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random: avoid initializing twice in credit race 

Since all changes of crng_init now go through credit_init_bits(), we can
fix a long standing race in which two concurrent callers of
credit_init_bits() have the new bit count >= some threshold, but are
doing so with crng_init as a lower threshold, checked outside of a lock,
resulting in crng_reseed() or similar being called twice.

In order to fix this, we can use the original cmpxchg value of the bit
count, and only change crng_init when the bit count transitions from
below a threshold to meeting the threshold.

Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
This commit is contained in:
Jason A. Donenfeld 2022-05-09 13:40:55 +02:00
parent e3d2c5e79a
commit fed7ef0616
1 changed files with 5 additions and 5 deletions
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10
drivers/char/random.c
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@ -822,7 +822,7 @@ static void extract_entropy(void *buf, size_t nbytes)
static void credit_init_bits(size_t nbits)
{
unsigned int init_bits, orig, add;
unsigned int new, orig, add;
unsigned long flags;
if (crng_ready() || !nbits)
@ -832,12 +832,12 @@ static void credit_init_bits(size_t nbits)
do {
orig = READ_ONCE(input_pool.init_bits);
init_bits = min_t(unsigned int, POOL_BITS, orig + add);
} while (cmpxchg(&input_pool.init_bits, orig, init_bits) != orig);
new = min_t(unsigned int, POOL_BITS, orig + add);
} while (cmpxchg(&input_pool.init_bits, orig, new) != orig);
if (!crng_ready() && init_bits >= POOL_READY_BITS)
if (orig < POOL_READY_BITS && new >= POOL_READY_BITS)
crng_reseed();
else if (unlikely(crng_init == CRNG_EMPTY && init_bits >= POOL_EARLY_BITS)) {
else if (orig < POOL_EARLY_BITS && new >= POOL_EARLY_BITS) {
spin_lock_irqsave(&base_crng.lock, flags);
if (crng_init == CRNG_EMPTY) {
extract_entropy(base_crng.key, sizeof(base_crng.key));