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Commit 0747ad15 authored by Jason A. Donenfeld's avatar Jason A. Donenfeld Committed by Greg Kroah-Hartman
Browse files

random: avoid initializing twice in credit race 



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: default avatarDominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: default avatarJason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 0baeec0e

drivers/char/random.c

+5 −5
Original line number Diff line number Diff line
@@ -818,7 +818,7 @@ 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)
@@ -828,12 +828,12 @@ 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));