[PATCH 5.15 20/66] crypto: drbg - prepare for more fine-grained tracking of seeding state

Greg Kroah-Hartman
2022-06-03T18:00:57Z

From: Nicolai Stange <nstange@suse.de>

commit ce8ce31b2c5c8b18667784b8c515650c65d57b4e upstream.

There are two different randomness sources the DRBGs are getting seeded
from, namely the jitterentropy source (if enabled) and get_random_bytes().
At initial DRBG seeding time during boot, the latter might not have
collected sufficient entropy for seeding itself yet and thus, the DRBG
implementation schedules a reseed work from a random_ready_callback once
that has happened. This is particularly important for the !->pr DRBG
instances, for which (almost) no further reseeds are getting triggered
during their lifetime.

Because collecting data from the jitterentropy source is a rather expensive
operation, the aforementioned asynchronously scheduled reseed work
restricts itself to get_random_bytes() only. That is, it in some sense
amends the initial DRBG seed derived from jitterentropy output at full
(estimated) entropy with fresh randomness obtained from get_random_bytes()
once that has been seeded with sufficient entropy itself.

With the advent of rng_is_initialized(), there is no real need for doing
the reseed operation from an asynchronously scheduled work anymore and a
subsequent patch will make it synchronous by moving it next to related
logic already present in drbg_generate().

However, for tracking whether a full reseed including the jitterentropy
source is required or a "partial" reseed involving only get_random_bytes()
would be sufficient already, the boolean struct drbg_state's ->seeded
member must become a tristate value.

Prepare for this by introducing the new enum drbg_seed_state and change
struct drbg_state's ->seeded member's type from bool to that type.

For facilitating review, enum drbg_seed_state is made to only contain
two members corresponding to the former ->seeded values of false and true
resp. at this point: DRBG_SEED_STATE_UNSEEDED and DRBG_SEED_STATE_FULL. A
third one for tracking the intermediate state of "seeded from jitterentropy
only" will be introduced with a subsequent patch.

There is no change in behaviour at this point.

Signed-off-by: Nicolai Stange <nstange@suse.de>
Reviewed-by: Stephan Müller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
crypto/drbg.c | 19 ++++++++++---------
include/crypto/drbg.h | 7 ++++++-
2 files changed, 16 insertions(+), 10 deletions(-)

--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1043,7 +1043,7 @@ static inline int __drbg_seed(struct drb
if (ret)
return ret;

- drbg->seeded = true;
+ drbg->seeded = DRBG_SEED_STATE_FULL;
/* 10.1.1.2 / 10.1.1.3 step 5 */
drbg->reseed_ctr = 1;

@@ -1088,14 +1088,14 @@ static void drbg_async_seed(struct work_
if (ret)
goto unlock;

- /* Set seeded to false so that if __drbg_seed fails the
- * next generate call will trigger a reseed.
+ /* Reset ->seeded so that if __drbg_seed fails the next
+ * generate call will trigger a reseed.
*/
- drbg->seeded = false;
+ drbg->seeded = DRBG_SEED_STATE_UNSEEDED;

__drbg_seed(drbg, &seedlist, true);

- if (drbg->seeded)
+ if (drbg->seeded == DRBG_SEED_STATE_FULL)
drbg->reseed_threshold = drbg_max_requests(drbg);

unlock:
@@ -1386,13 +1386,14 @@ static int drbg_generate(struct drbg_sta
* here. The spec is a bit convoluted here, we make it simpler.
*/
if (drbg->reseed_threshold < drbg->reseed_ctr)
- drbg->seeded = false;
+ drbg->seeded = DRBG_SEED_STATE_UNSEEDED;

- if (drbg->pr || !drbg->seeded) {
+ if (drbg->pr || drbg->seeded == DRBG_SEED_STATE_UNSEEDED) {
pr_devel("DRBG: reseeding before generation (prediction "
"resistance: %s, state %s)\n",
drbg->pr ? "true" : "false",
- drbg->seeded ? "seeded" : "unseeded");
+ (drbg->seeded == DRBG_SEED_STATE_FULL ?
+ "seeded" : "unseeded"));
/* 9.3.1 steps 7.1 through 7.3 */
len = drbg_seed(drbg, addtl, true);
if (len)
@@ -1577,7 +1578,7 @@ static int drbg_instantiate(struct drbg_
if (!drbg->core) {
drbg->core = &drbg_cores[coreref];
drbg->pr = pr;
- drbg->seeded = false;
+ drbg->seeded = DRBG_SEED_STATE_UNSEEDED;
drbg->reseed_threshold = drbg_max_requests(drbg);

ret = drbg_alloc_state(drbg);
--- a/include/crypto/drbg.h
+++ b/include/crypto/drbg.h
@@ -105,6 +105,11 @@ struct drbg_test_data {
struct drbg_string *testentropy; /* TEST PARAMETER: test entropy */
};

+enum drbg_seed_state {
+ DRBG_SEED_STATE_UNSEEDED,
+ DRBG_SEED_STATE_FULL,
+};
+
struct drbg_state {
struct mutex drbg_mutex; /* lock around DRBG */
unsigned char *V; /* internal state 10.1.1.1 1a) */
@@ -127,7 +132,7 @@ struct drbg_state {
struct crypto_wait ctr_wait; /* CTR mode async wait obj */
struct scatterlist sg_in, sg_out; /* CTR mode SGLs */

- bool seeded; /* DRBG fully seeded? */
+ enum drbg_seed_state seeded; /* DRBG fully seeded? */
bool pr; /* Prediction resistance enabled? */
bool fips_primed; /* Continuous test primed? */
unsigned char *prev; /* FIPS 140-2 continuous test value */