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random: unify early init crng load accounting

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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>
This commit is contained in:
Jason A. Donenfeld 2022-02-12 23:54:09 +01:00
parent afba0b80b9
commit da792c6d5f
1 changed files with 58 additions and 54 deletions
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112
drivers/char/random.c
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@ -386,7 +386,7 @@ static void crng_make_state(u32 chacha_state[CHACHA_STATE_WORDS],
* 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;
@ -437,72 +437,75 @@ static void crng_make_state(u32 chacha_state[CHACHA_STATE_WORDS],
}
/*
* 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.
*
* 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.
* 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).
*
* 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 (account)
len = min_t(size_t, len, CRNG_INIT_CNT_THRESH - crng_init_cnt);
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 (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
++base_crng.generation;
crng_init = 1;
if (account) {
crng_init_cnt += len;
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;
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;
}
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);
return len;
}
static void _get_random_bytes(void *buf, size_t nbytes)
@ -1018,7 +1021,7 @@ void add_device_randomness(const void *buf, size_t size)
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);
@ -1135,7 +1138,7 @@ void add_hwgenerator_randomness(const void *buffer, size_t count,
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;
@ -1298,7 +1301,8 @@ 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)) {