[PATCH v2 1/6] crypto: DRBG - prepare for more fine-grained tracking of seeding state

[Nicolai Stange]

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@xxxxxxx>
Reviewed-by: Stephan Müller <smueller@xxxxxxxxxx>
---
 crypto/drbg.c         | 19 ++++++++++---------
 include/crypto/drbg.h |  7 ++++++-
 2 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/crypto/drbg.c b/crypto/drbg.c
index f72f340a1321..6329c70e5bbb 100644
--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1043,7 +1043,7 @@ static inline int __drbg_seed(struct drbg_state *drbg, struct list_head *seed,
 	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_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_state *drbg,
 	 * 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)
@@ -1578,7 +1579,7 @@ static int drbg_instantiate(struct drbg_state *drbg, struct drbg_string *pers,
 	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);
diff --git a/include/crypto/drbg.h b/include/crypto/drbg.h
index c4165126937e..92a87b23ad2f 100644
--- 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 */
-- 
2.26.2