linux-kernel - [PATCH 3/6] crypto: DRBG - move dynamic ->reseed_threshold adjustments to __drbg

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Date:   Mon, 25 Oct 2021 11:25:22 +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 3/6] crypto: DRBG - move dynamic ->reseed_threshold adjustments to __drbg_seed()

Since commit 42ea507fae1a ("crypto: drbg - reseed often if seedsource is
degraded"), the maximum seed lifetime represented by ->reseed_threshold
gets temporarily lowered if the get_random_bytes() source cannot provide
sufficient entropy yet, as is common during boot, and restored back to
the original value again once that has changed.

More specifically, if the add_random_ready_callback() invoked from
drbg_prepare_hrng() in the course of DRBG instantiation does not return
-EALREADY, that is, if get_random_bytes() has not been fully initialized
at this point yet, drbg_prepare_hrng() will lower ->reseed_threshold
to a value of 50. The drbg_async_seed() scheduled from said
random_ready_callback will eventually restore the original value.

A future patch will replace the random_ready_callback based notification
mechanism and thus, there will be no add_random_ready_callback() return
value anymore which could get compared to -EALREADY.

However, there's __drbg_seed() which gets invoked in the course of both,
the DRBG instantiation as well as the eventual reseeding from
get_random_bytes() in aforementioned drbg_async_seed(), if any. Moreover,
it knows about the get_random_bytes() initialization state by the time the
seed data had been obtained from it: the new_seed_state argument introduced
with the previous patch would get set to DRBG_SEED_STATE_PARTIAL in case
get_random_bytes() had not been fully initialized yet and to
DRBG_SEED_STATE_FULL otherwise. Thus, __drbg_seed() provides a convenient
alternative for managing that ->reseed_threshold lowering and restoring at
a central place.

Move all ->reseed_threshold adjustment code from drbg_prepare_hrng() and
drbg_async_seed() respectively to __drbg_seed(). Make __drbg_seed()
lower the ->reseed_threshold to 50 in case its new_seed_state argument
equals DRBG_SEED_STATE_PARTIAL and let it restore the original value
otherwise.

There is no change in behaviour.

Signed-off-by: Nicolai Stange <nstange@...e.de>
---
 crypto/drbg.c | 30 +++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/crypto/drbg.c b/crypto/drbg.c
index 55fe8caf569a..6aad210f101a 100644
--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1047,6 +1047,27 @@ static inline int __drbg_seed(struct drbg_state *drbg, struct list_head *seed,
 	/* 10.1.1.2 / 10.1.1.3 step 5 */
 	drbg->reseed_ctr = 1;

+	switch (drbg->seeded) {
+	case DRBG_SEED_STATE_UNSEEDED:
+		/* Impossible, but handle it to silence compiler warnings. */
+		fallthrough;
+	case DRBG_SEED_STATE_PARTIAL:
+		/*
+		 * Require frequent reseeds until the seed source is
+		 * fully initialized.
+		 */
+		drbg->reseed_threshold = 50;
+		break;
+
+	case DRBG_SEED_STATE_FULL:
+		/*
+		 * Seed source has become fully initialized, frequent
+		 * reseeds no longer required.
+		 */
+		drbg->reseed_threshold = drbg_max_requests(drbg);
+		break;
+	}
+
 	return ret;
 }

@@ -1095,9 +1116,6 @@ static void drbg_async_seed(struct work_struct *work)

 	__drbg_seed(drbg, &seedlist, true, DRBG_SEED_STATE_FULL);

-	if (drbg->seeded == DRBG_SEED_STATE_FULL)
-		drbg->reseed_threshold = drbg_max_requests(drbg);
-
 unlock:
 	mutex_unlock(&drbg->drbg_mutex);

@@ -1534,12 +1552,6 @@ static int drbg_prepare_hrng(struct drbg_state *drbg)
 		return err;
 	}

-	/*
-	 * Require frequent reseeds until the seed source is fully
-	 * initialized.
-	 */
-	drbg->reseed_threshold = 50;
-
 	return err;
 }

-- 
2.26.2