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random: do crng pre-init loading in worker rather than irq 

Taking spinlocks from IRQ context is generally problematic for
PREEMPT_RT. That is, in part, why we take trylocks instead. However, a
spin_try_lock() is also problematic since another spin_lock() invocation
can potentially PI-boost the wrong task, as the spin_try_lock() is
invoked from an IRQ-context, so the task on CPU (random task or idle) is
not the actual owner.

Additionally, by deferring the crng pre-init loading to the worker, we
can use the cryptographic hash function rather than xor, which is
perhaps a meaningful difference when considering this data has only been
through the relatively weak fast_mix() function.

The biggest downside of this approach is that the pre-init loading is
now deferred until later, which means things that need random numbers
after interrupts are enabled, but before workqueues are running -- or
before this particular worker manages to run -- are going to get into
trouble. Hopefully in the real world, this window is rather small,
especially since this code won't run until 64 interrupts had occurred.

Cc: Sultan Alsawaf <sultan@kerneltoast.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
This commit is contained in:
Jason A. Donenfeld 2022-02-13 18:25:07 +01:00
parent abded93ec1
commit c2a7de4feb
1 changed files with 19 additions and 46 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

65
drivers/char/random.c
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@ -443,10 +443,6 @@ static void crng_make_state(u32 chacha_state[CHACHA_STATE_WORDS],
* 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
@ -455,19 +451,15 @@ static void crng_make_state(u32 chacha_state[CHACHA_STATE_WORDS],
* Returns the number of bytes processed from input, which is bounded
* by CRNG_INIT_CNT_THRESH if account is true.
*/
static size_t crng_pre_init_inject(const void *input, size_t len,
bool fast, bool account)
static size_t crng_pre_init_inject(const void *input, size_t len, bool account)
{
static int crng_init_cnt = 0;
struct blake2s_state hash;
unsigned long flags;
if (fast) {
if (!spin_trylock_irqsave(&base_crng.lock, flags))
return 0;
} else {
spin_lock_irqsave(&base_crng.lock, flags);
}
blake2s_init(&hash, sizeof(base_crng.key));
spin_lock_irqsave(&base_crng.lock, flags);
if (crng_init != 0) {
spin_unlock_irqrestore(&base_crng.lock, flags);
return 0;
@ -476,21 +468,9 @@ static size_t crng_pre_init_inject(const void *input, size_t len,
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);
}
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;
@ -1034,7 +1014,7 @@ void add_device_randomness(const void *buf, size_t size)
unsigned long flags, now = jiffies;
if (crng_init == 0 && size)
crng_pre_init_inject(buf, size, false, false);
crng_pre_init_inject(buf, size, false);
spin_lock_irqsave(&input_pool.lock, flags);
_mix_pool_bytes(&cycles, sizeof(cycles));
@ -1155,7 +1135,7 @@ void add_hwgenerator_randomness(const void *buffer, size_t count,
size_t entropy)
{
if (unlikely(crng_init == 0)) {
size_t ret = crng_pre_init_inject(buffer, count, false, true);
size_t ret = crng_pre_init_inject(buffer, count, true);
mix_pool_bytes(buffer, ret);
count -= ret;
buffer += ret;
@ -1295,8 +1275,14 @@ static void mix_interrupt_randomness(struct work_struct *work)
fast_pool->last = jiffies;
local_irq_enable();
mix_pool_bytes(pool, sizeof(pool));
credit_entropy_bits(1);
if (unlikely(crng_init == 0)) {
crng_pre_init_inject(pool, sizeof(pool), true);
mix_pool_bytes(pool, sizeof(pool));
} else {
mix_pool_bytes(pool, sizeof(pool));
credit_entropy_bits(1);
}
memzero_explicit(pool, sizeof(pool));
}
@ -1329,24 +1315,11 @@ void add_interrupt_randomness(int irq)
fast_mix(fast_pool->pool32);
new_count = ++fast_pool->count;
if (unlikely(crng_init == 0)) {
if (new_count >= 64 &&
crng_pre_init_inject(fast_pool->pool32, sizeof(fast_pool->pool32),
true, true) > 0) {
fast_pool->count = 0;
fast_pool->last = now;
if (spin_trylock(&input_pool.lock)) {
_mix_pool_bytes(&fast_pool->pool32, sizeof(fast_pool->pool32));
spin_unlock(&input_pool.lock);
}
}
return;
}
if (new_count & MIX_INFLIGHT)
return;
if (new_count < 64 && !time_after(now, fast_pool->last + HZ))
if (new_count < 64 && (!time_after(now, fast_pool->last + HZ) ||
unlikely(crng_init == 0)))
return;
if (unlikely(!fast_pool->mix.func))