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Message-Id: <20220223185511.628452-1-Jason@zx2c4.com>
Date: Wed, 23 Feb 2022 19:55:11 +0100
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
bigeasy@...utronix.de
Cc: "Jason A. Donenfeld" <Jason@...c4.com>,
Dominik Brodowski <linux@...inikbrodowski.net>,
Sultan Alsawaf <sultan@...neltoast.com>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Theodore Ts'o <tytso@....edu>
Subject: [PATCH] random: do crng pre-init loading in worker rather than irq
Taking spinlocks from IRQ context is problematic for PREEMPT_RT. That
is, in part, why we take trylocks instead. But apparently this still
trips up various lock dependency analyzers. That seems like a bug in the
analyzers that should be fixed, rather than having to change things
here.
But maybe there's another reason to change things up: 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: Dominik Brodowski <linux@...inikbrodowski.net>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Sultan Alsawaf <sultan@...neltoast.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Theodore Ts'o <tytso@....edu>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
drivers/char/random.c | 62 ++++++++++++-------------------------------
1 file changed, 17 insertions(+), 45 deletions(-)
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 536237a0f073..9fb06fc298d3 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -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;
@@ -1040,7 +1020,7 @@ void add_device_randomness(const void *buf, size_t size)
unsigned long flags;
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(buf, size);
@@ -1157,7 +1137,7 @@ 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;
@@ -1298,7 +1278,12 @@ static void mix_interrupt_randomness(struct work_struct *work)
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);
+ else
+ credit_entropy_bits(1);
+
memzero_explicit(pool, sizeof(pool));
}
@@ -1331,24 +1316,11 @@ 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))
--
2.35.1
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