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    Subject[PATCH 5.10 146/163] random: avoid initializing twice in credit race
    Date
    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
    @@ -823,7 +823,7 @@ 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)
    @@ -833,12 +833,12 @@ 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));

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