[PATCH 4.9 143/264] random: unify early init crng load accounting

Greg Kroah-Hartman
2022-06-23T16:56:42Z

From: "Jason A. Donenfeld" <Jason@zx2c4.com>

commit da792c6d5f59a76c10a310c5d4c93428fd18f996 upstream.

crng_fast_load() and crng_slow_load() have different semantics:

- crng_fast_load() xors and accounts with crng_init_cnt.
- crng_slow_load() hashes and doesn't account.

However add_hwgenerator_randomness() can afford to hash (it's called
from a kthread), and it should account. Additionally, ones that can
afford to hash don't need to take a trylock but can take a normal lock.
So, we combine these into one function, crng_pre_init_inject(), which
allows us to control these in a uniform way. This will make it simpler
later to simplify this all down when the time comes for that.

Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/char/random.c | 114 +++++++++++++++++++++++++-------------------------
1 file changed, 59 insertions(+), 55 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -385,7 +385,7 @@ static void crng_make_state(u32 chacha_s
* For the fast path, we check whether we're ready, unlocked first, and
* then re-check once locked later. In the case where we're really not
* ready, we do fast key erasure with the base_crng directly, because
- * this is what crng_{fast,slow}_load mutate during early init.
+ * this is what crng_pre_init_inject() mutates during early init.
*/
if (unlikely(!crng_ready())) {
bool ready;
@@ -436,72 +436,75 @@ static void crng_make_state(u32 chacha_s
}

/*
- * This function is for crng_init == 0 only.
+ * This function is for crng_init == 0 only. It loads entropy directly
+ * into the crng's key, without going through the input pool. It is,
+ * generally speaking, not very safe, but we use this only at early
+ * boot time when it's better to have something there rather than
+ * nothing.
+ *
+ * There are two paths, a slow one and a fast one. The slow one
+ * hashes the input along with the current key. The fast one simply
+ * xors it in, and should only be used from interrupt context.
+ *
+ * If account is set, then the crng_init_cnt counter is incremented.
+ * This shouldn't be set by functions like add_device_randomness(),
+ * where we can't trust the buffer passed to it is guaranteed to be
+ * unpredictable (so it might not have any entropy at all).
*
- * crng_fast_load() can be called by code in the interrupt service
- * path. So we can't afford to dilly-dally. Returns the number of
- * bytes processed from cp.
+ * Returns the number of bytes processed from input, which is bounded
+ * by CRNG_INIT_CNT_THRESH if account is true.
*/
-static size_t crng_fast_load(const void *cp, size_t len)
+static size_t crng_pre_init_inject(const void *input, size_t len,
+ bool fast, bool account)
{
static int crng_init_cnt = 0;
unsigned long flags;
- const u8 *src = (const u8 *)cp;
- size_t ret = 0;

- if (!spin_trylock_irqsave(&base_crng.lock, flags))
- return 0;
+ if (fast) {
+ if (!spin_trylock_irqsave(&base_crng.lock, flags))
+ return 0;
+ } else {
+ spin_lock_irqsave(&base_crng.lock, flags);
+ }
+
if (crng_init != 0) {
spin_unlock_irqrestore(&base_crng.lock, flags);
return 0;
}
- while (len > 0 && crng_init_cnt < CRNG_INIT_CNT_THRESH) {
- base_crng.key[crng_init_cnt % sizeof(base_crng.key)] ^= *src;
- src++; crng_init_cnt++; len--; ret++;
- }
- if (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
- ++base_crng.generation;
- crng_init = 1;
- }
- spin_unlock_irqrestore(&base_crng.lock, flags);
- if (crng_init == 1)
- pr_notice("fast init done\n");
- return ret;
-}

-/*
- * This function is for crng_init == 0 only.
- *
- * crng_slow_load() is called by add_device_randomness, which has two
- * attributes. (1) We can't trust the buffer passed to it is
- * guaranteed to be unpredictable (so it might not have any entropy at
- * all), and (2) it doesn't have the performance constraints of
- * crng_fast_load().
- *
- * So, we simply hash the contents in with the current key. Finally,
- * we do *not* advance crng_init_cnt since buffer we may get may be
- * something like a fixed DMI table (for example), which might very
- * well be unique to the machine, but is otherwise unvarying.
- */
-static void crng_slow_load(const void *cp, size_t len)
-{
- unsigned long flags;
- struct blake2s_state hash;
+ if (account)
+ len = min_t(size_t, len, CRNG_INIT_CNT_THRESH - crng_init_cnt);

- blake2s_init(&hash, sizeof(base_crng.key));
-
- if (!spin_trylock_irqsave(&base_crng.lock, flags))
- return;
- if (crng_init != 0) {
- spin_unlock_irqrestore(&base_crng.lock, flags);
- return;
+ if (fast) {
+ const u8 *src = input;
+ size_t i;
+
+ for (i = 0; i < len; ++i)
+ base_crng.key[(crng_init_cnt + i) %
+ sizeof(base_crng.key)] ^= src[i];
+ } else {
+ struct blake2s_state hash;
+
+ blake2s_init(&hash, sizeof(base_crng.key));
+ blake2s_update(&hash, base_crng.key, sizeof(base_crng.key));
+ blake2s_update(&hash, input, len);
+ blake2s_final(&hash, base_crng.key);
+ }
+
+ if (account) {
+ crng_init_cnt += len;
+ if (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
+ ++base_crng.generation;
+ crng_init = 1;
+ }
}

- blake2s_update(&hash, base_crng.key, sizeof(base_crng.key));
- blake2s_update(&hash, cp, len);
- blake2s_final(&hash, base_crng.key);
-
spin_unlock_irqrestore(&base_crng.lock, flags);
+
+ if (crng_init == 1)
+ pr_notice("fast init done\n");
+
+ return len;
}

static void _get_random_bytes(void *buf, size_t nbytes)
@@ -1014,7 +1017,7 @@ void add_device_randomness(const void *b
unsigned long flags;

if (!crng_ready() && size)
- crng_slow_load(buf, size);
+ crng_pre_init_inject(buf, size, false, false);

spin_lock_irqsave(&input_pool.lock, flags);
_mix_pool_bytes(buf, size);
@@ -1131,7 +1134,7 @@ void add_hwgenerator_randomness(const vo
size_t entropy)
{
if (unlikely(crng_init == 0)) {
- size_t ret = crng_fast_load(buffer, count);
+ size_t ret = crng_pre_init_inject(buffer, count, false, true);
mix_pool_bytes(buffer, ret);
count -= ret;
buffer += ret;
@@ -1294,7 +1297,8 @@ void add_interrupt_randomness(int irq)

if (unlikely(crng_init == 0)) {
if (new_count >= 64 &&
- crng_fast_load(fast_pool->pool32, sizeof(fast_pool->pool32)) > 0) {
+ crng_pre_init_inject(fast_pool->pool32, sizeof(fast_pool->pool32),
+ true, true) > 0) {
atomic_set(&fast_pool->count, 0);
fast_pool->last = now;
if (spin_trylock(&input_pool.lock)) {