linux-kernel - [PATCH 2/6] crypto: DRBG - track whether DRBG was seeded with !rng_is

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Message-Id: <20211025092525.12805-3-nstange@suse.de>
Date:   Mon, 25 Oct 2021 11:25:21 +0200
From:   Nicolai Stange <nstange@...e.de>
To:     Stephan Müller <smueller@...onox.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Torsten Duwe <duwe@...e.de>, linux-crypto@...r.kernel.org,
        linux-kernel@...r.kernel.org, Nicolai Stange <nstange@...e.de>
Subject: [PATCH 2/6] crypto: DRBG - track whether DRBG was seeded with !rng_is_initialized()

Currently, the DRBG implementation schedules asynchronous works from
random_ready_callbacks for reseeding the DRBG instances with output from
get_random_bytes() once the latter has sufficient entropy available.

However, as the get_random_bytes() initialization state can get queried by
means of rng_is_initialized() now, there is no real need for this
asynchronous reseeding logic anymore and it's better to keep things simple
by doing it synchronously when needed instead, i.e. from drbg_generate()
once rng_is_initialized() has flipped to true.

Of course, for this to work, drbg_generate() would need some means by which
it can tell whether or not rng_is_initialized() has flipped to true since
the last seeding from get_random_bytes(). Or equivalently, whether or not
the last seed from get_random_bytes() has happened when
rng_is_initialized() was still evaluating to false.

As it currently stands, enum drbg_seed_state allows for the representation
of two different DRBG seeding states: DRBG_SEED_STATE_UNSEEDED and
DRBG_SEED_STATE_FULL. The former makes drbg_generate() to invoke a full
reseeding operation involving both, the rather expensive jitterentropy as
well as the get_random_bytes() randomness sources. The DRBG_SEED_STATE_FULL
state on the other hand implies that no reseeding at all is required for a
!->pr DRBG variant.

Introduce the new DRBG_SEED_STATE_PARTIAL state to enum drbg_seed_state for
representing the condition that a DRBG was being seeded when
rng_is_initialized() had still been false. In particular, this new state
implies that
- the given DRBG instance has been fully seeded from the jitterentropy
  source (if enabled)
- and drbg_generate() is supposed to reseed from get_random_bytes()
  *only* once rng_is_initialized() turns to true.

Up to now, the __drbg_seed() helper used to set the given DRBG instance's
->seeded state to constant DRBG_SEED_STATE_FULL. Introduce a new argument
allowing for the specification of the to be written ->seeded value instead.
Make the first of its two callers, drbg_seed(), determine the appropriate
value based on rng_is_initialized(). The remaining caller,
drbg_async_seed(), is known to get invoked only once rng_is_initialized()
is true, hence let it pass constant DRBG_SEED_STATE_FULL for the new
argument to __drbg_seed().

There is no change in behaviour, except for that the pr_devel() in
drbg_generate() would now report "unseeded" for ->pr DRBG instances which
had last been seeded when rng_is_initialized() was still evaluating to
false.

Signed-off-by: Nicolai Stange <nstange@...e.de>
---
 crypto/drbg.c         | 12 ++++++++----
 include/crypto/drbg.h |  1 +
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/crypto/drbg.c b/crypto/drbg.c
index 8651bd2c5c82..55fe8caf569a 100644
--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1036,14 +1036,14 @@ static const struct drbg_state_ops drbg_hash_ops = {
  ******************************************************************/
 
 static inline int __drbg_seed(struct drbg_state *drbg, struct list_head *seed,
-			      int reseed)
+			      int reseed, enum drbg_seed_state new_seed_state)
 {
 	int ret = drbg->d_ops->update(drbg, seed, reseed);
 
 	if (ret)
 		return ret;
 
-	drbg->seeded = DRBG_SEED_STATE_FULL;
+	drbg->seeded = new_seed_state;
 	/* 10.1.1.2 / 10.1.1.3 step 5 */
 	drbg->reseed_ctr = 1;
 
@@ -1093,7 +1093,7 @@ static void drbg_async_seed(struct work_struct *work)
 	 */
 	drbg->seeded = DRBG_SEED_STATE_UNSEEDED;
 
-	__drbg_seed(drbg, &seedlist, true);
+	__drbg_seed(drbg, &seedlist, true, DRBG_SEED_STATE_FULL);
 
 	if (drbg->seeded == DRBG_SEED_STATE_FULL)
 		drbg->reseed_threshold = drbg_max_requests(drbg);
@@ -1123,6 +1123,7 @@ static int drbg_seed(struct drbg_state *drbg, struct drbg_string *pers,
 	unsigned int entropylen = drbg_sec_strength(drbg->core->flags);
 	struct drbg_string data1;
 	LIST_HEAD(seedlist);
+	enum drbg_seed_state new_seed_state = DRBG_SEED_STATE_FULL;
 
 	/* 9.1 / 9.2 / 9.3.1 step 3 */
 	if (pers && pers->len > (drbg_max_addtl(drbg))) {
@@ -1150,6 +1151,9 @@ static int drbg_seed(struct drbg_state *drbg, struct drbg_string *pers,
 		BUG_ON((entropylen * 2) > sizeof(entropy));
 
 		/* Get seed from in-kernel /dev/urandom */
+		if (!rng_is_initialized())
+			new_seed_state = DRBG_SEED_STATE_PARTIAL;
+
 		ret = drbg_get_random_bytes(drbg, entropy, entropylen);
 		if (ret)
 			goto out;
@@ -1206,7 +1210,7 @@ static int drbg_seed(struct drbg_state *drbg, struct drbg_string *pers,
 		memset(drbg->C, 0, drbg_statelen(drbg));
 	}
 
-	ret = __drbg_seed(drbg, &seedlist, reseed);
+	ret = __drbg_seed(drbg, &seedlist, reseed, new_seed_state);
 
 out:
 	memzero_explicit(entropy, entropylen * 2);
diff --git a/include/crypto/drbg.h b/include/crypto/drbg.h
index 92a87b23ad2f..3ebdb1effe74 100644
--- a/include/crypto/drbg.h
+++ b/include/crypto/drbg.h
@@ -107,6 +107,7 @@ struct drbg_test_data {
 
 enum drbg_seed_state {
 	DRBG_SEED_STATE_UNSEEDED,
+	DRBG_SEED_STATE_PARTIAL, /* Seeded with !rng_is_initialized() */
 	DRBG_SEED_STATE_FULL,
 };
 
-- 
2.26.2