[PATCH 4.9 158/264] random: do crng pre-init loading in worker rather than irq

Greg Kroah-Hartman
2022-06-23T17:01:48Z

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

commit c2a7de4feb6e09f23af7accc0f882a8fa92e7ae5 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/char/random.c | 65 ++++++++++++++------------------------------------
1 file changed, 19 insertions(+), 46 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -442,10 +442,6 @@ static void crng_make_state(u32 chacha_s
* 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
@@ -454,19 +450,15 @@ static void crng_make_state(u32 chacha_s
* 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;
@@ -475,21 +467,9 @@ static size_t crng_pre_init_inject(const
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;
@@ -1030,7 +1010,7 @@ void add_device_randomness(const void *b
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));
@@ -1151,7 +1131,7 @@ void add_hwgenerator_randomness(const vo
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;
@@ -1291,8 +1271,14 @@ static void mix_interrupt_randomness(str
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));
}

@@ -1325,24 +1311,11 @@ 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))