[PATCH 5.4 175/240] crypto: drbg - move dynamic ->reseed_threshold adjustments to __drbg_seed()

Greg Kroah-Hartman
2022-06-20T14:10:22Z

From: Nicolai Stange <nstange@suse.de>

commit 262d83a4290c331cd4f617a457408bdb82fbb738 upstream.

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@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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
crypto/drbg.c | 30 +++++++++++++++++++++---------
1 file changed, 21 insertions(+), 9 deletions(-)

--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1046,6 +1046,27 @@ static inline int __drbg_seed(struct drb
/* 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;
}

@@ -1094,9 +1115,6 @@ static void drbg_async_seed(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);

@@ -1532,12 +1550,6 @@ static int drbg_prepare_hrng(struct drbg
return err;
}

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