Merge tag 'random-5.18-rc5-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/crng/random

Pull random number generator fixes from Jason Donenfeld:

 - Eric noticed that the memmove() in crng_fast_key_erasure() was bogus,
   so this has been changed to a memcpy() and the confusing situation
   clarified with a detailed comment.

 - [Half]SipHash documentation updates from Bagas and Eric, after Eric
   pointed out that the use of HalfSipHash in random.c made a bit of the
   text potentially misleading.

* tag 'random-5.18-rc5-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/crng/random:
  Documentation: siphash: disambiguate HalfSipHash algorithm from hsiphash functions
  Documentation: siphash: enclose HalfSipHash usage example in the literal block
  Documentation: siphash: convert danger note to warning for HalfSipHash
  random: document crng_fast_key_erasure() destination possibility

2 files changed, 36 insertions, 19 deletions

diff --git a/Documentation/security/siphash.rst b/Documentation/security/siphash.rst
index bd9363025fcb..a10380cb78e5 100644
--- a/Documentation/security/siphash.rst
+++ b/Documentation/security/siphash.rst
@@ -121,26 +121,36 @@ 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.
 
-Danger!
-
-Do not ever use HalfSipHash 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.
-
-Generating a HalfSipHash key
-============================
+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:
+random numbers, either using get_random_bytes or get_random_once::
 
-hsiphash_key_t key;
-get_random_bytes(&key, sizeof(key));
+	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 HalfSipHash functions
-===============================
+Using the hsiphash functions
+============================
 
 There are two variants of the function, one that takes a list of integers, and
 one that takes a buffer::
@@ -183,7 +193,7 @@ You may then iterate like usual over the returned hash bucket.
 Performance
 ===========
 
-HalfSipHash is roughly 3 times slower than JenkinsHash. 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 HalfSipHash.
+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().
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 3a293f919af9..4c9adb4f3d5d 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -318,6 +318,13 @@ static void crng_reseed(bool force)
  * the resultant ChaCha state to the user, along with the second
  * half of the block containing 32 bytes of random data that may
  * be used; random_data_len may not be greater than 32.
+ *
+ * The returned ChaCha state contains within it a copy of the old
+ * key value, at index 4, so the state should always be zeroed out
+ * immediately after using in order to maintain forward secrecy.
+ * If the state cannot be erased in a timely manner, then it is
+ * safer to set the random_data parameter to &chacha_state[4] so
+ * that this function overwrites it before returning.
  */
 static void crng_fast_key_erasure(u8 key[CHACHA_KEY_SIZE],
 				  u32 chacha_state[CHACHA_STATE_WORDS],
@@ -333,7 +340,7 @@ static void crng_fast_key_erasure(u8 key[CHACHA_KEY_SIZE],
 	chacha20_block(chacha_state, first_block);
 
 	memcpy(key, first_block, CHACHA_KEY_SIZE);
-	memmove(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
+	memcpy(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
 	memzero_explicit(first_block, sizeof(first_block));
 }