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Subject[PATCH 4.19 177/234] crypto: drbg - make reseeding from get_random_bytes() synchronous
Date
From: Nicolai Stange <nstange@suse.de>

commit 074bcd4000e0d812bc253f86fedc40f81ed59ccc upstream.

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 16b369a91d0d ("random: Blocking API for accessing
nonblocking_pool")
commit 4c7879907edd ("crypto: drbg - add async seeding operation")

commit 205a525c3342 ("random: Add callback API for random pool
readiness")
commit 57225e679788 ("crypto: drbg - Use callback API for random
readiness")
commit c2719503f5e1 ("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
9a47249d444d ("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 <nstange@suse.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
[Jason: for stable, undid the modifications for the backport of 5acd3548.]
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 | 61 +++++++++-----------------------------------------
drivers/char/random.c | 2 -
include/crypto/drbg.h | 2 -
3 files changed, 11 insertions(+), 54 deletions(-)

--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1085,12 +1085,10 @@ static inline int drbg_get_random_bytes(
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;
@@ -1101,23 +1099,15 @@ static void drbg_async_seed(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;
+ goto out;

- __drbg_seed(drbg, &seedlist, true, DRBG_SEED_STATE_FULL);
-
-unlock:
- mutex_unlock(&drbg->drbg_mutex);
+ ret = __drbg_seed(drbg, &seedlist, true, DRBG_SEED_STATE_FULL);

+out:
memzero_explicit(entropy, entropylen);
+ return ret;
}

/*
@@ -1418,6 +1408,11 @@ static int drbg_generate(struct drbg_sta
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)
@@ -1510,44 +1505,15 @@ static int drbg_generate_long(struct drb
return 0;
}

-static int drbg_schedule_async_seed(struct notifier_block *nb, unsigned long action, void *data)
-{
- struct drbg_state *drbg = container_of(nb, struct drbg_state,
- random_ready);
-
- schedule_work(&drbg->seed_work);
- return 0;
-}
-
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.notifier_call = drbg_schedule_async_seed;
- err = register_random_ready_notifier(&drbg->random_ready);
-
- switch (err) {
- case 0:
- break;
-
- case -EALREADY:
- err = 0;
- /* fall through */
-
- default:
- drbg->random_ready.notifier_call = NULL;
- return err;
- }
-
- return err;
+ return 0;
}

/*
@@ -1641,11 +1607,6 @@ free_everything:
*/
static int drbg_uninstantiate(struct drbg_state *drbg)
{
- if (drbg->random_ready.notifier_call) {
- unregister_random_ready_notifier(&drbg->random_ready);
- cancel_work_sync(&drbg->seed_work);
- }
-
if (!IS_ERR_OR_NULL(drbg->jent))
crypto_free_rng(drbg->jent);
drbg->jent = NULL;
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -157,7 +157,6 @@ int __cold register_random_ready_notifie
spin_unlock_irqrestore(&random_ready_chain_lock, flags);
return ret;
}
-EXPORT_SYMBOL(register_random_ready_notifier);

/*
* Delete a previously registered readiness callback function.
@@ -172,7 +171,6 @@ int __cold unregister_random_ready_notif
spin_unlock_irqrestore(&random_ready_chain_lock, flags);
return ret;
}
-EXPORT_SYMBOL(unregister_random_ready_notifier);

static void __cold process_random_ready_list(void)
{
--- a/include/crypto/drbg.h
+++ b/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 notifier_block random_ready;
};

static inline __u8 drbg_statelen(struct drbg_state *drbg)

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