From 8fe9ac5ed215ade6f1cd23415b3474fbb6e3f7fa Mon Sep 17 00:00:00 2001 From: "Jason A. Donenfeld" Date: Mon, 9 May 2022 13:40:55 +0200 Subject: 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: Dominik Brodowski Signed-off-by: Jason A. Donenfeld Signed-off-by: Greg Kroah-Hartman --- drivers/char/random.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 6d20beb34971..0ad3fccb510b 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -821,7 +821,7 @@ 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) @@ -831,12 +831,12 @@ 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)); -- cgit v1.2.3