crypto: jitter - replace LFSR with SHA3-256

Using the kernel crypto API, the SHA3-256 algorithm is used as
conditioning element to replace the LFSR in the Jitter RNG. All other
parts of the Jitter RNG are unchanged.

The application and use of the SHA-3 conditioning operation is identical
to the user space Jitter RNG 3.4.0 by applying the following concept:

- the Jitter RNG initializes a SHA-3 state which acts as the "entropy
  pool" when the Jitter RNG is allocated.

- When a new time delta is obtained, it is inserted into the "entropy
  pool" with a SHA-3 update operation. Note, this operation in most of
  the cases is a simple memcpy() onto the SHA-3 stack.

- To cause a true SHA-3 operation for each time delta operation, a
  second SHA-3 operation is performed hashing Jitter RNG status
  information. The final message digest is also inserted into the
  "entropy pool" with a SHA-3 update operation. Yet, this data is not
  considered to provide any entropy, but it shall stir the entropy pool.

- To generate a random number, a SHA-3 final operation is performed to
  calculate a message digest followed by an immediate SHA-3 init to
  re-initialize the "entropy pool". The obtained message digest is one
  block of the Jitter RNG that is returned to the caller.

Mathematically speaking, the random number generated by the Jitter RNG
is:

aux_t = SHA-3(Jitter RNG state data)

Jitter RNG block = SHA-3(time_i || aux_i || time_(i-1) || aux_(i-1) ||
                         ... || time_(i-255) || aux_(i-255))

when assuming that the OSR = 1, i.e. the default value.

This operation implies that the Jitter RNG has an output-blocksize of
256 bits instead of the 64 bits of the LFSR-based Jitter RNG that is
replaced with this patch.

The patch also replaces the varying number of invocations of the
conditioning function with one fixed number of invocations. The use
of the conditioning function consistent with the userspace Jitter RNG
library version 3.4.0.

The code is tested with a system that exhibited the least amount of
entropy generated by the Jitter RNG: the SiFive Unmatched RISC-V
system. The measured entropy rate is well above the heuristically
implied entropy value of 1 bit of entropy per time delta. On all other
tested systems, the measured entropy rate is even higher by orders
of magnitude. The measurement was performed using updated tooling
provided with the user space Jitter RNG library test framework.

The performance of the Jitter RNG with this patch is about en par
with the performance of the Jitter RNG without the patch.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 163 insertions, 20 deletions

diff --git a/crypto/jitterentropy-kcapi.c b/crypto/jitterentropy-kcapi.c
index b9edfaa51b27..4b50cbc8a2fa 100644
--- a/crypto/jitterentropy-kcapi.c
+++ b/crypto/jitterentropy-kcapi.c
@@ -2,7 +2,7 @@
  * Non-physical true random number generator based on timing jitter --
  * Linux Kernel Crypto API specific code
  *
- * Copyright Stephan Mueller <smueller@chronox.de>, 2015
+ * Copyright Stephan Mueller <smueller@chronox.de>, 2015 - 2023
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -37,6 +37,8 @@
  * DAMAGE.
  */
 
+#include <crypto/hash.h>
+#include <crypto/sha3.h>
 #include <linux/fips.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
@@ -46,6 +48,8 @@
 
 #include "jitterentropy.h"
 
+#define JENT_CONDITIONING_HASH	"sha3-256-generic"
+
 /***************************************************************************
  * Helper function
  ***************************************************************************/
@@ -60,11 +64,6 @@ void jent_zfree(void *ptr)
 	kfree_sensitive(ptr);
 }
 
-void jent_memcpy(void *dest, const void *src, unsigned int n)
-{
-	memcpy(dest, src, n);
-}
-
 /*
  * Obtain a high-resolution time stamp value. The time stamp is used to measure
  * the execution time of a given code path and its variations. Hence, the time
@@ -91,6 +90,91 @@ void jent_get_nstime(__u64 *out)
 	*out = tmp;
 }
 
+int jent_hash_time(void *hash_state, __u64 time, u8 *addtl,
+		   unsigned int addtl_len, __u64 hash_loop_cnt,
+		   unsigned int stuck)
+{
+	struct shash_desc *hash_state_desc = (struct shash_desc *)hash_state;
+	SHASH_DESC_ON_STACK(desc, hash_state_desc->tfm);
+	u8 intermediary[SHA3_256_DIGEST_SIZE];
+	__u64 j = 0;
+	int ret;
+
+	desc->tfm = hash_state_desc->tfm;
+
+	if (sizeof(intermediary) != crypto_shash_digestsize(desc->tfm)) {
+		pr_warn_ratelimited("Unexpected digest size\n");
+		return -EINVAL;
+	}
+
+	/*
+	 * This loop fills a buffer which is injected into the entropy pool.
+	 * The main reason for this loop is to execute something over which we
+	 * can perform a timing measurement. The injection of the resulting
+	 * data into the pool is performed to ensure the result is used and
+	 * the compiler cannot optimize the loop away in case the result is not
+	 * used at all. Yet that data is considered "additional information"
+	 * considering the terminology from SP800-90A without any entropy.
+	 *
+	 * Note, it does not matter which or how much data you inject, we are
+	 * interested in one Keccack1600 compression operation performed with
+	 * the crypto_shash_final.
+	 */
+	for (j = 0; j < hash_loop_cnt; j++) {
+		ret = crypto_shash_init(desc) ?:
+		      crypto_shash_update(desc, intermediary,
+					  sizeof(intermediary)) ?:
+		      crypto_shash_finup(desc, addtl, addtl_len, intermediary);
+		if (ret)
+			goto err;
+	}
+
+	/*
+	 * Inject the data from the previous loop into the pool. This data is
+	 * not considered to contain any entropy, but it stirs the pool a bit.
+	 */
+	ret = crypto_shash_update(desc, intermediary, sizeof(intermediary));
+	if (ret)
+		goto err;
+
+	/*
+	 * Insert the time stamp into the hash context representing the pool.
+	 *
+	 * If the time stamp is stuck, do not finally insert the value into the
+	 * entropy pool. Although this operation should not do any harm even
+	 * when the time stamp has no entropy, SP800-90B requires that any
+	 * conditioning operation to have an identical amount of input data
+	 * according to section 3.1.5.
+	 */
+	if (!stuck) {
+		ret = crypto_shash_update(hash_state_desc, (u8 *)&time,
+					  sizeof(__u64));
+	}
+
+err:
+	shash_desc_zero(desc);
+	memzero_explicit(intermediary, sizeof(intermediary));
+
+	return ret;
+}
+
+int jent_read_random_block(void *hash_state, char *dst, unsigned int dst_len)
+{
+	struct shash_desc *hash_state_desc = (struct shash_desc *)hash_state;
+	u8 jent_block[SHA3_256_DIGEST_SIZE];
+	/* Obtain data from entropy pool and re-initialize it */
+	int ret = crypto_shash_final(hash_state_desc, jent_block) ?:
+		  crypto_shash_init(hash_state_desc) ?:
+		  crypto_shash_update(hash_state_desc, jent_block,
+				      sizeof(jent_block));
+
+	if (!ret && dst_len)
+		memcpy(dst, jent_block, dst_len);
+
+	memzero_explicit(jent_block, sizeof(jent_block));
+	return ret;
+}
+
 /***************************************************************************
  * Kernel crypto API interface
  ***************************************************************************/
@@ -98,32 +182,82 @@ void jent_get_nstime(__u64 *out)
 struct jitterentropy {
 	spinlock_t jent_lock;
 	struct rand_data *entropy_collector;
+	struct crypto_shash *tfm;
+	struct shash_desc *sdesc;
 };
 
-static int jent_kcapi_init(struct crypto_tfm *tfm)
+static void jent_kcapi_cleanup(struct crypto_tfm *tfm)
 {
 	struct jitterentropy *rng = crypto_tfm_ctx(tfm);
-	int ret = 0;
 
-	rng->entropy_collector = jent_entropy_collector_alloc(1, 0);
-	if (!rng->entropy_collector)
-		ret = -ENOMEM;
+	spin_lock(&rng->jent_lock);
 
-	spin_lock_init(&rng->jent_lock);
-	return ret;
-}
+	if (rng->sdesc) {
+		shash_desc_zero(rng->sdesc);
+		kfree(rng->sdesc);
+	}
+	rng->sdesc = NULL;
 
-static void jent_kcapi_cleanup(struct crypto_tfm *tfm)
-{
-	struct jitterentropy *rng = crypto_tfm_ctx(tfm);
+	if (rng->tfm)
+		crypto_free_shash(rng->tfm);
+	rng->tfm = NULL;
 
-	spin_lock(&rng->jent_lock);
 	if (rng->entropy_collector)
 		jent_entropy_collector_free(rng->entropy_collector);
 	rng->entropy_collector = NULL;
 	spin_unlock(&rng->jent_lock);
 }
 
+static int jent_kcapi_init(struct crypto_tfm *tfm)
+{
+	struct jitterentropy *rng = crypto_tfm_ctx(tfm);
+	struct crypto_shash *hash;
+	struct shash_desc *sdesc;
+	int size, ret = 0;
+
+	spin_lock_init(&rng->jent_lock);
+
+	/*
+	 * Use SHA3-256 as conditioner. We allocate only the generic
+	 * implementation as we are not interested in high-performance. The
+	 * execution time of the SHA3 operation is measured and adds to the
+	 * Jitter RNG's unpredictable behavior. If we have a slower hash
+	 * implementation, the execution timing variations are larger. When
+	 * using a fast implementation, we would need to call it more often
+	 * as its variations are lower.
+	 */
+	hash = crypto_alloc_shash(JENT_CONDITIONING_HASH, 0, 0);
+	if (IS_ERR(hash)) {
+		pr_err("Cannot allocate conditioning digest\n");
+		return PTR_ERR(hash);
+	}
+	rng->tfm = hash;
+
+	size = sizeof(struct shash_desc) + crypto_shash_descsize(hash);
+	sdesc = kmalloc(size, GFP_KERNEL);
+	if (!sdesc) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
+	sdesc->tfm = hash;
+	crypto_shash_init(sdesc);
+	rng->sdesc = sdesc;
+
+	rng->entropy_collector = jent_entropy_collector_alloc(1, 0, sdesc);
+	if (!rng->entropy_collector) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
+	spin_lock_init(&rng->jent_lock);
+	return 0;
+
+err:
+	jent_kcapi_cleanup(tfm);
+	return ret;
+}
+
 static int jent_kcapi_random(struct crypto_rng *tfm,
 			     const u8 *src, unsigned int slen,
 			     u8 *rdata, unsigned int dlen)
@@ -180,15 +314,24 @@ static struct rng_alg jent_alg = {
 		.cra_module             = THIS_MODULE,
 		.cra_init               = jent_kcapi_init,
 		.cra_exit               = jent_kcapi_cleanup,
-
 	}
 };
 
 static int __init jent_mod_init(void)
 {
+	SHASH_DESC_ON_STACK(desc, tfm);
+	struct crypto_shash *tfm;
 	int ret = 0;
 
-	ret = jent_entropy_init();
+	tfm = crypto_alloc_shash(JENT_CONDITIONING_HASH, 0, 0);
+	if (IS_ERR(tfm))
+		return PTR_ERR(tfm);
+
+	desc->tfm = tfm;
+	crypto_shash_init(desc);
+	ret = jent_entropy_init(desc);
+	shash_desc_zero(desc);
+	crypto_free_shash(tfm);
 	if (ret) {
 		/* Handle permanent health test error */
 		if (fips_enabled)