From: "Jason A. Donenfeld" Jason@zx2c4.com
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 --- drivers/char/random.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)
--- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -821,7 +821,7 @@ static void extract_entropy(void *buf, s
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) @@ -831,12 +831,12 @@ static void credit_init_bits(size_t nbit
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));