crypto: drbg - make reseeding from get_random_bytes() synchronous

get_random_bytes() usually hasn't full entropy available by the time DRBG
instances are first getting seeded from it during boot. Thus, the DRBG
implementation registers random_ready_callbacks which would in turn
schedule some work for reseeding the DRBGs once get_random_bytes() has
sufficient entropy available.

For reference, the relevant history around handling DRBG (re)seeding in
the context of a not yet fully seeded get_random_bytes() is:

  commit 16b369a ("random: Blocking API for accessing
                        nonblocking_pool")
  commit 4c78799 ("crypto: drbg - add async seeding operation")

  commit 205a525 ("random: Add callback API for random pool
                        readiness")
  commit 57225e6 ("crypto: drbg - Use callback API for random
                        readiness")
  commit c271950 ("random: Remove kernel blocking API")

However, some time later, the initialization state of get_random_bytes()
has been made queryable via rng_is_initialized() introduced with commit
9a47249 ("random: Make crng state queryable"). This primitive now
allows for streamlining the DRBG reseeding from get_random_bytes() by
replacing that aforementioned asynchronous work scheduling from
random_ready_callbacks with some simpler, synchronous code in
drbg_generate() next to the related logic already present therein. Apart
from improving overall code readability, this change will also enable DRBG
users to rely on wait_for_random_bytes() for ensuring that the initial
seeding has completed, if desired.

The previous patches already laid the grounds by making drbg_seed() to
record at each DRBG instance whether it was being seeded at a time when
rng_is_initialized() still had been false as indicated by
->seeded == DRBG_SEED_STATE_PARTIAL.

All that remains to be done now is to make drbg_generate() check for this
condition, determine whether rng_is_initialized() has flipped to true in
the meanwhile and invoke a reseed from get_random_bytes() if so.

Make this move:
- rename the former drbg_async_seed() work handler, i.e. the one in charge
  of reseeding a DRBG instance from get_random_bytes(), to
  "drbg_seed_from_random()",
- change its signature as appropriate, i.e. make it take a struct
  drbg_state rather than a work_struct and change its return type from
  "void" to "int" in order to allow for passing error information from
  e.g. its __drbg_seed() invocation onwards to callers,
- make drbg_generate() invoke this drbg_seed_from_random() once it
  encounters a DRBG instance with ->seeded == DRBG_SEED_STATE_PARTIAL by
  the time rng_is_initialized() has flipped to true and
- prune everything related to the former, random_ready_callback based
  mechanism.

As drbg_seed_from_random() is now getting invoked from drbg_generate() with
the ->drbg_mutex being held, it must not attempt to recursively grab it
once again. Remove the corresponding mutex operations from what is now
drbg_seed_from_random(). Furthermore, as drbg_seed_from_random() can now
report errors directly to its caller, there's no need for it to temporarily
switch the DRBG's ->seeded state to DRBG_SEED_STATE_UNSEEDED so that a
failure of the subsequently invoked __drbg_seed() will get signaled to
drbg_generate(). Don't do it then.

Signed-off-by: Nicolai Stange <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>

crypto/drbg.c

@@ -1087,12 +1087,10 @@ static inline int drbg_get_random_bytes(struct drbg_state *drbg,
 	return 0;
 }
 
-static void drbg_async_seed(struct work_struct *work)
+static int drbg_seed_from_random(struct drbg_state *drbg)
 {
 	struct drbg_string data;
 	LIST_HEAD(seedlist);
-	struct drbg_state *drbg = container_of(work, struct drbg_state,
-					       seed_work);
 	unsigned int entropylen = drbg_sec_strength(drbg->core->flags);
 	unsigned char entropy[32];
 	int ret;
@@ -1103,23 +1101,15 @@ static void drbg_async_seed(struct work_struct *work)
 	drbg_string_fill(&data, entropy, entropylen);
 	list_add_tail(&data.list, &seedlist);
 
-	mutex_lock(&drbg->drbg_mutex);
-
 	ret = drbg_get_random_bytes(drbg, entropy, entropylen);
 	if (ret)
-		goto unlock;
-
-	/* Reset ->seeded so that if __drbg_seed fails the next
-	 * generate call will trigger a reseed.
-	 */
-	drbg->seeded = DRBG_SEED_STATE_UNSEEDED;
-
-	__drbg_seed(drbg, &seedlist, true, DRBG_SEED_STATE_FULL);
+		goto out;
 
-unlock:
-	mutex_unlock(&drbg->drbg_mutex);
+	ret = __drbg_seed(drbg, &seedlist, true, DRBG_SEED_STATE_FULL);
 
+out:
 	memzero_explicit(entropy, entropylen);
+	return ret;
 }
 
 /*
@@ -1422,6 +1412,11 @@ static int drbg_generate(struct drbg_state *drbg,
 			goto err;
 		/* 9.3.1 step 7.4 */
 		addtl = NULL;
+	} else if (rng_is_initialized() &&
+		   drbg->seeded == DRBG_SEED_STATE_PARTIAL) {
+		len = drbg_seed_from_random(drbg);
+		if (len)
+			goto err;
 	}
 
 	if (addtl && 0 < addtl->len)
@@ -1514,45 +1509,15 @@ static int drbg_generate_long(struct drbg_state *drbg,
 	return 0;
 }
 
-static void drbg_schedule_async_seed(struct random_ready_callback *rdy)
-{
-	struct drbg_state *drbg = container_of(rdy, struct drbg_state,
-					       random_ready);
-
-	schedule_work(&drbg->seed_work);
-}
-
 static int drbg_prepare_hrng(struct drbg_state *drbg)
 {
-	int err;
-
 	/* We do not need an HRNG in test mode. */
 	if (list_empty(&drbg->test_data.list))
 		return 0;
 
 	drbg->jent = crypto_alloc_rng("jitterentropy_rng", 0, 0);
 
-	INIT_WORK(&drbg->seed_work, drbg_async_seed);
-
-	drbg->random_ready.owner = THIS_MODULE;
-	drbg->random_ready.func = drbg_schedule_async_seed;
-
-	err = add_random_ready_callback(&drbg->random_ready);
-
-	switch (err) {
-	case 0:
-		break;
-
-	case -EALREADY:
-		err = 0;
-		fallthrough;
-
-	default:
-		drbg->random_ready.func = NULL;
-		return err;
-	}
-
-	return err;
+	return 0;
 }
 
 /*
@@ -1646,11 +1611,6 @@ static int drbg_instantiate(struct drbg_state *drbg, struct drbg_string *pers,
  */
 static int drbg_uninstantiate(struct drbg_state *drbg)
 {
-	if (drbg->random_ready.func) {
-		del_random_ready_callback(&drbg->random_ready);
-		cancel_work_sync(&drbg->seed_work);
-	}
-
 	if (!IS_ERR_OR_NULL(drbg->jent))
 		crypto_free_rng(drbg->jent);
 	drbg->jent = NULL;

include/crypto/drbg.h

@@ -137,12 +137,10 @@ struct drbg_state {
 	bool pr;		/* Prediction resistance enabled? */
 	bool fips_primed;	/* Continuous test primed? */
 	unsigned char *prev;	/* FIPS 140-2 continuous test value */
-	struct work_struct seed_work;	/* asynchronous seeding support */
 	struct crypto_rng *jent;
 	const struct drbg_state_ops *d_ops;
 	const struct drbg_core *core;
 	struct drbg_string test_data;
-	struct random_ready_callback random_ready;
 };
 
 static inline __u8 drbg_statelen(struct drbg_state *drbg)