secure primitive into the kernel for developers to use. Then
> we can, bit by bit, port things over to it as deemed appropriate.
> 
> Dozens of languages are already using this internally for their hash
> tables. Some of the BSDs already use this in their kernels. SipHash is
> a widely known high-speed solution to a widely known problem, and it's
> time we catch-up.
...
> +u64 siphash24(const u8 *data, size_t len, const u8 key[SIPHASH24_KEY_LEN])
...
> +	u64 k0 = get_unaligned_le64(key);
> +	u64 k1 = get_unaligned_le64(key + sizeof(u64));
...
> +		m = get_unaligned_le64(data);

All these unaligned accesses are going to get expensive on architectures
like sparc64.

	David

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