[PATCH v6 4/5] crypto: drbg - use Jitter RNG to obtain seed

Stephan Mueller
2015-05-13T19:57:57Z

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 | 46 ++++++++++++++++++++++++++++++++++++++++------
include/crypto/drbg.h | 1 +
2 files changed, 41 insertions(+), 6 deletions(-)

diff --git a/crypto/drbg.c b/crypto/drbg.c
index 2b2738e..1224c0c 100644
--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1104,10 +1104,25 @@ 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);

@@ -1132,7 +1147,7 @@ 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;
/*
@@ -1173,6 +1188,10 @@ static inline void drbg_dealloc_state(struct drbg_state *drbg)
#endif
kzfree(drbg->seed_buf);
drbg->seed_buf = NULL;
+ if (drbg->jent) {
+ crypto_free_rng(drbg->jent);
+ drbg->jent = NULL;
+ }
}

/*
@@ -1248,14 +1267,29 @@ 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;

INIT_WORK(&drbg->seed_work, drbg_async_seed);

+ 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 46994b2..c3f208d 100644
--- a/include/crypto/drbg.h
+++ b/include/crypto/drbg.h
@@ -123,6 +123,7 @@ struct drbg_state {
struct work_struct 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