1 files changed, 199 insertions, 0 deletions
diff --git a/Documentation/security/siphash.rst b/Documentation/security/siphash.rst
new file mode 100644
index 0000000000..023bd95c74
--- /dev/null
+++ b/Documentation/security/siphash.rst
@@ -0,0 +1,199 @@
+===========================
+SipHash - a short input PRF
+===========================
+
+:Author: Written by Jason A. Donenfeld <jason@zx2c4.com>
+
+SipHash is a cryptographically secure PRF -- a keyed hash function -- that
+performs very well for short inputs, hence the name. It was designed by
+cryptographers Daniel J. Bernstein and Jean-Philippe Aumasson. It is intended
+as a replacement for some uses of: `jhash`, `md5_transform`, `sha1_transform`,
+and so forth.
+
+SipHash takes a secret key filled with randomly generated numbers and either
+an input buffer or several input integers. It spits out an integer that is
+indistinguishable from random. You may then use that integer as part of secure
+sequence numbers, secure cookies, or mask it off for use in a hash table.
+
+Generating a key
+================
+
+Keys should always be generated from a cryptographically secure source of
+random numbers, either using get_random_bytes or get_random_once::
+
+	siphash_key_t key;
+	get_random_bytes(&key, sizeof(key));
+
+If you're not deriving your key from here, you're doing it wrong.
+
+Using the functions
+===================
+
+There are two variants of the function, one that takes a list of integers, and
+one that takes a buffer::
+
+	u64 siphash(const void *data, size_t len, const siphash_key_t *key);
+
+And::
+
+	u64 siphash_1u64(u64, const siphash_key_t *key);
+	u64 siphash_2u64(u64, u64, const siphash_key_t *key);
+	u64 siphash_3u64(u64, u64, u64, const siphash_key_t *key);
+	u64 siphash_4u64(u64, u64, u64, u64, const siphash_key_t *key);
+	u64 siphash_1u32(u32, const siphash_key_t *key);
+	u64 siphash_2u32(u32, u32, const siphash_key_t *key);
+	u64 siphash_3u32(u32, u32, u32, const siphash_key_t *key);
+	u64 siphash_4u32(u32, u32, u32, u32, const siphash_key_t *key);
+
+If you pass the generic siphash function something of a constant length, it
+will constant fold at compile-time and automatically choose one of the
+optimized functions.
+
+Hashtable key function usage::
+
+	struct some_hashtable {
+		DECLARE_HASHTABLE(hashtable, 8);
+		siphash_key_t key;
+	};
+
+	void init_hashtable(struct some_hashtable *table)
+	{
+		get_random_bytes(&table->key, sizeof(table->key));
+	}
+
+	static inline hlist_head *some_hashtable_bucket(struct some_hashtable *table, struct interesting_input *input)
+	{
+		return &table->hashtable[siphash(input, sizeof(*input), &table->key) & (HASH_SIZE(table->hashtable) - 1)];
+	}
+
+You may then iterate like usual over the returned hash bucket.
+
+Security
+========
+
+SipHash has a very high security margin, with its 128-bit key. So long as the
+key is kept secret, it is impossible for an attacker to guess the outputs of
+the function, even if being able to observe many outputs, since 2^128 outputs
+is significant.
+
+Linux implements the "2-4" variant of SipHash.
+
+Struct-passing Pitfalls
+=======================
+
+Often times the XuY functions will not be large enough, and instead you'll
+want to pass a pre-filled struct to siphash. When doing this, it's important
+to always ensure the struct has no padding holes. The easiest way to do this
+is to simply arrange the members of the struct in descending order of size,
+and to use offsetofend() instead of sizeof() for getting the size. For
+performance reasons, if possible, it's probably a good thing to align the
+struct to the right boundary. Here's an example::
+
+	const struct {
+		struct in6_addr saddr;
+		u32 counter;
+		u16 dport;
+	} __aligned(SIPHASH_ALIGNMENT) combined = {
+		.saddr = *(struct in6_addr *)saddr,
+		.counter = counter,
+		.dport = dport
+	};
+	u64 h = siphash(&combined, offsetofend(typeof(combined), dport), &secret);
+
+Resources
+=========
+
+Read the SipHash paper if you're interested in learning more:
+https://131002.net/siphash/siphash.pdf
+
+-------------------------------------------------------------------------------
+
+===============================================
+HalfSipHash - SipHash's insecure younger cousin
+===============================================
+
+:Author: Written by Jason A. Donenfeld <jason@zx2c4.com>
+
+On the off-chance that SipHash is not fast enough for your needs, you might be
+able to justify using HalfSipHash, a terrifying but potentially useful
+possibility. HalfSipHash cuts SipHash's rounds down from "2-4" to "1-3" and,
+even scarier, uses an easily brute-forcable 64-bit key (with a 32-bit output)
+instead of SipHash's 128-bit key. However, this may appeal to some
+high-performance `jhash` users.
+
+HalfSipHash support is provided through the "hsiphash" family of functions.
+
+.. warning::
+   Do not ever use the hsiphash functions except for as a hashtable key
+   function, and only then when you can be absolutely certain that the outputs
+   will never be transmitted out of the kernel. This is only remotely useful
+   over `jhash` as a means of mitigating hashtable flooding denial of service
+   attacks.
+
+On 64-bit kernels, the hsiphash functions actually implement SipHash-1-3, a
+reduced-round variant of SipHash, instead of HalfSipHash-1-3. This is because in
+64-bit code, SipHash-1-3 is no slower than HalfSipHash-1-3, and can be faster.
+Note, this does *not* mean that in 64-bit kernels the hsiphash functions are the
+same as the siphash ones, or that they are secure; the hsiphash functions still
+use a less secure reduced-round algorithm and truncate their outputs to 32
+bits.
+
+Generating a hsiphash key
+=========================
+
+Keys should always be generated from a cryptographically secure source of
+random numbers, either using get_random_bytes or get_random_once::
+
+	hsiphash_key_t key;
+	get_random_bytes(&key, sizeof(key));
+
+If you're not deriving your key from here, you're doing it wrong.
+
+Using the hsiphash functions
+============================
+
+There are two variants of the function, one that takes a list of integers, and
+one that takes a buffer::
+
+	u32 hsiphash(const void *data, size_t len, const hsiphash_key_t *key);
+
+And::
+
+	u32 hsiphash_1u32(u32, const hsiphash_key_t *key);
+	u32 hsiphash_2u32(u32, u32, const hsiphash_key_t *key);
+	u32 hsiphash_3u32(u32, u32, u32, const hsiphash_key_t *key);
+	u32 hsiphash_4u32(u32, u32, u32, u32, const hsiphash_key_t *key);
+
+If you pass the generic hsiphash function something of a constant length, it
+will constant fold at compile-time and automatically choose one of the
+optimized functions.
+
+Hashtable key function usage
+============================
+
+::
+
+	struct some_hashtable {
+		DECLARE_HASHTABLE(hashtable, 8);
+		hsiphash_key_t key;
+	};
+
+	void init_hashtable(struct some_hashtable *table)
+	{
+		get_random_bytes(&table->key, sizeof(table->key));
+	}
+
+	static inline hlist_head *some_hashtable_bucket(struct some_hashtable *table, struct interesting_input *input)
+	{
+		return &table->hashtable[hsiphash(input, sizeof(*input), &table->key) & (HASH_SIZE(table->hashtable) - 1)];
+	}
+
+You may then iterate like usual over the returned hash bucket.
+
+Performance
+===========
+
+hsiphash() is roughly 3 times slower than jhash(). For many replacements, this
+will not be a problem, as the hashtable lookup isn't the bottleneck. And in
+general, this is probably a good sacrifice to make for the security and DoS
+resistance of hsiphash().