[PATCH 5/6] crypto: drbg - use Jitter RNG to obtain seed

[Stephan Mueller <smueller@chronox.de>]

During initialization, the DRBG now tries to allocate a handle of the
Jitter RNG. If such a Jitter RNG is available during seeding, the DRBG
pulls the required entropy/nonce string from get_random_bytes and
concatenates it with a string of equal size from the Jitter RNG. That
combined string is now the seed for the DRBG.

Written differently, the initial seed of the DRBG is now:

get_random_bytes(entropy/nonce) || jitterentropy (entropy/nonce)

If the Jitter RNG is not available, the DRBG only seeds from
get_random_bytes.

CC: Andreas Steffen <andreas.steffen@strongswan.org>
CC: Theodore Ts'o <tytso@mit.edu>
CC: Sandy Harris <sandyinchina@gmail.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
---
 crypto/drbg.c         | 44 ++++++++++++++++++++++++++++++++++++++------
 include/crypto/drbg.h |  1 +
 2 files changed, 39 insertions(+), 6 deletions(-)

diff --git a/crypto/drbg.c b/crypto/drbg.c
index cdc23c8..c98941a 100644
--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1125,10 +1125,25 @@ static int drbg_seed(struct drbg_state *drbg, struct drbg_string *pers,
 				 drbg->test_data.len);
 		pr_devel("DRBG: using test entropy\n");
 	} else {
-		pr_devel("DRBG: (re)seeding with %zu bytes of entropy\n",
-			 drbg->seed_buf_len);
+		/* Get seed from in-kernel /dev/urandom */
 		get_random_bytes(drbg->seed_buf, drbg->seed_buf_len);
-		drbg_string_fill(&data1, drbg->seed_buf, drbg->seed_buf_len);
+
+		/* Get seed from Jitter RNG */
+		if (!drbg->jent ||
+		    crypto_rng_get_bytes(drbg->jent,
+					 drbg->seed_buf + drbg->seed_buf_len,
+					 drbg->seed_buf_len)) {
+			pr_info("DRBG: could not obtain random data from Jitter RNG\n");
+			drbg_string_fill(&data1, drbg->seed_buf,
+					 drbg->seed_buf_len);
+			pr_devel("DRBG: (re)seeding with %zu bytes of entropy\n",
+				 drbg->seed_buf_len);
+		} else {
+			drbg_string_fill(&data1, drbg->seed_buf,
+					 drbg->seed_buf_len * 2);
+			pr_devel("DRBG: (re)seeding with %zu bytes of entropy\n",
+				 drbg->seed_buf_len * 2);
+		}
 	}
 	list_add_tail(&data1.list, &seedlist);
 
@@ -1153,7 +1168,7 @@ static int drbg_seed(struct drbg_state *drbg, struct drbg_string *pers,
 	 * Clear the initial entropy buffer as the async call may not overwrite
 	 * that buffer for quite some time.
 	 */
-	memzero_explicit(drbg->seed_buf, drbg->seed_buf_len);
+	memzero_explicit(drbg->seed_buf, drbg->seed_buf_len * 2);
 	if (ret)
 		goto out;
 	/*
@@ -1195,6 +1210,8 @@ static inline void drbg_dealloc_state(struct drbg_state *drbg)
 #endif
 	kzfree(drbg->seed_buf);
 	drbg->seed_buf = NULL;
+	crypto_free_rng(drbg->jent);
+	drbg->jent = NULL;
 }
 
 /*
@@ -1270,12 +1287,27 @@ static inline int drbg_alloc_state(struct drbg_state *drbg)
 		ret = -EFAULT;
 		goto err;
 	}
-	/* ensure we have sufficient buffer space for initial seed */
+	/*
+	 * Ensure we have sufficient buffer space for initial seed which
+	 * consists of the seed from get_random_bytes and the Jitter RNG.
+	 */
 	drbg->seed_buf_len = ((drbg->seed_buf_len + 1) / 2) * 3;
-	drbg->seed_buf = kzalloc(drbg->seed_buf_len, GFP_KERNEL);
+	drbg->seed_buf = kzalloc(drbg->seed_buf_len * 2, GFP_KERNEL);
 	if (!drbg->seed_buf)
 		goto err;
 
+	drbg->jent = crypto_alloc_rng("jitterentropy_rng", 0, 0);
+	if(IS_ERR(drbg->jent))
+	{
+		pr_info("DRBG: could not allocate Jitter RNG handle for seeding\n");
+		/*
+		 * As the Jitter RNG is a module that may not be present, we
+		 * continue with the operation and do not fully tie the DRBG
+		 * to the Jitter RNG.
+		 */
+		drbg->jent = NULL;
+	}
+
 	return 0;
 
 err:
diff --git a/include/crypto/drbg.h b/include/crypto/drbg.h
index e4980a1..fabf102 100644
--- a/include/crypto/drbg.h
+++ b/include/crypto/drbg.h
@@ -122,6 +122,7 @@ struct drbg_state {
 	struct random_work seed_work;	/* asynchronous seeding support */
 	u8 *seed_buf;			/* buffer holding the seed */
 	size_t seed_buf_len;
+	struct crypto_rng *jent;
 	const struct drbg_state_ops *d_ops;
 	const struct drbg_core *core;
 	struct drbg_string test_data;
-- 
2.1.0