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random: avoid initializing twice in credit race

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commit fed7ef061686cc813b1f3d8d0edc6c35b4d3537b upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Jason A. Donenfeld 2022-05-09 13:40:55 +02:00 committed by Greg Kroah-Hartman
parent 87fccf8de1
commit 39cd44bdb5
1 changed files with 22 additions and 26 deletions
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48
drivers/char/random.c
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@ -265,7 +265,6 @@ static void crng_reseed(void)
unsigned long flags;
unsigned long next_gen;
u8 key[CHACHA20_KEY_SIZE];
bool finalize_init = false;
extract_entropy(key, sizeof(key));
@ -282,28 +281,10 @@ static void crng_reseed(void)
++next_gen;
WRITE_ONCE(base_crng.generation, next_gen);
WRITE_ONCE(base_crng.birth, jiffies);
if (!crng_ready()) {
if (!crng_ready())
crng_init = CRNG_READY;
finalize_init = true;
}
spin_unlock_irqrestore(&base_crng.lock, flags);
memzero_explicit(key, sizeof(key));
if (finalize_init) {
process_random_ready_list();
wake_up_interruptible(&crng_init_wait);
kill_fasync(&fasync, SIGIO, POLL_IN);
pr_notice("crng init done\n");
if (unseeded_warning.missed) {
pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
unseeded_warning.missed);
unseeded_warning.missed = 0;
}
if (urandom_warning.missed) {
pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
urandom_warning.missed);
urandom_warning.missed = 0;
}
}
}
/*
@ -819,7 +800,7 @@ 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)
@ -829,13 +810,28 @@ 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)
crng_reseed();
else if (unlikely(crng_init == CRNG_EMPTY && init_bits >= POOL_EARLY_BITS)) {
if (orig < POOL_READY_BITS && new >= POOL_READY_BITS) {
crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */
process_random_ready_list();
wake_up_interruptible(&crng_init_wait);
kill_fasync(&fasync, SIGIO, POLL_IN);
pr_notice("crng init done\n");
if (unseeded_warning.missed) {
pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
unseeded_warning.missed);
unseeded_warning.missed = 0;
}
if (urandom_warning.missed) {
pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
urandom_warning.missed);
urandom_warning.missed = 0;
}
} else if (orig < POOL_EARLY_BITS && new >= POOL_EARLY_BITS) {
spin_lock_irqsave(&base_crng.lock, flags);
/* Check if crng_init is CRNG_EMPTY, to avoid race with crng_reseed(). */
if (crng_init == CRNG_EMPTY) {
extract_entropy(base_crng.key, sizeof(base_crng.key));
crng_init = CRNG_EARLY;